Gas and Galaxy Evolution Abstracts

This page contains all the abstracts that have been received, sorted alphabetically. The date of last update is given at the bottom. I've made an attempt to format the abstracts in a readable fashion, but have not tried to reproduce all the nuances of LaTeX; nor have I included figures. The abstract book should look much better.

If you wish to change your title and/or abstract text, or if there is an error in what is given here, please let me know, via email to vla2000@nrao.edu.

Allen, Ron (STScI)
Cold Molecular Gas, PDRs and the Origin of HI in Galaxies
Ron Allen
In the current "standard" model for star formation out of the interstellar gas in galaxies, the basic construction material is taken to be large clouds of atomic hydrogen. These clouds aggregate into higher-density complexes of gas and dust, and turn molecular. Stars then form out of the molecular gas.

I will discuss the evidence for a contrary, "heretical" view, in which the basic construction material is cold molecular gas with a turbulent, generally-fractal structure, illuminated with UV photons from nearby young stars which have formed directly from the molecular gas. The physics of photodissociation regions provides a natural explanation for the appearance of CO emission from the surfaces of the clouds, and of HI envelopes around them. In this picture, the HI in galaxy spiral arms is a product of the star formation process, not a precursor to it.

Baker, Andrew (MPE)
Molecular Gas Kinematics in Nearby Active Galaxies
Andrew J. Baker
Using the Owens Valley Radio Observatory millimeter array, I have mapped the molecular gas in seven nearby Seyfert and LINER galaxies at 1'' (~100 pc) resolution. Each system shows a massive nuclear gas concentration, and most show strong evidence for noncircular or noncoplanar gas motions. I will present detailed kinematic models of three examples-- NGC 1068, NGC 5005, and NGC 7479-- in which warps and/or bars will drive steady or episodic fuelling of star formation and accretion flows at smaller scales. I will discuss the implications of these results for the evolution of active galaxies' circumnuclear regions.

Barnes, Josh (IfA)
Gas Dynamics in Galaxy Mergers
Joshua E. Barnes
In interacting and merging galaxies, gas is subject to direct hydrodynamic effects as well as tidal effects. One consequence of such interactions is the rapid inflows of gas which may fuel central starbursts; this is known as stoking the furnace'. However, gas dynamics is not limited to inflows; a modest survey of both equal-mass and unequal-mass encounters shows a variety of features, including gas bridges, extended rings produced by infall from tidal tails, and counter-rotating central gas disks.

Beijersbergen, Marco (Kapteyn Astronomical Institute)
Radial U band Luminosity Functions in Coma
M. Beijersbergen, H. Hoekstra, J.M. van der Hulst, & P.G. van Dokkum
Clusters of galaxies are important for studies of galaxy evolution; the galaxy population in clusters is very different from the population in the field. This means that galaxy formation and evolution are a strong function of environment. We have used the Wide Field Camera on the Isaac Newton Telescope to map 5.2 square degrees in B and R and 1.3 square degrees in U of the Coma cluster. Obtaining deep photometry of a large part a cluster gives the interesting possibility to study the LF as function of position within the cluster. Galaxies in the core region are expected to have a different merger history than the galaxies populating the outskirts. One then also expects different shaped LFs for dense and less dense regions in a cluster. In this poster we present the data and concentrate on the radial U band LFs.

Bland-Hawthorn, Joss (Anglo-Australian Observatory)
The Galactic Halo Ionizing Field
J. Bland-Hawthorn
Since 1996, we (in collaboration with P.R. Maloney) have proposed that a significant fraction (>5%) of ionizing photons escape the Galactic disk into the halo. Moreover, we have shown that the H$\alpha$ flux from an HI cloud can be used to infer the external UV field and therefore the cloud's distance from the Galaxy. This method has recently been used by two groups to show that many high-velocity clouds are distributed throughout the halo on tens of kiloparsec scales. Given the potential importance of the H$\alpha$ distance method, we present evidence for UV escape, correct an earlier error, discuss some complicating factors, and show the likely impact on the Galactic halo field.

The Magellanic Stream H-alpha detections remain an important constraint on the shape and normalization of the halo UV field. Using revised O star fluxes, to produce 200 mR in H-alpha, the UV escape fraction would need to be (4%, 10%, 20%) for a Stream distance of (25, 40, 55) kpc. There is extenuating evidence in favour of a low escape fraction, but this seems to argue for an unconventional distance. If all detectable clouds really are photoionized, the large H-alpha scatter between clouds might indicate that the projected Stream has significant depth along the line of sight.

Detecting Dark Matter in High Velocity Clouds
J. Bland-Hawthorn, G.F. Lewis, B. Gibson, & M. Putman
Many high-velocity HI clouds (HVC) are now believed to be scattered throughout the Galactic halo on scales of tens of kiloparsecs. Some of these clouds appear to contain substantial HI masses (>10^6 M_sun). It has been suggested that these structures may be associated with dark matter mini haloes' accreting onto the Galactic halo. For a compact HVC along the sight line to a more distant galaxy, we demonstrate that pixel lensing provides a crucial test for the presence of a grainy dark halo. The detection of pixel lensing will provide an independent means to map the mass distribution within HVCs.

Borne, Kirk (Goddard Space Flight Center)
The Ultra-Luminous IR Galaxy Population
K.D. Borne, H. Bushouse, R.A. Lucas, & L. Colina
Results will be presented from an on-going Hubble Space Telescope (HST) survey of a large sample of ULIRGs (Ultra-Luminous IR Galaxies). New ground-based observations are now being used to complement the HST data and to assist in the interpretation of these complex objects. Spectroscopic results will be presented for several ULIRGs on the nature of the luminous gas-rich knots that are ubiquitous among these galaxies. A summary of the morphological results from our HST survey will demonstrate the rich dynamical diversity within the ULIRG population, nearly 100% of which are merger and/or collision remnants. The consequences of this diversity may also be applicable to the interpretation of the distant Submm/FIR sources and their subsequent evolution: there are multiple emission sources and complex dynamical histories among these galaxies.

Braun, Robert (NFRA)
Compact High-Velocity Clouds in the Local Group and the NGC 628 Group
Robert Braun and W. Butler Burton
A distinct sub-class of anomalous velocity HI emission features has emerged from recent high quality surveys of the Local Group environment, namely the compact high velocity clouds (CHVCs). A program of high-resolution imaging with the Westerbork array and the Arecibo telescope has begun to provide many insights into the nature of these objects. Elongated core components with a velocity gradient consistent with rotation (V_Rot~15 km/sec) are seen in many objects. Comparison of volume and column densities has allowed the first distance estimates to be made (600+/-300~kpc). The objects appear to be strongly dark-matter dominated with dark-to-gas mass ratios of 30--50 implied if the typical distance is 700~kpc. Distant counterparts of the CHVCs have been sought in the NGC 628 galaxy group in a deep, wide-field survey with a 5\sigma mass sensitivity over 32 km/sec of 2x10^6 M_sun. Tentative detections of 10's of objects have been made, most of which have no optical counterpart in the POSS2 images.

Bravo-Alfaro, Hector (Universidad de Guanajuato, Mexico)
The Dynamics of Coma from a VLA HI Survey
H. Bravo-Alfaro, J. H. van Gorkom, V. Cayatte, & C. Balkowski
We carried out three HI surveys with the VLA inside one Abell radius of the Coma cluster, detecting 19 spiral galaxies. The goal of our HI imaging of clusters is twofold: first, study the interaction between individual galaxies and the cluster environment, and look for a connection with the SB--PSB phenomena; and second, investigate the dynamical state of the cluster. In this work we present results concerning the last issue, based on the distribution of detected and non detected spiral galaxies in Coma. We compare this distribution with the position of groups as reported from optical surveys. We detect gas rich and gas poor groups of galaxies inside Coma, that can be isolated in space and velocity. We suggest that the most HI poor groups (center, E and SW) have gone through the cluster center, while the HI rich groups (NE) are currently falling.

HI Distribution in Blue Compact Dwarf Galaxies
H. Bravo-Alfaro, E. Brinks, & D. Kunth
We present observations and preliminary results from an HI VLA survey of two dwarf galaxies which are currently experiencing a burst of star formation: Haro 2 (Mk 33) and Haro 4 (Mk 36). We analyze the distribution and kinematics of the neutral gas, in order to relate the HI component to star formation events.
We estimate the gas column density and compare this with the empirical critical star formation threshold density. The possibility of a correlation between this empirical threshold and metallicity is still a matter of debate; this makes the study of these HII galaxies of special interest, as they display low metallicities.

Briggs, Frank (University of Groningen)
Prospects for Radio Detection of the Epoch of Reionization and the Very First Stars
Frank Briggs
The reionization of the intergalactic medium that takes place at redshift greater than 5, when the first stars or AGN ignite, may create a distinctive spatial and spectral signature in the radio background. Detection of a global signal might be measured in the near term with current technology and existing telescopes. Measurement of structures associated with the first pockets of ionization around stars requires a new telescope such as a Compact Lofar or SKA.

Buote, David (UC Santa Cruz)
Oxygen Absorption in Cooling Flow Galaxies and Groups: Evidence for Warm Ionized Gas
David A. Buote
From analysis of the ROSAT PSPC spectra of 10 of the brightest cooling flow galaxies and groups with low Galactic column densities we have detected strong intrinsic absorption over energies ~0.4-0.8 keV in half of the sample. Since no excess absorption is indicated for energies below ~0.4 keV the most reasonable model for the absorber is warm, collisionally ionized gas with T=10^{5-6} K where ionized states of oxygen dominate the absorption. The ionized absorber accommodates the negligible atomic and molecular H inferred from HI and CO observations and the mass deposited by the cooling flows.

Carignan, Claude (Université de Montréal)
Dark Matter Distribution Laws
C. Carignan and S. Blais-Ouellette
Spiral galaxies are known to present an important mass discrepancy between their dynamical and visible masses. The commonly accepted hypothesis is to assume a spherical halo of unseen matter in addition to the stars and gas. The exact density distribution of these halos have become an important issue. On one side, N-body simulations of CDM cosmological evolution have now reached sufficient resolution to predict the dark halos density profiles down to the inner parts of spiral galaxies and almost always show dense cuspy halos. On the other side, observations of dwarf late type spiral galaxies present a shallow inner rotation curve compatible with a flat density core. Those different density laws will be confronted with HI rotation curves of spiral and dwarf galaxies.

Carilli, Chris (NRAO/Socorro)
High redshift radio galaxies: Beacons to Biased Hierarchical Galaxy Formation Within Large Scale Structure (LSS)
C.L. Carilli, G. Miley, H. Rottgering, J. Kurk, L. Pentericci, K. Menten, F. Bertoldi, K. Kohno, & R. Kawabe
We summarize evidence showing that some high redshift radio galaxies, and other AGN, inhabit regions of gas and galaxy over-densities indicative of cluster or proto-cluster environments. Evidence includes:
1. clustering of Ly-dropout galaxies, and submm-selected dust and CO emitting massive starburst galaxies, on Mpc scales,
2. rotating, 100 kpc-scale Ly-alpha halos and Ly-alpha absorption,
3. disturbed radio source morphologies and spectroscopic signatures of kinematic interaction between radio jets and the ambient medium,
4. extreme values of Faraday rotation, comparable to observed values in low redshift cooling flow' cluster atmospheres,
5. in one case, (possibly extended) X-ray emission with a luminosity expected for a massive cluster atmosphere.
Illustrative examples are given using the extensively studied radio galaxies PKS 1138-262 at z=2.2 and 4C 41.17 at z=3.8, and contrasted with Cygnus A where appropriate. Drawing an analogy to the recently observed clustering of Ly-dropout galaxies in blind surveys by Steidel et al., we speculate that high redshift radio galaxies are beacons to highly biased heirarchical galaxy formation within LSS (b=6 for Omega_M=1, or b=2 for Omega_M=0.2). The LSS will eventually separate from the Hubble flow and collapse to form a rich cluster at z<2, and the individual Ly-dropout and submm-selected galaxies will evolve into cluster spheroidal galaxies, including the giant elliptical host galaxy of the AGN. However, the possible X-ray atmosphere associated with PKS 1138--262, and the 1000 km/sec velocity dispersion of the associated Ly-dropout galaxies, could indicate virialized LSS and hence require Omega_M<1. We will also present data on a few high-z QSOs which indicate massive starbursts co-eval with the AGN activity, and conclude with a summary of the capabilities of the EVLA for the study of high-z star forming galaxies.

Chang, Tzu-Ching (Columbia University)
A Search for HI in Spectroscopically Selected E+A Galaxies
T.-C. Chang
We present the results of the HI line and the radio continuum observations of five nearby E+A galaxies. These galaxies are selected from a unique sample of 21 E+A's identified by Zabludoff et al. (1996) in their spectroscopic search for E+A galaxies using the Las Campanas Redshift Survey, where over 11,000 nearby galaxies were examined. We have detected HI emission in one E+A galaxy, EA 1, with a total flux of 0.30+/-0.02 Jy km s^-1 and a HI mass of 3.4+/-0.3 x 10^9 Msun. The HI morphology and kinematics of EA 1 suggest a galaxy-galaxy merger. A lower limit to the time since the merger happened is ~8 x 10^8 years, which approximately agrees with the general interpretation that starbursts ceased within the last ~ Gyr in E+A's. If the other E+A's had the HI properties of EA 1, the closest of the sample galaxies, we would have detected them. Therefore, we conclude that E+A galaxies have a wide range of HI masses. None of the galaxies were detected in radio continuum emissions. The upper limits on the radio power are ~10^{21} h^{-2} W/Hz, which exclude the possibility that these galaxies have dust-obscured, on-going starbursts.

Charlton, Jane (Penn State)
QSO Absorption Line Constraints on Intragroup High Velocity Clouds
J.C. Charlton, C.W. Churchill, & J.R. Rigby
We show that the number statistics of moderate redshift MgII and Lyman limit absorbers may rule out the hypothesis that high velocity clouds are infalling intragroup material. There cannot be a substantial population of large, intragroup clouds in groups at redshift z~0.5 because such clouds would contribute to the cross--section for MgII and Lyman limit absorption. Luminous galaxies already account for nearly all of this absorption. The discrepancies between the Local Group HVC population and the statistics of MgII and Lyman limit absorbers can only be reconciled if most of the extragalactic HVC analogs are within 200 kpc of galaxies, and not at large throughout the groups.

Chaves, Tara (Queen's University)
Neutral Hydrogen in NGC 2613
T.A. Chaves and J.A. Irwin
Studies of edge-on spiral galaxies have revealed a number of cases in which kpc scale HI features extend from the disk into the halo region (King and Irwin 1997). Large arc-like features, usually referred to as expanding supershells, can require huge input energies on the order of 10^{55} ergs. Several models have been developed to explain these large energy requirements. The massive edge-on spiral galaxy NGC 2613 shows evidence of supershells which, if confirmed, would be among the largest known. We will present CnB and D array HI maps to investigate the presence of supershells in NGC 2613.

Chen, Hsiao-Wen (Carnegie Observatories)
Extended CIV Envelopes Around Galaxies at z<1
Hsiao-Wen Chen, Kenneth M. Lanzetta, John K. Webb, & Xavier Barcons
We report the discovery of extended C IV gaseous envelopes around galaxies of a wide range of luminosity and morphological type. First, we demonstrate that the C IV absorption systems arise in individual galaxies by showing that there exist a statistically significant excess of galaxy and C IV absorber pairs of velocity difference Delta v< 250 km/sec and impact parameter rho<100 h^{-1} kpc and distinct boundaries in the extended gaseous envelopes around galaxies, outside of which almost no C IV absorption is detected to within the sensitivity of the absorption line survey. Next, considering only galaxy and absorber pairs that are likely to be physically associated and rejecting galaxy and absorber pairs within 3000 km/sec of the background QSOs (which are likely to be associated with the QSOs), we find that (1) galaxies of all morphological types and of luminosity brighter than 0.03 L_{B_*} appear to possess extended C IV gas to approximately 100 h^{-1} kpc and (2) the extent of C IV gas scales with galaxy B-band luminosity as R\propto L_B^{0.5+/-0.1} but does not depend strongly on galaxy mean surface brightness, redshift, or morphological type, and (3) the covering factor of the C IV clouds in the extended gaseous envelopes approaches unity, although there is a large scatter in the mean number of clouds encountered along a line of sight. The most significant implication of the study is that galaxies of a wide range of luminosity and morphological type are surrounded by chemically enriched gas that extends for at least ~100 h^{-1} kpc.

Chengalur, Jayaram N. (TIFR)
GMRT Observations of HI in Absorption
Jayaram N. Chengalur
The Giant Meterwave Radio Telescope (GMRT) is currently operational at 5 wavebands, 1000-1420 MHz, 610 MHz, 327 MHz, 233 MHz and 150 MHz, i.e. redshift ranges 0-0.5, 1.2-1.4, 3-3.7 5-5.25 and 7.8-8.5 for the 21cm HI line. Over the last several months, observations of redshifted HI have been conducted using both the prototype 8 station correlator as well as the first side band of the final 30 station correlator. These observations have focused on 21cm absorption from damped lyman alpha systems. Damped lyman alpha systems are believed to be the progenitors of spiral galaxies. Detailed studies of these systems hence have an important bearing on our understanding of galaxy formation. We discuss the implications of our observations for the nature of damped lyman alpha systems. In addition other on going GMRT 21cm observations will be described briefly.

Churchill, Chris (Penn State)
Kinematics & Multiple Ionization Phases of Extended Gaseous Structures Around z~1 Galaxies
Chris Churchill
From quasar absorption line studies, evidence is mounting that there are several gaseous phases of gas in and surrounding z~1 galaxies. For a sample of 45, intermediate--redshift galaxies, the the data reveal that the high ionization C IV 1548, 1550 absorption strengths and kinematics (FOS/HST data) are srongly correlated with the velocity spreads and kinematic complexities of the low ionization Mg II 2796, 2803 absorption (HIRES/Keck data). For the large majority of the systems, only multiphase, chemical photoionization models can reproduce the highly detailed data.

Drawing from WFPC2/HST images of the galaxies and local examples, the likelihood that the gas is located in (i) extended HI disks, (ii) Galactic--like coronae above and below the disk planes, and (iii) in extended stripped gas from local interactions (as opposed to residing in a stable, quiescent, "extended halo") is discussed. The data are then compared and contrasted to global galactic evolution and star formation rate models from z~4 to the present epoch in order to place the gaseous conditions in z~1 galaxies in cosmological context.

Clarke, Tracy (NRAO/Socorro)
Intermediate Velocity Clouds -- Distances and Infall Models
T.E. Clarke, M. Gladders, C. Burns, A. Attard, et al.
We will present the first set of distance measurements from the David Dunlap Observatory's Intermediate Velocity Cloud (DDO IVC) survey. This ongoing project aims to measure the distances to all accessible IVCs using the interstellar absorption line method. The distance measurements are essential to our understanding of the origins and dynamics of the IVCs, as the distances set most of the physical parameters of the clouds. We will discuss the survey strategy, and highlight differences between our techniques and those used previously in IVC distance studies. We will present distance estimates for approximately 10 northern IVCs (the exact number being dependent on ongoing observations). We will also discuss the constraints our current distance estimates place on IVC infall models.

Clemens, Marcel (University of Cambridge)
Star Formation in Interacting Galaxies: NGC 3395/6
M. S. Clemens, P. Alexander, & G. Winter
We are studying the response of the ISM to galaxy/galaxy interactions with a particular view to constraining the relative timescales of infall and star formation. We have reproduced the observed HI dynamics and morphology in the closely interacting, starburst galaxies NGC 3395/6 by means of an N-body simulation including both non-collisional stellar and collisional gas particles. By including star formation in our models, and constraining the star formation rate over the interaction timescale observationally, we aim to constrain the ISM and dynamical parameters which are important in defining the distribution and efficiency of interaction induced star formation.

Collins, Joseph (University of New Mexico)
The Ionization of, and Physical Conditions in Diffuse Ionized Gas Halos
Joseph A. Collins and Richard J. Rand
Using spectroscopic data for a selection of edge-on spiral galaxies, we attempt to address the issue of how DIG halos are energized. We concentrate on the well -studied galaxies NGC 5775, data for which is presented in a companion poster by Rand, and NGC 891, though we briefly discuss results for two other galaxies, NG C 4302 and UGC 10288. Composite photo-ionization/shock models are generally better at explaining runs of line ratios in these galaxies than photo-ionization models alone. Models of line ratios in NGC 5775 require a greater contribution from shocks for filamentary regions than non-filamentary regions to explain the run of [OIII]/H-alpha. In either case, however, the run of [SII]/[NII] is not well fit by the models. Runs of [SII]/H-alpha, [NII]/H-alpha, and [SII]/[NII] in each of the four galaxies are consistent with a photo-ionization model without shocks, but with an increasing gas temperature with z. However, derived ionization fractions of O^{++} in NGC 5775 are unusually high unless a secondary source of ionization is invoked. In addition, the increase of this fraction with z along H-alpha filaments strongly suggests an increased contribution from a secondary ionizing source away from the midplane. The run of [OIII]/H-alpha in NGC 891 is generally consistent with an increase in temperature with z.

Daly, Ruth A. (Penn State University)
Non-Eddington Limited Outflows From AGN
R.A. Daly
It is quite common to assume that emission from AGN will be Eddington limited. Generally, this means assuming the luminosity is proportional to the energy available, or the mass of the central black hole, so the total time the source is active is independent of the mass of the black hole, or the luminosity of the source.

The question of whether emission from all massive black holes is Eddington limited will be investigated. An empirical study of the relationship between luminosity, total lifetime, and total energy available for a sample of powerful radio sources will be presented. It will be shown that the total lifetime varies substantially from source to source. It will also be shown that this lifetime depends rather strongly on the total energy available. Another way to put this is to state that the luminosity is not proportional to the energy available, but is proportional to the energy available to the second or third power.

These results may have implications for models designed to explain highly collimated outflows from AGN. Constraints on such models suggested by these results will be discussed.

Darling, Jeremy (Cornell University)
The Arecibo OH Megamaser Survey and the Galaxy Merger Rate
J. Darling and R. Giovanelli
We present the current results of a survey for OH megamasers (OHMs) underway at the Arecibo Observatory. The survey is 2/3 complete and has produced a high OHM detection rate (1 in 6) from a redshift-selected sample of IRAS galaxies, identifying 35 new OHMs in luminous infrared galaxies to add to the sample of 55 found in the literature. The OHM fraction in luminous infrared galaxies shows a strong bias for the most FIR-luminous hosts, indicating that OHMs can be used to measure the high luminosity tail of the luminous IR galaxy luminosity function for redshifts spanning the epoch of major galaxy mergers (0.5<z<5). This survey will provide a low-redshift (0.1<z<0.3) calibration of the OHM luminosity function to the galaxy merger rate which can be applied to blind OHM surveys in order to measure the galaxy merger rate as a function of cosmic time. The survey has also made the first detection of strong variability in OHMs. The variability appears over time scales of months in individual spectral features rather than in broad-band modulation which could be attributed to antenna calibration or pointing errors. Variability in OHMs constrains the sizes of the variable and quiescent spectral features, regardless of the source of modulation (intrinsic to the source or due to propagation effects). Variability in OHMs, particularly those with 0.1<z<1.0, will provide a powerful tool for understanding the small-scale physical settings and mechanisms of masers which can be observed at cosmological distances.

Dettmar, Ralf-Juergen (Ruhr-University Bochum)
Tracers of the Interstellar Disk-Halo Connection in Spiral Galaxies
R.-J. Dettmar, J. Rossa, & R. Tullmann
Evidence for the presence of various phases of the interstellar medium in the halos of "normal" spiral galaxies is summarized and it is demonstrated that the presence of gaseous halos correlate with the star formation in the disk. This is indicative of a large scale exchange of matter between disks and halos as described in various theoretical scenarious such as galactic fountains, chimneys, and winds. It is demonstrated that diffuse (warm) ionized gas (DIG or WIM) in general is a good and easily observable tracer for halo gas.

We present some recent detections of DIG in the halos of edge-on galaxies from a small sample observed in H-alpha. The ionization problem is briefly rediscussed using a new set of long-slit spectra covering diagnostic emission lines in the blue. In addition, we present some first results of radio-polarization observations which indicate a correlation of the magnetic field structure with individual features in the DIG distribution.

Dole, Hervé (Inst. d'Astrophysique Spatiale, Orsay)
Galaxy Evolution as Seen from Space with ISO
H. Dole
The Infrared Space Observatory (ISO) probe for the first time the evolution of galaxies in the mid and far infrared range. We review ISO deep surveys performed at 15, 90 and 170microns and their cosmological implications, and report the first detection of fluctuations of the Cosmic Infrared Background at 170microns in the FIRBACK survey. We also present the current status of the multiwavelength (from visible to radio range) follow-up designed to constrain the nature of these infrared sources. All these results led us to build a simple model of galaxy evolution which fits all the mid-infrared to submillimetre existing data, and allows scientific preparation of forthcomming instruments: SIRTF, ALMA, Planck, FIRST.

Duc, Pierre-Alain (CNRS/CEA-Saclay)
HI Recycling: The Formation of Tidal Dwarf Galaxies
Pierre-Alain Duc
Galactic collisions trigger a number of phenomena, such as transportation inward of gas from distances of up to kiloparsecs to the nucleus fueling a central starburst or nuclear activity. The inverse process, the ejection of material into the intergalactic medium by tidal forces, has been much less studied. However, detailed HI maps of interacting systems have recently shown that a fraction as large as 90% of of the gaseous component of colliding galaxies can be expelled into the intergalactic medium as a result of tidal forces. Part of these tidal debris might be dispersed in the intergalactic medium or recondense to form a new generation of galaxies: the so--called tidal dwarf galaxies. I will review the properties of these gas-rich, star forming, recycled objects and show that they provide in the nearby universe instances of young forming galaxies.

Dwarakanath, K.S. (Raman Research Institute)
GMRT HI Absorption Detection from the Unusual Galaxy in Abell 2125 (z=0.246)
K.S. Dwarakanath and F.N. Owen
We have detected the HI 21 cm line absorption from the unusual galaxy C153 in the galaxy cluster Abell 2125 with the recently completed Giant Meterwave Radio Telescope. The HI absorption is at a redshift of 0.2533, with an optical depth of 0.36. The full width at half minimum of the absorption line is 100 km/sec. The estimated column density of atomic Hydrogen is 0.7x10^{22} (T_{s}/100) cm^{-2}. The redshift of the HI absorption is larger than the values estimated for this galaxy using optical lines by as much as 400 km/sec. This system appears complex as indicated by its optical morphology and the different systemic velocities of different lines. Its proximity to the cluster core and/or its interaction with a close-by galaxy might be responsible for its disturbed nature. The implications are explored.

Eder, Jo Ann (Arecibo Observatory)
21-cm Spectroscopy with the Upgraded Arecibo Telescope
J. Eder and K. O'Neil
The recent upgrade of the Arecibo radio telescope included a Gregorian reflector system, an expanded suite of receivers and a flexible new auto/cross-correlation spectrometer. The resulting improved sensitivity and expanded frequency range available have led to greater capability in observing gas in galaxies. Early test observations led to the detection of HI in dwarf galaxies that had previously been undetected with the pre-upgrade Arecibo telescope. The frequency agility and the advanced features of the correlator allow a broad range of redshift space to be sampled simultaneously. Also, highly redshifted emission (and absorption) from the gas in galaxies can now be observed.

We will discuss the specifications of the current system. Several Arecibo results are presented in posters at this meeting from the two years since the upgraded telescope was commissioned. We will report on additional experiments which have been successfully completed. Constraints on the observation of highly redshifted 21 cm emission due to the RFI environment will also be outlined.

Edge, Alastair (University of Durham)
Molecular Gas in Cooling Flows - Detected at Last!
Alastair Edge
I will present JCMT and IRAM 30m spectra that show CO(1-0) through to CO(4-3) detections that imply molecular gas masses in the range of 10^{10} to 10^{11} M_sun are present in the cores of eleven cooling flow clusters. Additional OVRO interferometry of two of these systems shows the CO emission is compact (<5"). The implications of these detections for cooling flows and future X-ray observations will be reviewed.

Englmaier, Peter (MPI)
Gas Flow in the Antennae Galaxies NGC 4038/9
P. Englmaier
The prominent interacting galaxy pair NGC 4038/9 contains many active star-forming regions and is continuously forming new globular clusters. We present a self-consistent n-body model for this system which includes a SPH gas component. The model explains the apparent concentration of gas in the so-called overlap region between the two nuclei as a bridge of stars and gas connecting the two galaxies. Projected on the sky, the bridge appears as a dense spot of gas.

Evrard, Augustus (Univ. of Michigan)
Intracluster Gas -- The Missing Baryons and Their Stories
A.E. Evrard
In galaxy clusters, the missing' baryons have been visible for decades. The hot intracluster gas confined within the deepest potential wells represents the bulk of the baryonic material within clusters. Empirical and computational evidence support a picture in which the gas has had a relatively simple thermodynamic history involving an early epoch of pre-heating' that preceeds hierarchical gravitational collapse into the cluster potential. I will review the evidence supporting such a picture and define its limitations. In addition, I will show predictions for the population of distant cluster potential wells expected within a flat, Omega_Lambda = 0.7 universe derived from N-body simulations of the Hubble Volume.

Ferreras, Ignacio (NAPL, Oxford)
Mass-to-Light Ratios and the Star Formation History of Cluster Early-type Galaxies
I. Ferreras and J. Silk
The slope of the correlation between mass to light ratios and masses in elliptical galaxies (closely related to the tilt of the Fundamental Plane with respect to the virial expectation) is explored with a simple model which reduces the evolution to four parameters: the timescale and epoch of infall, the fraction of ejected gas in outflows, and the star formation efficiency. The last two parameters can explain the color-magnitude (CM) relation, but they are degenerate even with respect to the predictions of the CM relation at moderate redshift. It is the evolution with redshift of the slope between mass to light ratios and masses which can break this degeneracy, thereby disentangling the effects of age and metallicity.

Fraternali, Filippo (University of Bologna)
The HI Halo of NGC 2403
F. Fraternali, T. Oosterloo, R. Sancisi, & G. van Moorsel
In recent years 21-cm line observations of a few nearby spiral galaxies have brought new evidence on the vertical extent and kinematics of the HI and have indicated the presence of a few kpc thick HI layer surrounding the differentially rotating disk. Our study of the spiral galaxy NGC 2403 has indicated the presence of an HI "halo" component with the following characteristics: 1) a mass of about 3x10^8 Msun, or about 1/10 of the total HI mass, 2) a slower (about 20 km/s) rotation than the disk, 3) a possible general infall motion towards the central regions. The origin of this gas is not clear: it could be galactic fountain or intergalactic.

Freeman, Ken (Mt. Stromlo)
Dark Matter, an HI Perspective
Ken Freeman
I plan to review some of the current problems in the study of dark matter in galaxies, with emphasis on the contribution of HI observations.

Gay, Pamela (University of Texas - Austin)
The Tex0x Survey: Using Radio Signposts to Find Intermediate Redshift Galaxy Clusters
P.L. Gay and G.J. Hill
The advent of sensitive large-area radio maps (e. g. NVSS & FIRST) offers astronomers several new approaches for finding galaxy clusters. Above the disk of the Milky Way, the majority of radio sources are identified with galaxies, and in several instances these galaxies have serendipitously been identified as members of moderate-redshift clusters. The Texas-Oxford (TexOx) Survey team hopes to maximize the probability of finding galaxy clusters by using radio source overdensities as cluster signposts. We are imaging 7'-square fields centered on overdensities of 5 or more sources (S >= 2.3 mJy) in a 6' radius (Fig. 1) using the McDonald Observatory 2.7 m telescope. We have HET time for spectroscopic conformation of optically identified clusters. Clusters confirmed to date have redshifts of: 0.41, 0.49, 0.51 and 0.61.

We find that radio source overdensities provide efficient guides for locating galaxy clusters. Application of our technique to the region that was surveyed optically in the Palomar Distant Cluster Survey (PDCS - Postman et al. 1996) re-discovers 8 (7.5 %) of 107 clusters and finds no extraneous non-cluster overdensities. Our technique also finds 18 (9.9 %) of 182 clusters in the X-ray selected sample of Vilkhlinin et al. (1998). This leads us to believe that we can identify roughly 8 % of the total cluster population in the redshift regime covered by these samples (z < 1.2). These "radio-active" clusters do not form an unbiased sample however. We find that 14 (43.8 %) of 32 clusters in Dressler's (1985) sample of Butcher-Oemler (BO) clusters are radio-active, indicating that we are particularly sensitive to BO clusters. These clusters are all rich in blue galaxies. The excess blue galaxies are thought to be infalling field galaxies or sub-cluster galaxies undergoing star formation triggered by interaction with the intracluster medium (Caldwell and Rose 1997) and galaxy harassment (Rakos et al. 1997). Spatially, both blue galaxies and radio galaxies tend to be found in the outskirts of clusters (Fig. 4), and work by Owen et al. (1999) and our group indicates that there is a connection between a cluster's radio activity and blue galaxy fraction.

In summary, we find that we can efficiently discover moderate redshift (z>0.3) clusters by examining radio-source over densities. We believe our technique finds ~ 8% of the cluster population with a bias towards clusters that have a high fraction of blue galaxies. We are using our cluster sample to study the relationship between radio activity and blue galaxy excess.

Caldwell, N. and Rose, J.A. 1997. AJ 113, 492
Dressler, A., Gunn, J.E. and Schneider, D.P. 1985. ApJ 294, 70
Owen, F. et al. 1999. AJ 118, 633
Postman, M. et al. 1996. AJ 111, 615
Rakos, K., Odell, A., and Schombert, J. 1997. ApJ, 490, 184
Vikhlinin, A. et al. 1998. ApJ 502,558

Georgakakis, Antonis (University of Birmingham)
Cold gas and star formation in a merging galaxy sequence
Antonis Georgakakis, D. A. Forbes, & R. P. Norris
We explore the evolution of the cold gas (molecular and neutral hydrogen) and star-formation activity during galaxy interactions, using a merging galaxy sequence comprising both pre- and post-merger candidates. We find evidence for a decreasing fraction of cold gas mass from early interacting systems to merger remnants, attributed to neutral hydrogen conversion into other forms (e.g. stars, hot gas) and molecular hydrogen depletion due to on-going star-formation. Additionally, we find that the star-formation efficiency starts at a level comparable to isolated spirals, peaks around nuclear coalescence and decreases at post-mergers to a level similar to that of normal ellipticals.

Ghosh, Tapasi (Arecibo Observatory, NAIC)
Neutral Hydrogen in Seyfert Galaxies - A Test for the Unification Scheme
T. Ghosh, J. Eder, C.J. Salter, & D.H. Frierson
We present an unbiased survey of HI in Seyfert galaxies to test theories designed to unify the Seyfert-1 (broad-emission line) and Seyferts-2 (narrow-line) classes of active galaxies. These theories, which explain apparent differences between the two subclasses as due to orientation effects, can be examined by comparing various HI properties derived from \lambda 21-cm spectra. If the Unification Scheme is strictly applicable to all seyfert galaxies, and if the galaxy's HI disk is aligned with the accretion disk, then Sy-1 HI spectra should have the narrower velocity widths, while total derived HI masses and surface densities should be comparable for the two classes; HI emission being orientation independent. During the summer of 1999, the Arecibo telescope was used to observe a sample of 61 Seyfert galaxies with redshifts <=0.044 which were previously unobserved or undetected at \lambda 21-cm. HI was detected in 39 galaxies (64 %), of which 20 are Sy-1 and 19 Sy-2. Typical spectra of a few objects belonging to both classes are shown in Fig.1. The distributions of HI mass and surface density were found to be similar for the two classes, consistent with orientation independence. However, the distributions of the velocity widths were not significantly different either, which could indicate random alignment of the HI and accretion disks. Further, the ratio of the continuum flux density (orientation dependent for cor-jet emission) and the HI mass (orientation independent) did not display significant differences between the two classes contrary to the expectation based on the Unified Scheme.

Do High-Velocity Clouds Really Fuel Galactic Star Formation?
Tantalizing evidence has been presented which supports the suggestion that a large population of extragalactic gas clouds permeate the Local Group, a population which has now been associated with the Galactic High-Velocity Clouds (HVCs). Playing devil's advocate, I will comment upon both the strengths and weaknesses of this picture, concentrating on our new HST GHRS+STIS and FUSE metallicity analyses of several HVCs seen in absorption toward background QSOs. Particular attention will be paid to HVC Complex~C, where the sulfur abundance along one sightline (Mrk 290) (0.09 Z_sun), line-of-sight velocity (-120 km/sec), and inferred thermal pressure have motivated the suggestion that this cloud represents a prototypical example of the postulated source of infalling low metallicity gas that fuels the bulk of star formation in the Milky Way. We discuss how our mapping of the abundance distribution and local ionization environment with HST and FUSE, via the use of what is now five background probes spread across the face of the HVC, might modify this still-controversial picture of Galactic evolution.

Gilbank, David (University of Durham)
Optical vs. X-ray Selection for Finding Clusters of Galaxies
David G. Gilbank, Richard G. Bower, & Francisco Javier Castander
At intermediate redshift, optical and X-ray approaches are equally competitive methods of discovering galaxy clusters. However, the properties of the discovered clusters, and their member-galaxies, may be strongly dependent on the search method used. To directly compare the systematic differences in mass, dynamical structure, galaxy evolution, etc. we have undertaken an optical cluster survey (using a matched-filter'' technique) which overlaps with regions of deep ROSAT X-ray exposures. Early results indicate that we have optically-detected cluster candidates, missed in the X-ray. These are high-significance candidates, unlikely to be due to projection effects. We are currently undertaking follow-up spectroscopy and IR imaging.

Hameed, Salman (NMSU)
The Role of Interactions in the Evolution of Early-type Spiral Galaxies
S. Hameed, L. Young, D. Thilker, & D. Blank
For the past several years we have been studying the star forming properties of a complete sample of 57 nearby early-type(Sa-Sab) spiral galaxies. Contrary to popular perception, our H-alpha images have revealed a significant number(25-30%) of Sa-Sab galaxies with giant HII regions, and star formation rates comparable to the most prolifically star forming Sc galaxies. The HII region luminosity functions of high star forming early-type spirals appear to be different compared to those galaxies with low star formation rates. The difference in the properties of star forming regions may point to corresponding differences in massive star formation triggering mechanisms.

In recent years it has been suggested that interactions may play an important role in the formation and subsequent evolution of early-type spirals (e.g. Schweizer 1990; Pfenniger 1991; Mihos & Hernquist 1994). We suspect that early-type spirals that hosts giant HII regions or nuclear starbursts, either had a recent interaction, or are presently going through one. Since neutral hydrogen is one of the best tracers of a past interaction, we tested this hypothesis by obtaining HI maps of some of these high star forming early-type spirals. Here, we present preliminary results of the correlation between star formation rates, shape of HII region luminosity functions, and HI morphology of Sa-Sab galaxies.

Haynes, Martha P. (Cornell University)
Kinematic Evidence for Minor Mergers in Isolated, Normal Sa Galaxies
Martha P. Haynes
The Sa galaxies are a heterogenous class, including gas-rich and gas-poor disks and large and small-bulged systems. In contrast to their later spiral counterparts, Sa's typically occupy higher density environments and require little or no dark matter. In order to uncover the reason for the diversity of the Sa class, we have conducted a comprehensive study of the dynamics and kinematics of a sample of nearby, morphologically normal, relatively isolated Sa galaxies, combining BVRI and H-alpha imaging, long-slit optical spectroscopy and HI synthesis mapping. In about half the sample, evidence of kinematic separation of ionized gas components is found in the long-slit spectroscopic data. The degree and circumstances of the distinct kinematics vary from complete counterrotation of all of the gas from the stars to nuclear gas disks decoupled from the stars to anomalous velocity central gas components that may be indicative of misaligned bars or oval distortions. In 8 of the 9 objects mapped in HI, the neutral gas extends far beyond the optical radius. In general, the HI surface density is very low, and the outer HI is patchy and asymmetric or found in a distinct ring, exterior to the optical edge. While the overall HI velocity fields are dominated by circular motions, strong warps are suggested in the outer regions by bending of the minor axis isovelocity contours and/or systematic shifts in position angle between inner and outer rings. The multiwavelength evidence can be interpreted in terms of the kinematic "memory" of past minor mergers in objects that otherwise exhibit no morphological signs of interaction.

Hearn, Nathan C. (Univ. of Illinois/Urbana-Champaign)
Gas Flows and Shock Production in Impacted Disk Galaxies: Results from Numerical Simulations
N.C. Hearn and S.A. Lamb
The gaseous component of a galactic disk can be affected dramatically by a collision with another galaxy. The induced internal motion leads to long-lasting morphological changes in the large-scale structure of the gas. Density waves in the form of expanding rings or long arcs are generated in both the gas and collisionless (stellar) components, and propagate through the disk. These density enhancements are much more pronounced in the gas than in the stellar component. We use a series of numerical 3-D, n-body/hydrodynamical simulations, in which a gas-rich disk galaxy is impacted by an elliptical galaxy traveling parallel to the disk's rotation axis, to study the time-dependent properties of the gas density wave (such as shock strengths and propagation speeds), as well as of the underlying medium. These studies have implications for global star formation, and lead to an understanding of observed asymmetries and warps in some gaseous galactic disks.

Higdon, James (Kapteyn Astronomical Institute)
The Neutral ISM and Starburst Triggering in Ring Galaxies
J. Higdon, J. Wallin, & R. Rand
We have combined H-alpha and aperture synthesis HI maps in a sample of nine ring galaxies to investigate both their evolution and the factors regulating starburst activity. The objects span a wide range in diameter, star formation rate (SFR), optical morphology, and HI mass. The gas rings show a great deal of structure regardless of their age or local SFR, and can attain very high HI surface densities (Sigma_{HI, peak}=100 M_sun pc^{-2}, 3 kpc resolution). The interior disks and nuclei are HI poor in all cases. Very massive intruders'' can severely distort the gas rings in their earliest stages of development, though symmetric and high Sigma_HI rings may still form given sufficient time. Star formation triggering in the rings most likely results from the confinement of 90% of the pre-collision galaxy's ISM for >100 Myr timescales, while threshold effects in the gas poor interiors can explain the observed suppression of massive star formation there. Starburst activity is strongly correlated with the attainment of very high HI surface densities in the outer rings. We also present estimates of atomic-to-molecular gas mass ratio and star formation efficiency in a ring starburst.

Hoffman, G. Lyle (Lafayette College)
The Fine Structure of DDO 154
G. Lyle Hoffman and Jacqueline van Gorkom
C array HI synthesis mapping of the irregular galaxy DDO 154 reveals considerable structure in the HI disk even well outside the central part of the galaxy where stars have formed. A couple of prominent quasi-spherical cavities are examined in particular. They are typical of holes in other well-resolved irregular galaxies, consistent with supernova-driven evacuations, but lie entirely outside the stellar portion of the galaxy.

The improved resolution of the central rotation curve does not significantly change the mass models that have been published by other authors previously. We are able to present a velocity dispersion map, which shows relatively uniform dispersions of 6 or 7 km/s through the outer regions of the gas disk.

Hopkins, Andrew (Univ. of Pittsburgh)
Estimates of SFR density between 0.7<z<1.8
A. Hopkins, A. Connolly, & A. Szalay
The star formation rate in galaxies between 0.7 < z < 1.8 is analysed using Near Infrared Camera and Multi-Object Spectrograph (NICMOS) grism spectral observations. We detect 163 galaxies in an area of ~4.4 square arcminutes, 37 of which show possible H-alpha emission. We extend the observed H$\alpha$ luminosity function (LF) in this redshift range to luminosities a factor of two fainter than earlier work, and are consistent in the region of overlap. Using the H-alpha LF, we estimate a star formation rate (SFR) density in this redshift range of 0.166 M_sun/yr Mpc^{-3} (H_0=75 km/sec/Mpc), consistent with other estimates based on emission lines, and supporting the order of magnitude increase in SFR density between z=0 and z=1. Our measurement of SFR density is a factor of ~2-3 greater than that estimated from UV data, comparable to the factor observed locally, implying little evolution in the relative extinctions between UV and H-alpha out to z~1.3.

van der Hulst, Thijs (Kapteyn Astronomical Institute)
WHISP
J.M. van der Hulst, T.S. van Albada, & R. Sancisi
WHISP (Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies) is an extensive study of the neutral hydrogen component of disk galaxies carried out at the Kapteyn Institute. The WHISP project started in 1992 and makes use of the Westerbork Synthesis Radio Telescope for the HI imaging. About 230 galaxies have been observed to date. Further information about the project and overviews of the results for the galaxies fully reduced to date are given on the Web (URL: http://www.astro.rug.nl/~whisp). The final aim is to image a total of about 400 galaxies, selected from the UGC with a declination north of 20 degrees and a size larger than 1.5 arcmin.

The scientific applications of such a survey are numerous: studies of the systematics of rotation curves (and hence dark matter content) of galaxies as a function of galaxy type and luminosity, studies of the HI morphologies as a function of galaxy type and luminosity, studies of the stability of warps in the HI disks.

An overview will be given of the large variety of HI properties found among the galaxies observed to date and of the various projects (finished or underway) making use of WHISP data.

Huchtmeier, Walter (MPIfR)
The HI Content of Galaxies in the IC342/Maffei Group
W.K. Huchtmeier, I.D. Karachentsev, V.E. Karachentseva, & D.J. Westpfahl
The number of known galaxies in the local universe, i.e. within a distance of 10 Mpc increased from 179 (Kraan-Korteweg and Tammann 1979) to actually ~340 due to extended redshift surveys, and to optical and HI searches for low surface brightness dwarf galaxies. Due to their location in the Zone of Avoidance (extinction by our own Galaxy) galaxies in the area of IC342 have been realized to form a group only recently. Actually there are 21 probable members known, 13 of which have been detected since 1994. We present HI-data of this group, single dish profiles for 11 dwarfs, and VLA line data for Cassiopeia 1 and for MB1. The relative HI content of these galaxies is as expected for their morphological type.

Irwin, Judith (Queen's University, Canada) i
GMRT Observations of M 82 and NGC 3079
J.A. Irwin and D.J. Saikia
The two edge-on spiral galaxies, M 82, and NGC 3079, were chosen as good test cases for observations using India's new Giant Metre-Wave Radio Telescope (GMRT) during its commissioning phase. Each of these galaxies have known nuclear outflows. In M 82, the outflow appears to be starburst driven and for NGC 3079, the outflow is related to an active galactic nucleus.

These galaxies were observed at 327 MHz, and 610 MHz data were obtained for NGC 3079 as well. In both galaxies, the nuclear outflows have been detected and we compare the results with maps at other wavebands. For M 82, there are interesting spatial correlations with the B-I dust distribution as well as the HI, and for NGC 3079, we detect emission farther from the disk than previously observed.

Katz, Neal (Univ. of Massachusetts/Amherst)
Simulations of Galaxy Formation in Cold Dark Matter Models
Neal S. Katz
I will present hydrodynamical simulations of galaxy formation in CDM models concentrating on global galaxy properties such as the mass function, the luminosity function, galaxy clustering, galaxy star formation rates, and peculiar velocities. I will compare these with both low redshift observations and observations at high redshift such as Lyman break galaxies, the newly discovered sub mm sources and damped Lyman alpha systems. I will stress radio detectable properties.

Kaufman, Michele (Ohio State University)
Effects of Grazing Encounters on Spiral Galaxies
Michele Kaufman, Curtis Struck, Bruce G. Elmegreen, Debra M. Elmegreen, Elias Brinks, Magnus Thomasson, & Mario Klaric
We have radio, optical, and CO observations and computer models of three recent, nearly--grazing, prograde, in--plane galaxy encounters. In all three systems, the prograde galaxy (IC 2163, NGC 2535, NGC 5394) has two long tidal arms with high arm/interarm contrast, an intrinsically oval disk, and enhanced gas turbulence. Some features that will be discussed: a) 100 km/sec streaming motions of the HI gas, b) the origin of numerous parallel filaments seen with HST in the tidal tail of IC 2163, c) unusually massive HI clouds in the galaxies, d) a gravitational wake in the extended HI envelope of NGC 2535.

Kilborn, Virginia (The University of Melbourne)
An Extragalactic HI Cloud with No Optical Counterpart
V.A. Kilborn, L. Staveley-Smith, & R.L. Webster
An isolated HI cloud, which we believe to be extragalactic, has been found in the HIPASS survey. No optical counterpart to this object has been found to deep limits. The HI mass of the object, HIPASS J1712-64, is very low, ~1.7x10^7 M_sun, at distance of ~3.2 Mpc. HIPASS J1712-64 seems to be a binary system similar to, but much less massive than, HI 1225+01 (the Virgo HI Cloud), and has a size of at least 15 kpc. The peak column density for HIPASS J1712-64 is only 3.5x10^{19} cm^{-2}, which is estimated to be a factor of 2 below the critical threshold for star formation in this object. Although the properties of HIPASS J1712-64 are not consistent with it being a High Velocity Cloud (HVC), membership of the recently recognised class of Compact HVCs or the possibility of ejection from the interacting Magellanic Cloud/Galaxy system cannot be ruled out.

Kim, Sungeun (UIUC/CfA)
Interstellar Environment in the Large Magellanic Cloud
Sungeun Kim, L. Staveley-Smith, M.A. Dopita, K.C. Freeman, M. Bessell, Y.-H. Chu, R.J. Sault, D. McConnell, & M.J. Kesteven
We present the results of a global high-resolution HI and H-alpha survey of the Large Magellanic Cloud. These data provide a new insight into the process of feedback between the disk and halo gas in disk galaxies.

Koribalski, Baerbal (ATNF)
HIPASS Bright Galaxies Survey
B.S. Koribalski
We present HI parameters and optical identifications of the brightest galaxies detected in the HI Parkes All-Sky Survey (HIPASS) and the HI shallow survey of the Zone-of-Avoidance (ZOA). Special attention will be given to the Sculptor group as well as other galaxies and HI clouds at low systemic velocities.

Lamb, Susan (University of Illinois)
Colliding Galaxies, Global Gas Flows, and Star Formation
Susan A. Lamb and Nathan C. Hearn
Strong, interpenetrating collisions between comparable mass galaxies have a very significant effect upon the subsequent large-scale morphology, gas dynamics, and star formation rate, apparently for a period of time much larger than the dynamical time (a few times 10^8 years for an L* galaxy) on which a collision takes place. Although such collisions are rare in the local universe, there are a few spectacular examples that provide nearby laboratories for studying phenomena that were once much more common. The rate of collisions between galaxies was likely very much higher in the past, with the number of collision events far outnumbering the merger rate when the relative velocity between galaxies was high. These collisions in the early Universe played an important role in the formation of the galactic structure seen today. Collisions involving gas-rich disk galaxies can initiate rapid and extensive star formation in the disk, which is induced in regions of high gas density produced where gas streams collide. Density waves in extended H I disks can produce very extended tidal tails, leading to the possibility of dwarf galaxy formation. Our three-dimensional numerical simulations of collisions between comparable mass galaxies demonstrate that the gas volume density in the disk can be increased by large factors, both in the nucleus and in pronounced features that form well away from the galactic center. The final morphology can be slightly, or highly asymmetric, with a considerable fraction of the galactic material dispersed to distances large compared to the original galactic radius. I present comparisons between multi-wavelength observations of star-forming, impacted galaxies and our 3-D numerical simulations of galaxy collisions involving disk galaxies. I illustrate some of the effects mentioned above using a video of our simulations, containing comparisons to observations.

Lane, Wendy (Kapteyn Astron. Inst./Univ. of Pittsburgh)
HI 21cm Absorption at Moderate Redshifts
W.J. Lane and F.H. Briggs
We present results from an extensive search for HI 21cm absorption at moderate redshifts. The survey was limited to systems with Mg II absorption at redshifts in the range 0.2<z<1.0, which lie in front of bright radio quasars (S>0.5 Jy at 1400 MHz). Narrow band observations centered at the metal-line redshifts were made with the UHF-high system and, when possible, the DZB correlator at the WSRT. In addition to identifying several new absorbing systems, optical depth limits of a few percent at velocity resolutions of about 6 km/sec are obtained for most of the non-detections. The incidence of low redshift 21cm absorbers is calculated.

The detection rate is lower than that found in a comparable UV survey for damped Ly-alpha absorption lines in MgII absorbers (Rao and Turnshek, astro-ph/9909164). The 21cm search is complementary to the damped Ly-alpha survey, and by combining information from both we are able to better understand physical conditions in these high neutral column density absorbers. We find that the 21cm objects represent a cold subset of high neutral column density absorbers, but are not necessarily the highest column density systems. Observations in the optical, infrared, and radio are used to identify and characterize the galaxies which give rise to the absorption.

Lee, Siow-Wang (Astro. Dept., Univ. of Toronto)
The Disk-Halo Interface of NGC 5775
S.-W. Lee, J.A. Irwin, R.-J. Dettmar, C.R. Cunningham, & G. Golla
We present a multi-wavelength study of the disk-halo interface of the edge-on galaxy, NGC 5775. This interacting galaxy is known to host a number of high-latitude, kpc-scale HI supershells. Our previous study shows that about 10,000 supernovae are required to produce these shells. This unusually large energy requirement cannot be reconciled with either the starburst scenario or the impacting cloud scenario in this galaxy.

Whatever disturbance that produced the supershells is bound to register in the other components of the interstellar medium (ISM). By studying the similarity or differences in the distributions of the various components of the ISM, and the connections, if any, between the disk and halo gas distributions, we attempt to piece together a scenario for the formation of the supershells. We present HI, 6 cm and 20 cm VLA maps, high-resolution IRAS 12micron and 25micron maps, CO J=1-0 data from SEST, CO J=2-1 data from the JCMT and R-band and H-alpha images from the NTT.

Li, Yuexing (Columbia Univ.)
The Multi-phase ISM of Elliptical Galaxy M86
Yuexing Li
We present the first VLA observation of the neutral hydrogen distribution of M86 in Virgo cluster, together with X-ray emission. We find a total HI mass of 7.17 x 10^7 Msun, and a X-ray mass of 6.52 x 10^9 Msun. The HI mass is smaller than the previous single dish observation. The HI is mainly located at south of the optical center of M86, while the hot X-ray plasm spreads out far to northwest along the optical major axis. The distribution of the cold and hot gas shows a strong interaction between the ISM of M86 and the intracluster medium of Virgo cluster, and that M86 is undergoing a strong Ram-pressure stripping.

Lim, Jeremy (Academia Sinica Inst. of Astr. & Astrophys.)
HI Imaging of Low-z QSO Host Galaxies
Jeremy Lim and Paul T.P. Ho
What triggers quasi-stellar objects (QSOs) at low redshifts? What is the nature of their host galaxies? We present results of an ongoing program to image in neutral atomic hydrogen (HI) gas the host galaxies of all QSOs at redshifts z<0.07 visible from the VLA. Our results reveal dramatic evidence for ongoing or remnant tidal interactions in virtually all of the QSO host galaxies so far detected in HI, even when no such interactions are readily if at all visible in the optical. Rather than being the end-product of a merger, our results suggest that QSOs can be found in a broad range of interacting environments. These results also show that many of the most luminous AGNs at low-z are not the descendents of their much more luminous counterparts at high-z, but represent a newly triggered population of AGNs. The typical HI gas content of the detected QSO host galaxies is comparable to that of our own Milky Way galaxy.

Linder, Suzanne (INAOE)
The Evolution of Lyman-alpha Absorbing Galaxies
S.M. Linder
Ly-alpha absorbers at low and intermediate redshifts are powerful tools for studying the gaseous extent of galaxies and the distribution of gas relative to galaxies. Studies of the relationship between absorbers and galaxies often involve surveys out to redshifts ~0.8 while the UV background is often assumed to evolve quite rapidly at lower redshifts in simulations. While no evolution has been reported recently in the absorption properties of galaxies (with given luminosities; Chen et al. 1999), there is likely to be at least some evolution of the UV ionizing background and in the optical properties of galaxies with given gaseous properties, so that a more complex evolutionary process is likely to be occurring. I will discuss possible implications for the gaseous evolution of galaxies and ways for detecting such evolution after taking into account observational selection effects.

Lockman, Felix J. (NRAO/Green Bank)
A Giant HI Cloud in the Inner Galaxy: A New Type of Interstellar Cloud?
F.J. Lockman, A.R. Minter, G.I. Langston & J.A. Lockman
New high-resolution HI observations have revealed a giant HI cloud in the inner Galactic plane. It is about 170x130 pc in extent and contains more than 10^5 solar masses of atomic hydrogen. The outer portions of the cloud appear in emission while the inner core is in self-absorption against background HI. Analysis of the brightness and opacity of the cloud indicates that it must have at least two components: a cold core near 25 K and an outer shell that is hotter by at least an order of magnitude. The cloud is outlined in weak, anomalous, 1720 MHz OH emission. The cloud is associated with, but not coincident with, a giant molecular cloud seen in CO, but it shows little if any evidence of recent star formation. It is possible that this cloud is a transition region between the hot ISM and a giant molecular cloud, but against this view is the fact that the cloud appears remarkably regular and self-contained. Evidence suggests that objects with these properties are quite common in the inner Galaxy.

Malhotra, Sangeeta (The Johns Hopkins University)
Large Area Lyman Alpha survey at z=4.5
J. Rhoads, S. Malhotra, A. Dey, B. Jannuzi, D. Stern, & H. Spinrad
The first burst of star-formation in galaxies is expected to announce itself by strong Lyman-alpha emission. Lyman-alpha emitters at high redshift have now been discovered, but at lower line luminosities than expected from early models. Understanding why the observed Lyman-alpha line sources are much fainter than the predictions would help us understand galaxy and star-formation at these epochs. We report on the first confirmed sources from the Large Area Lyman Alpha (LALA) survey at z=4.5 and discuss how such a large, systematic survey can help us determine star-formation and large scale structure when the universe was one-tenth its current age.

Mashchenko, Sergey (Université de Montréal)
The Impact of External Tidal Fields on the ISM of Dwarf Spheroidals
Sergey Mashchenko and Claude Carignan
We have performed the analysis of the tidal field for the Local Group of galaxies. To calculate the average projection of the strongest stretching'' tidal force axis for each galaxy onto the plane of sky, and corresponding standard deviation, the Monte-Carlo strategy has been adopted. It has been found that in two dwarf spheroidal galaxies (dSph) --- Sculptor and Tucana --- the associated HI gas follows closely the projected tidal axis direction. For the Sculptor dwarf we also see the velocity gradient in HI distribution along the tidal axis. The presence of two symmetrical clouds in Sculptor galaxy favours the tidal picture over the ram pressure gas stripping scenario. To test the intuitive idea that fragments of expanding HI shells resulted from supernovae type Ia explosions in dSph's should be preferentially removed from the galaxy along the strongest tidal force direction the 2D hydrodynamic simulations have been conducted. The preliminary results of the simulations seem to confirm the idea. Our findings emphasize the importance of a tidal field impact on evolution of dwarf spheroidal galaxies, and might help to explain the existence of such isolated dSph's as Tucana and Cetus dwarfs.

Mathews, William G. (University of California, Santa Cruz)
Evolution of Gas in Elliptical Galaxies
William G. Mathews
Recent work on the origin and evolution of hot interstellar gas in elliptical galaxies will be briefly reviewed. The observed interstellar density, temperature and metallicity profiles can be understood with simple models for the origin and evolution of elliptical galaxies. X-ray observations can be used to determine the stellar mass to light ratio, mass profiles of dark halos and central magnetic fields. X-ray images of rotating elliptical galaxies are nearly circular at the half-light radius, indicating that interstellar gas is cooling over a large volume. The cooled gas is continuously forming stars with a bottom-heavy IMF. If these young stars produce some of the visible light, then the fundamental plane is less disturbed and the young apparent ages of some massive elliptical galaxies can be understood. If time permits, the flow of interstellar gas into central black holes will also be discussed.

Matthews, Lynn (NRAO/Charlottesville)
The Interstellar Medium of Low Surface Brightness Galaxies
L.D. Matthews
I will describe highlights from several ongoing programs aimed at studying the interstellar medium (ISM) in low surface brightness (LSB) galaxies. In particular, I will describe how unique information can be obtained through studies of edge-on systems.

Recent observations of edge-on LSB galaxies obtained with the Hubble Space Telescope and the WIYN telescope have shown that while the dust contents of these galaxies are low, they are not dust-free. In combination with Monte Carlo simulations, I use these observations to estimate the amount of internal extinction and reddening in individual LSB galaxies. Based on these models I will also illustrate possible effects of dust on the observed morphologies, disk color gradients, and rotation curves. Finally, I will report on some of the first ever detections of CO emission from LSB spirals.

Search for a Relationship between the Local Ly-alpha Forest and Galaxies: First Results
K.M. McLin, J.T. Stocke, R.J. Weymann, & S.V. Penton
We present preliminary results from our pencil-beam galaxy redshift survey toward bright AGN observed with HST/GHRS and STIS. The AGN observed have been chosen to be nearby enough, z<=0.1, so that we can observe even very faint dwarf galaxies, M_B~-16, between us and them. Our HST survey probes absorbers down to N_{HI}~10^{12.5} cm^{-2}, comparable to the lowest column densities observed at high z. The purpose of our galaxy survey is to study what relationship, if any, exists between the nearby low column density Ly-alpha clouds discovered in our HST absorber survey and galaxies along the lines of sight to the AGN. In particular, this survey tests the hypothesis that the Ly-alpha forest is composed of extended galaxy halos. Our preliminary analysis seems to rule this out for the vast majority of absorbers. We do see evidence that some of the absorbers are associated with the larger scale galaxy distribution, supporting earlier HI observations. Additionally, on the basis of bright galaxy surveys, eight absorbers along five sightlines studied by us lie in galaxy voids (nearest galaxy >= 2 h^{-1} Mpc away). Our much deeper survey fails to find any nearby dwarf galaxies, solidifying the isolation of these absorbers.

Meiksin, Avery (University of Edinburgh)
Possible Fate of Gas in the Universe
Avery Meiksin
The success of large-scale structure theories provides a foundation for predicting the fate of gas in the universe from the recombination epoch until today. The predictions are illustrated with models of the intergalactic medium as revealed through the Lyman-alpha forest. Wider implications are discussed, including the possibility of shifting the observational basis of our understanding of galaxy formation from the optical toward the radio.

Mihos, Chris (Case Western Reserve University)
Do Mergers Make "Normal" Ellipticals?
C. Mihos
The merger hypothesis says that elliptical galaxies are built through mergers of spirals. However, spiral mergers at high redshift may have significantly different properties than spiral mergers in the field today. These differences may translate to differences in the final properties of the elliptical galaxy population. Mergers can imprint several signatures on the forming elliptical, and here we look at differences in ellipticals in the context of differences in the merging progenitors.

Gas/Star Offsets in Tidal Tails
C. Mihos
Kiloparsec-scale offsets are sometimes observed between the gaseous and stellar components in tidal debris. These observations have led to models in which offsets are explained by hydrodynamic interactions between the tidal gas and a hot gas component, either an X-ray halo or outflowing starburst wind. In fact, some of these gas/star offsets can be simply explained by differences between the initial distribution of gas and stars in the progenitor disks, coupled with dissipation in the gasous component. Because HI disks are more extended than their stellar counterparts, they contain more angular momenta; once the tidal tails are launched, the high angular momenta gas tends to lead the low angular momenta stellar component. This kinematic evolution explains the observed gas/star offsets in interacting galaxies without invoking any additional (unseen) gaseous component.

Miller, Eric (Univ. of Michigan)
High Velocity Gas in Nearby Galaxies
E.D. Miller, J.N. Bregman, & B.P. Wakker
Two theories for the origin of the high velocity clouds (HVCs) have been suggested: that they are local to the Galaxy (~10 kpc distant), produced by the stripping of gas from dwarf galaxies and by galactic fountains; and that they are associated with the Local Group (~1 Mpc distant), representing gas remaining from galaxy formation. If the former scenario is correct, there should be significant concentrations of high velocity gas around external galaxies.

We present deep HI 21-cm observations of two nearby spiral galaxies, M51 and M83. These observations are 2--10 times more sensitive than previous data taken more than a decade ago, enabling us to detect Galaxy-like HVC complexes and streams in these galaxies over a fairly wide field of view. We detect several HVC candidates, identify the constraints these new observations place on the amount of high velocity gas present, and finally discuss the implications of our results on the various HVC formation mechanisms.

Miller, Neal (NRAO/NMSU)
A Search for Dust-Enshrouded Star Formation in E+A Galaxies
N.A. Miller and F.N. Owen
E+A galaxies are classically thought of as post-starburst, but recent observations suggest this may not always be the case. Their optical spectra indicate an older stellar population (~K stars) but also show the strong Balmer absorption bands representative of A stars. While these young stars are present, the emission lines associated with current star formation are absent. However, deep radio observations of intermediate redshift clusters have surprisingly found that some E+A's are radio sources, with radio luminosities suggestive of low levels of star formation. One possible explanation is that at least some E+A's are still forming stars, but the optical signatures of such star formation are heavily obscured by dust.

We have performed radio observations of 15 of the 21 E+A galaxies identified from the Las Campanas Redshift Survey (LCRS) by Zabludoff et al. In contrast to the cluster E+A's, these galaxies are relatively nearby and are almost exclusively field galaxies. Our observations are sensitive enough to detect radio luminosities of 2x10^{21} W/Hz, equivalent to a massive star formation rate of 0.5 M_sun/yr. Of the 15 observed E+A's, two were detected. This is roughly the same fraction of E+A radio detections as found in the intermediate redshift clusters. While the possibility that these detections represent weak AGN can not be ruled out entirely, these results suggest that the optical picture of E+A's as post-starburst systems is incomplete and that some E+A's could harbor current star formation in heavily dust-obscured regions.

Mundell, Carole (ARI)
The Gaseous Environment of Seyfert Galaxies
C.G. Mundell
\lambda 21-cm HI studies are enjoying a renaissance as a result of high-resolution absorption techniques and improvements in sensitivity and resolution for HI emission imaging, but to date, relatively few detailed synthesis imaging studies of HI in Seyfert galaxies have been carried out. While single-dish HI studies are important in detecting and measuring galaxy gas content and overall kinematics, only synthesis observations (e.g., using the VLA) have sufficient angular resolution to distinguish individual gas features from one another, such as disk gas from tidal tail/intragroup gas, dwarf galaxies from main disks and non-linear gas dynamics in bars, which have previously produced uninterpretable complex kinematics in low resolution, single dish, HI studies. Here I will summarise results from an on-going imaging project to study the detailed distribution and kinematics of HI in Seyferts (with angular resolution ranging from 5 ''-60''). So far we have imaged approximately 9 Seyferts (NGC3227, NGC3982, NGC4051, NGC4151, NGC4939, NGC5033, NGC5506, NGC7469, NGC7674) and although these galaxies form a rather heterogeneous sample, I shall summarise their properties and describe the need for future, statistically significant imaging studies.

Neff, Susan (NASA/GSFC)
Massive Star Formation in NGC4038/4039: HII Regions and Supernova Remnants in "the Antennae"
The nearest merger, NGC4038/4039 (The Antennae,'' Arp244), is undergoing a starburst apparently triggered and driven by the ongoing interaction. The system provides an excellent laboratory to study star formation processes because well-studied and nearby (21 Mpc assumed here). Models of the interaction by e.g. Mihos et al. 1993 suggest that the interaction's first periapse occurred ~200Myr ago, and that final coalescence will occur in another ~100Myr. Recent HST V and I images by Whitmore et al. (1999) resolve star-forming knots into several thousand blue compact clusters, with ages <5 Myr for the youngest clusters.

We used the VLA at 6 and 4 cm to obtain high resolution radio images of the star-formation regions in NGC4038/4038 (Figure 1). Compact radio emitting regions were identified, and their spectral indices used to differentiate between thermal HII regions and supernova remnants. We identify 115 individual compact 6cm radio sources and 63 individual 4cm sources in the system, to a limiting luminosity of ~2x10^18 W/Hz (or ~4 times the luminosity of Cas A). Among the sources strong enough to provide good spectral information, 1/3 have flat radio spectra (presumably dominated by HII regions) and 2/3 have steep spectra (apparently dominated by synchrotron emission from supernova remnants). Compact radio sources produce ~12% and ~25% of the 6cm and 4cm radio emission in the system, respectively.

For the thermal sources, we derive typical masses of 10^4 to 10^5 M_sun in massive young stars and ionized gas masses of 10^3--10^4 Msun. The strongest thermal radio emission occurs between the galaxies, at an optically unremarkable location near but not coincident with an extremely red cluster detected by Whitmore and Schweizer (1995). The radio peak is at a location of intense star formation, as detected in the mid-IR (Mirabel et al. 1998), and of dense molecular gas (Stanford et al. 1990). From the steep-spectrum sources, we derive a total system supernova rate of 0.2--0.3/yr, about an order of magnitude larger than that expected from the currently observed O-star population. Taken together, these suggest that that a large number of O stars may have formed in the system in a short-lived burst, less than 10^6 yr in duration and about 3--4 Myr ago.

O'Neil, Karen (Arecibo Observatory)
The Gas Content and Stellar Population of Red Low Surface Brightness Galaxies
Karen O'Neil
In a previous survey we discovered the first sample of very red (B-V>=1) low surface brightness galaxies. The properties of these systems are intriguing, as they may include some of the highest gas mass-to-light ratio galaxies known, and galaxies with HI velocity widths which do not adhere to the standard Tully-Fisher relation. In this poster we present recent results of our studies of these enigmatic systems, including optical spectroscopy, neutral hydrogen content, and multi-band photometry.

Oey, Sally (STScI)
A New Look at Chemical Evidence for Gas Infall in Galaxies
M.S. Oey
Most models for galactic chemical evolution incorporate gas infall or outflow in order to match observed metallicity distributions. I present a simple, analytic model for inhomogeneous chemical evolution that assumes no mixing, but incorporates a contamination filling factor Q. Preliminary results show good agreement with both the Galactic disk and halo, representing relatively evolved and unevolved systems, respectively. This suggests that infall and outflow may not strongly affect relative metallicity distributions.

Oosterloo, Tom (NFRA)
HI in Early-Type Galaxies
T.A. Oosterloo
Early-type galaxies are generally considered to be gas poor, but it has become clear in the past few years that many galaxies of this type do have an ISM that is similar to that in spirals, the main difference being that they have less of it. The amount of gas present in a galaxy during its formation and its further evolution is a key factor in understanding the structure of a galaxy, hence it is important to understand the gas properties of early-type galaxies, even if they are not gas rich as spiral galaxies.

I will briefly discuss the properties of the HI in early-type galaxies with particular emphasis on the differences between low-luminosity and more luminous galaxies and what this could imply for differences in evolution for these two classes of galaxies. I will also discuss a few examples of early-type galaxies with very regular HI structures and what we can learn about the dark matter properties of these galaxies.

Owen, Frazer (NRAO/Socorro)
Radio Evidence for Evolution of Galaxies in Rich Clusters
F.N. Owen
At z=0, below a 1400 MHz radio luminosity of 10^{23} W/Hz, the radio luminosity function is dominated by emission due due to star-formation processes. Furthermore, the radio luminosity is known to be a good estimator of a galaxy's the star-formation rate (SFR) from H-alpha and FIR studies. We have used this relation to study the current SFR's of galaxies in rich clusters without the complication of dust obscuration.

Studies of about 50 rich very clusters with 0.02 < z < 0.41 show that the distribution of galaxies with SFR > 5 M_sun/yr are much more broadly distributed than the radio AGN population and the red galaxy population with core radii of ~500 kpc, very much like the blue galaxy population. For the star-forming radio population no correlation is found with the Butcher-Oemler blue fraction and an anti-correlation appears to exist with richness. At these low radio luminosities, there is a strong increase in the radio fraction with redshift. Based on a small number of objects there appears to be a higher radio fraction in clusters currently undergoing a cluster-cluster merger.

Several results suggest that strong star-formation is occurring in some galaxies with post-starburst (E+A) spectra, suggesting that dust can sometimes obscure the optical signatures of this activity. Also results for one object suggest that cluster tidal effects may be important in simulating activity and global evolution of cluster galaxies.

Peck, Alison (MPIfR)
Circumnuclear HI in the Compact Symmetric Object 1946+708
A.B. Peck, G.B. Taylor, & K.R. Menten
In the last several years, a number of compact extragalactic radio sources have been found to exhibit neutral hydrogen absorption at or near the systemic velocities of their host galaxies. Models proposed to explain this phenomenon involve a circumnuclear torus of gas and dust. The orientation of this structure determines whether or not the central engine is obscured. Understanding the spatial distribution and kinematics of the HI detected toward the central parsecs of these sources provides an important test of this model and of unified schemes for AGN.

We present results of Global VLBI Network observations of the redshifted 21 cm HI line toward the Compact Symmetric Object 1946+708 (z=0.101). This source is of particular interest because it exhibits bi-directional motion measurable on timescales of a few years. The resulting unique information about the geometry of the continuum source greatly assists in the interpretation of the HI distribution.

We find significant structure in the gas on parsec scales. The peak column density of the HI occurs near the center of activity of the source, as does the highest velocity dispersion (FWHM~ 350 to 400 km/sec). The distribution of gas in 1946+708 is strongly suggestive of a circumnuclear torus of atomic material with one or more additional compact clumps of gas along the line of sight to the approaching jet.

Pedlar, Alan (NRAL)
MERLIN & VLA Observations of HI Absorption in Messier 82
Alan Pedlar, Karen Wills, Tom Muxlow, & M. Das
We present MERLIN and VLA 21cm atomic hydrogen (HI) absorption observed against the central region of the starburst galaxy M82. The MERLIN observations (0.1 arcsec beam=1.5pc) are mainly against individual supernova remnants, whereas the VLA observations delineate absorption against the diffuse background. By comparing the H1 absorption with molecular (CO) and ionised ([NeII]) gas in M82 it is possible to compare the dynamics and structure of the ionised, neutral and molecular gas in greater detail than any previous starburst studies. The position-velocity plots of all three components show significant departures from circular motion which we interpret as gas flows within a bar potential seen 10 degrees from edge-on. The ionised gas shows a steeper gradient and smaller linear extent than the neutral gas (H1 and CO) which in the bar model would imply that the ionised gas is largely following the X2 orbits and the neutral gas is largely confined to the outer X1 orbits. Although we see clear evidence in our H1 observations for the CO 'expanding superbubble' feature proposed by Weiss et al (1999), we note that this can also be interpreted in terms of non-circular motions characteristic of bars.

Petric, Andreea (New Mexico Tech)
Line Widths and the Universal HI Profile in NGC 1058
Andreea Petric and Michael Rupen
We use HI observations to trace the vertical motions of the neutral gas in the face-on spiral NGC 1058. The combination of high sensitivity and low inclination permits the accurate measurement of line profiles at high spatial and spectral resolution. Although the width of the line profile varies from one beam to the next (FWHM = 14 - 30 km/sec), some global trends are evident. The lines are broader in the central 1.5 arc-min where star formation is most vigorous. However, outside this region the linewidths are uniformly smaller in the spiral arms than in the interarm regions; and there is a general anticorrelation between H_${\alpha}$ emission and broad profiles. The line profiles are not Gaussian, and hence cannot solely be determined by single temperature thermal broadening. The observed shape of the line profiles does seem to be universal, in the sense that profiles are identical throughout most of the galaxy when scaled by their FWHM. The surprise is that the universal shape is seen in diverse environments: both within and beyond the stellar disk, in quiescent or actively star forming regions. However, in a few locations the profiles are assymmetric thus deviating from this universal shape. These asymmetric profiles tend to have large FWHMs 25-30 km/sec and do not seem to be associated with any obvious star formation.

Pfenniger, Daniel (University of Geneva)
The Dynamics of Massive Gaseous Disks
D. Pfenniger
The galactic dark halos are almost systematically assumed to belong to the very peculiar class of ellispoidal or spheroidal shapes, despite the fact that the famous disk-halo conspiracy'' applies only to these but not to disk flaring shapes. Here, to follow the hypothesis that outer galactic disks can contain a substantial amount of cold gas (Pfenniger & Combes 1994), we explore with high resolution N-body simulations the dynamical consequences of assuming maximum disks also in the outer galactic disks. Substantial differences occur between fully consistent and live disks and the often retained steady dark halos.

Phookun, Bikram (Raman Research Institute)
Kinematic and Morphological Lopsidednes in the HI Distributions of Spiral Galaxies
Bikram Phookun
Spiral Galaxies are almost axisymmetric - but not quite. Deviations from axisymmtery can reflect a number of factors, one of which is the gravitational potential. We use the deviations from axisymmetry in the HI velocity fields of a number of galaxies to estimate the lopsidedness of their gravitational potentials. We then examine whether the latter can account for the morphological lopsidedness of the HI distributions in these galaxies.

Pihlstroem, Ylva (Onsala Space Observatory)
Probing the Neutral Gas in High-z Sources
Y.H. Pihlstroem, J.E. Conway, R.C. Vermeulen, & G.B. Taylor
To increase the statistics of high redshift sources with associated HI absorption, a survey to detect redshifted HI in northern sky sources is being performed using the WSRT. The recent upgrade of the WSRT enables studies of the redshifted 21cm HI line for 0.2<z<1.0. Here we report on results from this survey, concentrating on the sample of Compact Steep Spectrum sources (CSS) and Compact Symmetric Objects (CSOs).

The First Scientific Results from the EVN MKIV Correlator
Y.H. Pihlstroem, J.E. Conway, H.J. van Langevelde, & R.T. Schilizzi
High-resolution and high-sensitivity EVN observations of the 21cm HI line in the FRI radio galaxy NGC4261 locates the absorbing gas to be in front of the counterjet. This is consistent with for example the orientation of the HST disk in this object. The absorption is likely to reside in a thin, atomic circumnuclear disk possibly feeding the central engine. The data presented are from the first scientific project processed on the new EVN MKIV data processor at JIVE.

Pisano, D. J. (University of Wisconsin/Madison)
Assessing the State of Galaxy Formation
D.J. Pisano and Eric M. Wilcots
Hierarchical galaxy formation models predict that galaxy assembly should continue into the present day as small clumps of gas fall onto galaxies. We present the results of a comprehensive survey of nearby, quiescent, non-peculiar, extremely isolated galaxies to assess the state of galaxy assembly in the local universe. We find low-mass, HI companions around 6 of 34 galaxies surveyed, which could be debris associated with galaxy assembly. In addition, we find severely lopsided HI distributions around an additional 6 galaxies, which could indicate recent accretion events. The lack of gas around these galaxies suggests that galaxy assembly has concluded, although perhaps only recently.

Poggianti, Bianca Maria (Osservatorio di Padova)
Environmental Effects on Galaxy Evolution in Clusters
B. M. Poggianti and J. van Gorkom
During the first part of my talk I will review the optical observations of galaxies in distant clusters up to z=0.6 discussing:
1) the evolution of the distant spiral population into the S0 population that dominates clusters at low-z.
2) the star formation properties and the connection between the changes in galaxy structure and the evolution of the stellar populations.

The second part of my talk will focus on the link between the HI content and the stellar populations + morphologies. I will present the results of a multiwavelength study of the cluster A2670 (z=0.08) where 42 galaxies have been detected in HI and for which radio-continuum data and optical images and spectra have been secured. The positions and HI masses & morphologies of these galaxies are extremely interesting signs of the gas absence/deficiency in the central regions of the cluster and reveal the existence of distinct subsystems of galaxies that are segregated spatially and in velocity. A detailed analysis of the optical spectra allows to identify starburst, star-forming, post-starburst and passive galaxies. I will show the star formation properties of the HI-normal and HI-deficient galaxies and discuss their dependence on the location within the cluster. I will use A2670 and - probably - Coma as local cluster examples to compare with the results at intermediate redshift.

Putman, Mary (MSSSO/ATNF)
High-Velocity Clouds of the South
Mary E. Putman
Our Galaxy's formation and evolution is strongly linked to halo objects such as high-velocity clouds (HVCs). HVCs are distributed over the entire sky, but most studies have focused on the northern complexes due to limited southern HI data. HVC-reduced HI Parkes All-Sky Survey data, combined with H-alpha observations, provide new insight into positive velocity HVCs; in particular, those which have resulted from the interaction of the Magellanic Clouds with the Milky Way and the mysterious compact high-velocity clouds (CHVCs). The protogalactic nature of CHVCs, and the HVC H-alpha detections and non-detections are discussed in terms of the clouds' three-dimensional HI environment. The results are gradually revealing the structure of the Galaxy's HI halo.

Rand, Rich (Univ. of New Mexico)
Key Results on the Ionized Gas Halo of NGC 5775
Richard Rand
We present key results from deep spectra of the diffuse ionized gas (DIG) halo of the edge-on galaxy NGC 5775. [N II] \lambda6583 has been detected up to z~13 kpc above the plane in one of two vertically oriented long slits -- making this the spiral galaxy with the greatest spectroscopically detected halo extent in emission. Diagnostic line ratios, measurable up to z~8 kpc, indicate clear departures from pure, dilute photo-ionization scenarios. Modeling of the ionization sources is presented in an accompanying poster by Collins & Rand. Velocities of the gas in both slits approach the systemic velocity of the galaxy at several kpc above the plane. We interpret this trend as a decrease in rotation velocity with z, with essentially no rotation at heights of several kpc. Such a trend was observed in the edge-on galaxy NGC 891, but here much more dramatically. This falloff is presumably due to the gravitational potential changing with z, but will also depend on the hydrodynamic nature of the disk-halo cycling of gas and projection effects.

Rao, Sandhya (Univ. of Pittsburgh)
Evolution of Damped Lyman-alpha Systems
Sandhya Rao
We present results from an efficient, non-traditional survey to discover damped Ly-alpha (DLA) absorption systems with neutral hydrogen column densities N_{HI}>=2x10^{20} atoms cm^{-2} and redshifts z<1.65. Contrary to previous studies at higher redshift that showed a decrease in the cosmological mass density of neutral gas in DLA absorbers, \Omega_{DLA}, with time, our results indicate that \Omega_{DLA} is consistent with remaining constant from redshifts z~4 to z~0.5. There is no evidence that \Omega_{DLA} is approaching the value at z=0. Other interesting results from the survey will also be presented and their implications about the nature of DLA galaxies will be discussed.

Ratay, Doug (University of Florida)
HI Observations of NGC 1784
Douglas L. Ratay and Stephen T. Gottesman
We present Very Large Array C and D-configuration 21cm observations of the barred spiral galaxy, NGC 1784. We find HI separated from the main body of the galaxy by 1'. This HI is in a seemingly retrograde orbit. We also find that the velocity field of NGC 1784 is decidedly non-circular. These properties may be the result of an interaction with a minor companion galaxy. However, no companion is visually obvious or has been catalogued.

Revaz, Yves (Geneva Observatory)
N-body Simulations of Warped Galaxies
Y. Revaz
Two methods generating warped galaxies with N-body simulations are presented. One uses an external potential as a disturber while the other is based on material accretion. Results of both methods are compared. A particular attention is given on the shape of the line of node.

Rigby, Jane (Penn State)
Weak MgII Absorbers at z~1: Low N(HI) HVCs, SNR in Dwarf Galaxies, or Pop III Remnants?
J.R. Rigby, J.C. Charlton, & C.W. Churchill
Using photoionization models, we investigate weak MgII absorbers,'' narrow (3--7 km/sec), high metallicity systems not directly associated with >0.05 L^* galaxies. These systems do not produce Lyman limit breaks and thus have metallicities >=0.1 solar; they must account for a substantial fraction of N(HI)~ 10^{16} cm^{-2} Ly-alpha forest clouds.

We identify a subclass with relatively high FeII, constrained to be ~10 pc. For dN/dz = 0.35 as observed, these small clouds must outnumber galaxies by roughly six orders of magnitude. They may be old Type Ia SNe shells in low-luminosity structures such as dwarf galaxies or faded Pop III star clusters embedded in CDM mini--halos. We also identify a low FeII subclass; these clouds may be low N(HI) high velocity clouds at z~1.

Roberts, Mort (NRAO/Charlottesville)
Asymmetry in NGC 1637
M.S. Roberts, D.E. Hogg, & E. Schulman
The asymmetry in the optical image of the isolated galaxy NGC1637 is unmistakable in that it possesses a well-defined third arm. To explore the asymmetry we have mapped the galaxy in both HI and CO. The distribution in HI and CO does reflect the asymmetry but, surprisingly, the global HI profile is symmetric. We attribute this to the large extent of the HI envelope. Modelling of the kinematics of the HI shows that the kinematic center is displaced from the center defined by the optical position. Such a displacement may result from an eccentric perturbation in the disk, of the type that may produce an additional m=1 mode.

Rupen, Michael (NRAO/Socorro)
The Expanded Very Large Array
Michael P. Rupen and the EVLA Science Team
The Expanded Very Large Array (EVLA) will build on the existing infrastructure of the VLA to yield order-of-magnitude improvements in all important observational parameters, including sensitivity, spatial and spectral resolution, and frequency coverage. The EVLA will provide microJansky continuum sensitivity, with continuous frequency coverage from 1 to 50 GHz. With a minimum of 16,384 channels even at the widest (8 GHz) bandwiths, blind' spectral line searches will become possible, and every observation of extragalactic line emission will yield deeper radio continuum images than any now available. We briefly describe a few sample projects to illustrate the power of the instrument, and discuss current progress towards its realization.

Sadler, Elaine M. (Univ. of Sydney)
HI in Elliptical Galaxies -- New Results from HIPASS
Elliptical galaxies have a notoriously low detection rate in HI, so that targeted HI studies of these galaxies are time-consuming and often unrewarding (unless one uses other clues, such as the presence of dust, to select promising targets). Now that the Parkes HI multibeam survey (HIPASS) has covered the whole southern sky, we can use the results to measure the HI content of several thousand elliptical and S0 galaxies in the local Universe, and hence derive the HI mass function from a large and homogeneous data set.

Sansom, Anne (Univ. of Central Lancashire)
Gas in Ellipticals with Fine Structure
A.E. Sansom, J.E. Hibbard, & F. Schweizer
We investigate trends of the cold and hot gas content in early-type galaxies with the degree of optical morphological peculiarity, as measured by the fine-structure index. We report VLA HI mapping observations of five such galaxies. These, plus observations from the literature are used to track cold gas content. Archival ROSAT data are used to quantify the hot gas content. There is no clear trend of cold gas content with fine structure. However, we find a clear trend of X-ray excess (measuring the hot gas content) with fine structure. Only galaxies with low fine structure are strong X-ray emitters. This is consistent with the build-up of hot gas haloes over several gigayears.

Sarma, Anuj (University of Kentucky)
VLA HI Observations of the Zeeman Effect in Centaurus A (NGC 5128)
A.P. Sarma, T.H. Troland, & M.P. Rupen
We place upper limits on the line-of-sight magnetic field toward the nucleus and inner jets in Centaurus A based on Zeeman effect HI absorption studies. The HI absorption curves also reveal a hitherto unreported feature --- a broad, low-intensity, redshifted absorption feature which extends up to about 635 km/sec in heliocentric velocity, and is seen only toward the nucleus. Our interpretations of this feature are also presented.

Schiminovich, David (Caltech)
David Schiminovich
Beginning with the first HI map of the Antennae (van der Hulst 1979), HI images and kinematics of galaxies with extended stellar tails have provided strong support for the Toomre & Toomre (1972) model of tidal feature formation during the merger of two galaxy disks. I will discuss recent HI studies of mergers and merger remnants which, in combination with increasingly realistic simulations, are being used to refine our understanding of the history of interactions, tidally-induced star formation and gaseous evolution in the local universe.

Schinnerer, Eva (Caltech)
Warps and Bars Traced by the Molecular Gas in Nearby Active Galaxies
E.Schinnerer, A.Eckart, L.Tacconi, R.Genzel, N.Scoville, & D.Downes
Sub-arcsecond mm-interferometric observations of the molecular emission lines in a number of nearby AGN reveal complex molecular gas kinematics in the inner 100 pc to 1 kpc inconsistent with circular motion in the inclined host galaxy disks. The kinematics imply either a bar potential or a warped thin gas disk.

A detailed analysis of the distribution and kinematics of the molecular gas in the inner 300 pc for two galaxies (NGC 3227 and NGC 1068) indicates that their nuclear molecular gas disks are probably warped. Position-velocity diagrams show rising rotation curves at r<=15 pc (~0.2'') -- the enclosed masses are >=2x10^7 M_sun (NGC 3227) and >=10^8 M_sun (NGC 1068). This is the first time that such a compact central mass concentration has been interferometrically detected using the emission of the cold molecular gas in its vicinity.

In the case of the Seyfert 2 NGC 1068 the warped molecular gas disk is oriented edge-on at a radius of ~70 pc providing material for the obscuration of the AGN nucleus. The most likely cause of the warping is the interaction of the ionization cone with the gas disk linking the host galaxy and the AGN itself.

Sellwood, Jerry (Rutgers University)
Does Dark Matter Make Sense?
J.A. Sellwood
The properties of dark matter, whatever it is, are constrained by the observed properties of halos, especially those deduced from HI rotation curves. The radial density profiles, shapes and estimated phase space densities of actual halos all provide constraints on the nature of DM, and are increasingly able to rule out the simpler ideas. If DM really exists, it now seems unlikely to be a simple, collisionless relic particle, but must have some bizarre additional properties.

Simkin, Susan (NSF)
HI Environment of Markarian 315
Susan M. Simkin and John W. MacKenty
Markarian 315 is a Seyfert 1.5 galaxy with an ionized gas tail, ~70~h^{-1} kpc in length, which extends towards the NNW out to a projected distance of ~45 kpc from its nucleus. We have mapped the extended HI gas associated with this galaxy using the C array of the VLA. Contrary to expectations, we find that the HI emitting gas is {\bf not} associated with the ionized gas tail but instead lies in a cloud of mass ~10^8 to 10^9 M_sun which is located on the opposite (SSE) side of the galaxy at a projected distance of ~30 galaxies. The disturbed morphology and kinematics of the HI gas tends to support a merger hypothesis for the origin of this object's nuclear fuel but modeling of the merger and the detailed feeding mechanism require more extensive observations than are presently available.

Smette, Alain (NASA/GSFC, NOAO)
HST-STIS Observations of the HeII Gunn-Peterson Effect
Alain Smette, Sarah. R. Heap, Gerard M. Williger, Ed B. Jenkins, Todd Tripp, & Antoinette Songaila
We present the results of the HST/STIS GTO program dedicated to the observations of the HeII Gunn--Peterson effect. 3 QSOs were observed.
(a) Q0302--003 clearly shows the GP HeII effect over the range 2.78<z<3.28. Near the QSO, the HeII Ly-alpha absorption is produced by discrete clouds with no significant diffuse gas, consistent with a HeII proximity effect in which the QSO fully ionizes He in the diffuse IGM but not in denser clouds. The average HeII Ly-alpha opacity at z=3.15 is \tau>4.8, which requires a diffuse gas component as well as a soft UV background with significant stellar contribution. High HeII Ly-alpha transmission at z=3.05 most likely arises in a region where He is doubly ionized by an AGN. At $z < 2.87$, the average HeII Ly-alpha opacity \tau=1.88, much lower that at z>3, which is consistent with an increase of the UV background hardness.
(b) The spectrum of HE2347--4342 presents associated absorption explaining the absence of obvious proximity effect. Two low-opacity regions are detected extending over 16 and 3 comoving h^{-1} Mpc. Due to an unusally high dark count--rate, we cannot improve on the lower--limits on the opacity in the high opacity regions obtained by Reimers et al. (1997).
(c) The spectrum of PKS1935-692 presents a HeII proximity effect and a region of high transmission at z=3.10. The STIS spectrum also reveals the presence of a z=0.30 DLA system which limits its useful range for GP studies. We show how a simple model based on the observed HI Ly-alpha forest can reproduce all the features observed in the spectra of these 3 QSOs.

Smith, Beverly (East Tennessee State University)
Interstellar Gas and Star Formation in the NGC 4410 Galaxy Group
Beverly J. Smith, Megan Donahue, & John Stocke
We present new radio continuum, 21 cm HI, 2.6 mm CO, and optical data for the NGC 4410 galaxy group, and compare with available X-ray maps. This group contains the peculiar radio galaxy NGC 4410A, which has a distorted double-lobed radio structure. NGC 4410A is strongly interacting with three nearby galaxies, and has a peculiar ring-like optical morphology, a number of extremely luminous H~II regions, as well as tidal tails and bridges variously visible in optical, HI, and X-ray light. In this paper, we discuss how the gravitational interaction has affected the interstellar matter, the star formation rate, and the radio lobes in this system.

Staveley-Smith, Lister (ATNF)
HIPASS Results on the Gaseous Environments of Galaxies
L. Staveley-Smith
The HI Parkes All-Sky Survey allows the neutral gaseous environment and content of large samples of galaxies to be studied in an unbiased manner. We present recent HIPASS results relating to the neutral gas density in the local Universe, the volume density of low-mass HI clouds, the HI mass function and its variation with environment, and high-velocity gas in the immediate vicinity of our own Galaxy. Finally, the HI environment of the massive Great Attractor complex is examined.

The Local Ly alpha Forest: HI in Nearby Intergalactic Space
John T. Stocke
Detecting H I using redshifted Ly alpha absorption lines is ~10^7 times more sensitive than using the 21 cm line. I will review recent discoveries of H I Ly-alpha absorbers made with the Hubble Space Telescope (HST) which has allowed us a first-look at gas in intergalactic space locally (i.e., between us and the ''Great Wall''). Despite its mere 2.4m aperture, these new HST observations allow the detection of absorbers as rarefied as those found using Keck at high-z (~ log N_{H I}=12.3 cm^{-2}). New results that will be discussed include: the evolution of absorbers with redshift, the location of absorbers relative to galaxies (including the two-point correlation function for absorbers), the metallicity of absorbers far from galaxies and the discovery of hot, shock-heated(?) (~10^{5-6} K) absorbers. The importance of VLA H I observations to discover the nearest galaxies to these absorbers will be stressed. This work is supported by observing grants from HST. The Colorado local Ly \alpha forest research group includes: J.M. Shull, M. Giroux, S. Penton, B. Gibson, K. McLin and J. Tumlinson. Primary collaborators eleswhere include: R. J. Weymann (Carnegie), J. van Gorkom (Columbia) and C. Carilli (VLA).

Swaters, Rob (Carnegie Institution)
Dark Matter in Late-Type Dwarf Galaxies
R.A. Swaters
HI rotation curves are presented for a sample of 35 dwarf and LSB galaxies. Their rotation curves have shapes similar to those of spiral galaxies. Despite their lower amplitudes, the rotation curves rise steeply until about two disk scale lengths, and start to reach a flat part at larger radii. Mass models based on these rotation curves are consistent with a wide range in dark matter properties: on the one hand, these rotation curves are consistent with dwarf galaxies being dominated by dark matter. On the other, they may also have dark matter properties similar to those of spiral galaxies.

A Gallery of Dwarf Galaxies
R.A. Swaters
For a sample of 75 late-type dwarf galaxies R-band surface photometry and HI synthesis imaging are presented. In addition to a general study of the optical and HI properties, these data have been used for a study of the rotation curve and dark matter properties of these galaxies. In this poster, the main results are presented. The radial extent of HI in dwarf galaxies, relative to the optical extent, is similar to that found in spiral galaxies. On the other hand, towards lower luminosities and surface brightnesses, dwarf galaxies become richer in HI. Often, the distribution of HI, and sometimes the kinematics, are found to be lopsided. The rotation curves, after correction for beam smearing, are generally found to have shapes similar to those of spiral galaxies. The shape of the rotation curves in the inner parts is correlated with the central concentration of light, suggesting that the luminous matter knows' about the dark matter. Mass modeling based on these rotation curves shows that the stellar disk can be scaled to explain the inner parts of the rotation curves, but the required stellar M/L values are high. For reasonable M/L values, high density halos with steep inner slopes are found. In particular, these rotation curves are found to be consistent with predictions from CDM simulations.

Thilker, Dave (NRAO/Socorro)
A High-resolution VLA Mosaic of HI in M33
David A. Thilker, Robert Braun, & René Walterbos
We report on sensitive, wide-field VLA observations of HI in M33. Our new, 6-pointing mosaic clearly highlights the filamentary nature of the neutral atomic interstellar medium (ISM). Indeed, this map represents the highest resolution HI survey for a spiral galaxy other than the Milky Way. Using the CS and B configurations of the VLA, we attained sensitivity to HI structure on scales down to 20~pc. We note detailed correspondence between the neutral ISM and ionized gas traced by wide-field H$\alpha$ observations. The new M33 HI datacube will be used for a morphological study of the relation between massive stars and the ISM, with emphasis on the formation and evolution of expanding shells.

Thomas, Helen (Cambridge University)
Extended dust emission in NGC7465
H.C. Thomas
As part of a study into the gas and dust content of nearby galaxies, we have observed the NGC7465 group of galaxies using SCUBA (at 850microns) and compared this with VLA HI observations and a B-Band image of the galaxies taken with the INT. I find that NGC7465 has extended sub-mm emission stretching out to at least twice the optical radius on the eastern side of the galaxy. Peaks in the extragalactic dust correspond to peaks in the neutral hydrogen column density implying that the dust is well mixed with the HI. Combined with faint tidal features present on the B-Band image, these data show strong evidence for the stripping of the ISM due to a tidal interaction.

Tripp, Todd (Princeton University)
Hot and Highly Ionized Gas in Galaxies and Galaxy Groups: High Resolution Spectroscopy of O VI Absorbers Toward Low-Redshift QSOs
T.M. Tripp, B.D. Savage, & E.B. Jenkins
We report the discovery of a surprisingly large number of weak intervening O VI absorption line systems located near galaxy groups in spectra of the low redshift QSOs H1821+643 and PG0953+415 observed with the Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer. We first show that these absorbers may harbor a substantial fraction of the baryons in the low redshift universe. We then discuss the physical conditions implied by the absorption line properties. In some cases the presence of broad components in the H I Ly-alpha profiles indicates that the gas is hot and collisionally ionized, but in most cases there is clear evidence that these are multiphase complex absorbing media. We briefly discuss the implications for galaxy evolution.

Turnshek, David (University of Pittsburgh)
Damped Ly-Alpha Galaxies
David Turnshek
Some results from an imaging program to identify low-redshift (0.09<z<1.63) damped Ly-alpha (DLA) galaxies are presented. The standard paradigm that was widely accepted a decade ago, that DLA galaxies are the progenitors of luminous disk galaxies, is now being seriously challenged. The indisputable conclusion from imaging studies at low redshift is that the morphological types of DLA galaxies are mixed and that they span a range in luminosities. Other pieces of evidence which support this conclusion include the DLA neutral hydrogen column density distribution, abundances, and metal-line kinematics.

Imaging Parsec-Scale Gas in Seyfert Nuclei using VLBA Observations of Free-Free Absorption
J.S. Ulvestad, J.M. Wrobel, A.S. Wilson, A.L. Roy, H. Falcke, T.P. Krichbaum, & E.J.M. Colbert
Seyfert galaxies generally contain steep-spectrum radio sources, which are thought to be caused by optically thin radio jets. Radio and optical imaging show that these jets are generally confined to the narrow-line regions of the host galaxies, with typical sizes of hundreds of parsecs or less.

Some Seyfert galaxies contain compact cores that can be imaged using VLBI techniques, and a number of the stronger objects have now been imaged using the VLBA. These objects show a variety of structures on scales ranging from less than a parsec to tens of parsecs. Of considerable interest is the fact that a number of galaxies (e.g., NGC 1068, Mrk 231, and Mrk 348) contain components that show low-frequency cutoffs or turnovers. In some cases, these cutoffs are almost certainly due to free-free absorption caused by the gas surrounding a central black hole. In Mrk 231 and Mrk 348, the 15 GHz radio sources are apparently one-sided on 1-milliarsecond scales (less than 1 parsec), despite apparent speeds that are much less than c. The column densities required to account for free-free absorption at 15~GHz on such scales are ~10^{24}~cm^{-2}, similar to the X-ray absorbing columns seen in the same galaxies. Thus, it is likely that multi-frequency VLBA imaging probes the conditions in the nuclear tori that ultimately feeds the supermassive black holes in Seyfert galaxies.

Urbanik, Marek (Jagiellonian Univ.)
Evolution of Interstellar Magnetic Fields in Spiral and Irregular galaxies - Recent VLA Observations
M. Urbanik, M. Soida, K. Chyzy, & R. Beck
Our three-frequency D and C-array VLA study of the flocculent galaxy NGC 4414 revealed a coherent pattern of polarization B-vectors with a substantial radial component. As this galaxy lacks density waves and nonaxisymmetric flows, the radial magnetic field resisting the differential rotation shear is best explained if the dynamo process were the main agent driving the magnetic field evolution.

The "classical" turbulent dynamo requires strong Coriolis forces caused by the disk rotation. However, our VLA study of the very slowly rotating dwarf irregular NGC~4449 revealed surprisingly strong dynamo-like magnetic fields, too. Their explanation needs non-standard dynamos, e.g. driven by Parker instabilities.

VLA Observations of Large-scale Interstellar Magnetized Shocks
M. Urbanik, K. Chyzy, M. Soida, & R. Beck
In the strongly barred spiral NGC 1097 Beck et al. recently detected jumps in the magnetic field direction along the bar. Spiral galaxies without strong bars usually do not show such jumps associated with large-scale shocks. However, our VLA observations at 8.44 and 4.85 GHz of the tidally interacting spiral NGC 3627 reveal a sudden turn of the magnetic field direction shifted by ~1 kpc upstream of the dust lane. This shift challenges classical density wave models. We also observed NGC 4254, a spiral swept by the intracluster gas. In this object we also found shock-like magnetic field jumps accompanied by Faraday rotation gradients, suggesting sharp, three-dimensional bending of the field.

van Zee, Liese (HIA/NRC)
Constraints on BCD Evolution: HI Distribution and Kinematics
L. van Zee, J.J. Salzer, & E.D. Skillman
Blue compact dwarf (BCD) galaxies are small galaxies which are currently undergoing a strong burst of star formation. Given their low luminosity, low metallicity nature, it has been hypothesized that these objects are short lived phenomena, and may be galaxies which undergo episodic bursts of star formation. The ultimate fate of the BCD class has been debated for the last three decades. New results on the gaseous and stellar kinematics of BCDs will be presented which show that BCDs have significant angular momentum, and thus cannot evolve passively into dwarf elliptical galaxies. Rather, BCDs in a quiescent phase must be found as a subset of the general class of dwarf irregular galaxies.

Veilleux, Sylvain (University of Maryland)
New Results from a Superwind Survey of Nearby Galaxies
S. Veilleux, G. Cecil, J. Bland-Hawthorn, & P. Shopbell
Active galactic nuclei and nuclear starbursts may severely disrupt the gas phase of galaxies through deposition of a large amount of mechanical energy in the centers of galaxies. As a result, a large-scale galactic wind ("superwind") that encompasses much of the central regions of these galaxies may be created. Depending upon the extent of the halo and its density and upon the wind's mechanical luminosity and duration, the wind may ultimately blow out through the halo and into the intergalactic medium. The effects of these superwinds may be far-reaching. I will present the most recent results from our multiwavelength survey of spatially resolved superwinds in nearby galaxies, and discuss the implications in the context of on-going surveys of distant galaxies with 8-meter class telescopes.

Verdes-Montenegro, Lourdes (Instituto de Astrofisica de Andalucia)
What does HI tell us about compact groups of galaxies?
L. Verdes-Montenegro
I will present a global analysis of HI properties for 16 compact groups of galaxies based on our new VLA observations of a dozen Hickson Compact Groups along with the published data from the literature. Morphological and kinematical information is used to examine the status of tidal interactions within the groups, and the atomic gas content and other global properties are examined in order to evaluate the impact of tidal interactions on the evolution of the individual galaxies.

Verdoes Kleijn, Gijs (Leiden Observatory)
Gas and Dust in the Nuclei of Nearby Radio-Loud Ellipticals
Gijs Verdoes Kleijn, Tim de Zeeuw, & Stefi Baum
In a complete sample of 21 nearby Fanaroff & Riley Type I (FR I) galaxies we detected 11 small (~1'') disks, 7 lanes of dust and emission-gas and 13 nuclear point sources using HST/WFPC2. We discuss the various possible origins for the dust and gas. Correlations between the central ISM, nuclear point sources and the properties of the radio-jets are interpreted. We present preliminary results from our on-going HST/STIS survey to determine black hole masses. Finally, we discuss the implication of these results for the relation between FR I and normal galaxies.

Verheijen, Marc (NRAO/Socorro)
Ursa Major: A Cluster in Formation at Zero Redshift?
Marc Verheijen
The spiral rich Ursa Major cluster of galaxies consists of a gravitationally bound overdensity of about 90 identified cluster members. It classifies as an Abell cluster of the poorest richness class and the current morphological mix of galaxies resembles that of low density regions. A Westerbork survey of pointed HI observations and a blind HI survey with the VLA reveal that many systems are involved in weak and strong interactions. Notably, the brighest lenticulars in Ursa Major are located in groups within the cluster. The extensive HI filaments associated with these lenticulars can be interpreted as the smoking guns of lenticular formation through the tidally induced mutation of normal spirals. Future merging of the galaxies in these groups may finally lead to the formation of ellipticals that are usually found in high density regions.

The Low-Mass End of the HI Mass Function in Ursa Major
Marc Verheijen
The Ursa Major cluster, at a distance of 18.5 Mpc, was surveyed blindly by both the VLA and the CFHT. The VLA observations comprised 54 pointings, surveying roughly 15% of the cluster volume with a 6-sigma HI mass limit of 5x10^6 Msun at a velocity resolution of 20 km/s. In HI, only 10 new dwarf galaxies were detected above the 6-sigma limit. Applying the appropriate volume corrections shows that the HIMF in UMa is flat and possibly falling at the low-mass end. All HI detections have obvious optical counterparts in the wide-field CFHT images on which dwarfs with M(R)=-11 can be identified. The optically selected candidates not detected in HI are mostly of the dwarf spheroidal type.

Mapping the z=0.19 HI Absorption Against PKS1830-211
Marc Verheijen
The HI absorption feature in PKS1830-211 at z=0.19 is observed with the VLA in conjunction with the VLBA antenna at Pie Town (PT). The PT antenna was linked in real time to the VLA correlator through a new experimental state-of-the-art fiber optics link while the observations at 1191MHz accessed the lowest frequencies available in the 20cm band. The VLA-PT baselines allowed us to separate the two bright radio images in the Einstein ring at 1191 MHz, spatially resolving the HI absorption feature.

We find similar absorption profiles against the two components, separated by 42 km/s. Each profile has an optical depth of tau=0.02 which corresponds to an HI column density of 1.2x10^{20} atoms/cm^2 for a spin temperature of 100K. Deconvolved velocity dispersions are 12 km/s for the NE and 7 km/s for the SW component. These properties are typical for a normal galactic HI disk.

Besides a red galaxy near the center of the Einstein ring, a recent HST-NICMOS image by Lehar et al (1999) also shows a bluer galaxy (G2) at 6.2 kpc to the south (H0=75 km/s/Mpc, q0=0.5, z=0.19). We identify G2 as the z=0.19 absorber since two other candidate galaxies in the HST image have impact parameters of 32 and 38 kpc at z=0.19. The NIR morphology of G2 suggests a near face-on late type galaxy which implies that the 42 km/s velocity difference hints at a rather massive system.

Vogt, Nicole (Institute of Astronomy)
Structure of Intermediate-Redshift Spirals
Nicole Vogt
We review the status of current observations of the fundamental parameters of intermediate redshift (0.3 < z < 1.3) galaxies. Recent advances in instrumentation of 8-10m class telescopes have made possible detailed measurements of galaxy kinematics and mass, in both the optical and the infrared passbands. By studying such well known star formation indicators as [OII] 3727A (in the optical) and H-alpha (redshifted to the infrared), the internal velocity structure of galaxies can be traced through this entire redshift regime. The combination of throughput and optimum seeing conditions yields spectra which can be combined with high resolution multiband imaging to explore the evolution of galaxies of various morphologies, and to place constraints on current models of galaxy formation.

Vollmer, Bernd (Obs. de Paris, Meudon)
Gasdynamics of Cluster Spiral Galaxies: Comparison between observations and simulations
Bernd Vollmer
I present a numerical N-body model to simulate the neutral gas component of a spiral galaxy entering a cluster. As the galaxy falls into the potential well of the cluster the gas clouds are affected by the ram pressure due to the hot intracluster medium whose density increases with decreasing distance to the cluster center. The clouds can even be pushed out of the galaxy (ram pressure stripping).

We studied this effect in the Virgo cluster with the help of the N-body model and a dynamical model of the galaxy trajectories within the cluster. We show that the simulations can reproduce nicely the observed HI properties of the spiral galaxies. We thus can retrace the histories of a considerable number of Virgo spirals. The influence of ram pressure stripping on star formation is discussed. Furthermore, I will present multi wavelength observations of two peculiar cluster spiral galaxies.

Weiner, Ben (Carnegie Observatories)
Distance Constraints for High-Velocity Clouds from Optical Emission
Benjamin J. Weiner, Stuart N. Vogel, & T.B. Williams
We report results from a survey of $\sim 20$ high velocity clouds for faint diffuse optical emission lines in H-alpha and [N II], using a Fabry-Perot at the Las Campanas 2.5-m. A few small complexes are "bright," with H-alpha emission from 100--400 milli-Rayleighs and high [N II]/H-alpha. Several HVCs are very faint in H-alpha, with brightnesses from 20 to 50 mR. A simple model for the ionizing flux emergent from the Galaxy, normalized by the large Northern "bright" HVCs that have known distances and H-alpha fluxes (80-200 mR), yields distances for the faint HVCs of 15 to ~60 kpc, in the outer Galactic halo. However, if H-alpha can be produced by other mechanisms (as seems to happen in the Magellanic Stream), these HVC distances could be lower limits. Independent of the mechanism, the faint HVCs must be at least in the outer halo, too far away to be produced by a Galactic fountain, and some have impressive H I masses.

Weistrop, Donna (Univ. of Nevada, Las Vegas)
HST Observations of Star Formation in Interacting Galaxies: NGC 4194, the "Medusa"
D. Weistrop, D. Eggers, C.H. Nelson, & M.E. Kaiser
NGC 4194 (Mrk 201), also known as the `Medusa'' galaxy, is a blue compact galaxy with strong far infrared and radio emission. Ground-based visible wavelength observations exhibit a distorted image with a tidal tail and regions of strong star formation. A population of massive O and early B stars is evident from the IUE spectra (Kinney, et al. 1993). Broadband red HST images exhibit lumpy HII regions and knots (Malkan et al. 1998). CO emission is observed within 15'' of the center (Casoli et al. 1992, Elfhag, et al. 1996). The central starburst is apparently produced by a galaxy merger.

As part of an investigation of star formation in interacting galaxies, the Space Telescope Imaging Spectrograph on HST was used to observe NGC 4194 in ultraviolet and visible wavelengths. Ultraviolet images were obtained in two wavebands (\lambda_c~1595\AA, FWHM ~206\AA,~ and \lambda_c~2010\AA, FWHM~681\AA). One visible band was observed (\lambda_c~7230 \AA, FWHM ~2000\AA). The data were reduced and analyzed using standard IRAF and STSDAS packages as well as customized software routines.

We have identified individual star forming knots at HST resolution, approximately 0.05 arcsec in the ultraviolet and 0.10 arcsec in the visible. We present sizes and luminosities for the knots, and the knot luminosity function. Current starburst models are compared with our data to constrain stellar ages and populations. Knot characteristics as a function of location in the galaxy will be discussed.

This work has been supported in part by NASA, under contract NAS5-31231.

Welch, Gary (Saint Mary's University)
The Interstellar Medium of M32
G.A. Welch and L.J. Sage
The elliptical galaxy M32 is one of several small satellites of the giant spiral M31, first-ranked member of the Local Group. We have used the VLA to search for atomic hydrogen in M32, thereby complementing our earlier attempt to detect molecular gas in this galaxy. Wispy foreground emission, primarily at -33 and -50 km/s, extends across the primary beam. Curiously, a relatively compact, isolated feature at -74 km/s lies on top of the optical image of M32. Given the latitude of -22 degrees this is likely to be a small, nearby HI cloud. No emission is seen near the -205 km/s optical velocity of M32, implying a 1-sigma upper limit of 2400 solar masses of atomic hydrogen near this velocity. M32 contains no more than about 8500 solar masses of cool gas. We discuss the implications of this result for the recent evolutionary history of M32.

Wong, Tony (UC/Berkeley)
Radial Gas Flows in Spiral Galaxies
Tony Wong and Leo Blitz
The CO and HI velocity fields of 7 nearby spiral galaxies are presented, based on new BIMA observations at 6" resolution and previously published VLA observations at 15"-20" resolution. The goal of this project is to detect or put constraints on radial gas flows that may have a significant impact on the evolution of spiral galaxies. Simple models of axisymmetric radial inflow and gas streaming in elliptical orbits are compared with the data.

Wootten, Al (NRAO/Charlottesville)
The Atacama Large Millimeter Array: Imaging Gas and Dust in Distant Galaxies
Alwyn Wootten
The Atacama Large Millimeter Array (ALMA), a project of the National Radio Astronomy Observatory and the European Southern Observatory, will be built over the coming decade in Northern Chile. ALMA will be a revolutionary telescope, operating at millimeter and submillimeter wavelengths and comprised of an array of individual antennas each 12 meters in diameter that work together to make precision images of astronomical objects. The goal of the ALMA Project is an array of 64 antennas that can be positioned as needed over an area 12 kilometers in diameter so as to give the array a zoom-lens capability. ALMA will image the universe with unprecedented sensitivity and sharpness at millimeter and submillimeter wavelengths. The energy density of radiation from both the Milky Way and from the diffuse extragalactic background peaks in the submillimeter. Aside from Cosmic Microwave Background photons, submillimeter photons are the most abundant photons in the Universe. Detailed imaging at these wavelengths will be a major step for astronomy, making it possible to study the origins of galaxies, and the state of gas and dust in those forming and interacting distant galaxies.

Xu, Cong (IPAC)
Molecular, Cosmic Ray and Ionized Gases Associated with the IGM Starburst in Stephan's Quintet
Cong Xu, Yu Gao, Nanyao Lu, & Jim Condon
New observations of the recently discovered intra-group medium (IGM) starburst in Stephan's Quintet (Xu, Sulentic, and Tuffs, 1999, ApJ 512, 178) are presented. These include the high resolution (~6'') BIMA observations of the CO emission of the molecular gas, new VLA radio continuum observations (~5'', both the intensity and the polarization) which can constrain the magnetic field and the Cosmic-Ray electrons associated with the starburst, and new optical spectroscopic observations of emission lines of the ionized gas using the Palomar 200'' telescope. These observations will shed new light on the mechanism of this unique starburst.

Yan, Lin (The Carnegie Observatories)
Faint Galaxies at z~1-2
Lin Yan
I will present the results of the NICMOS/HST parallel grism and imaging surveys. We detect a sample of H-alpha emisson line galaxies selected in the near-IR band from the NICMOS grism data. This sample allows us to derive the H-alpha luminosity function and global star formation density at z~1.3. Comparison with the SFR measured using the stellar UV continuum, we can infer the dust extinction in distant faint galaxies. In addition, I will present a large survey of EROs, with a specific emphasis on the deep 20cm VLA obsevations. Selecting faint optical-IR galaxies detected in the micro-Jy 20cm maps could be an efficient method of discovering distant sub-mm sources.

Young, Lisa (NMT)
Star Formation and the Interstellar Medium in Dwarf Galaxies
L.M. Young, L. van Zee, R.C. Dohm-Palmer, & K.Y. Lo
Color-magnitude diagrams of the stars within the nearest dwarf galaxies are giving detailed information about the star formation histories of these galaxies, and even about spatial variations in the star formation history. In other words, the color-magnitude diagrams are telling us how these dwarf galaxies have evolved; however, they don't tell us why things happened they way they did. To understand that, we must look at the properties of the interstellar medium. We argue that high spatial and spectral resolution observations of the atomic interstellar medium in some nearby dwarfs can tell us about the physical properties and the phase structure of the interstellar medium. We put this information together with the spatially resolved star formation history to understand what conditions are necessary for star formation in the dwarfs and why star formation starts and stops.

Molecular Gas in Elliptical Galaxies
L.M. Young
In recent years it has become clear that a significant number of elliptical galaxies contain molecular gas. The properties of this molecular gas offer vital clues to the evolution of ellipticals. For example, the gas distribution and kinematics help to reveal the origin of the gas and its fate--- whether it was acquired in a merger or interaction, and whether it will be stripped by an intracluster medium, among other possibilities. Also, because they contain molecular gas, ellipticals may be forming stars at the present time. The molecular gas distribution can be compared with radio continuum and FIR emission to give a better understanding of star formation in ellipticals. This poster presents preliminary results of a program to map the CO distribution and kinematics in a modest sample of ellipticals. The sample galaxies are located in a variety of environments; some have known AGN, and some do not. We use the molecular gas to probe the evolution of elliptical galaxies as a function of their environment and their properties.

Yun, Min S. (NRAO/Socorro)
Radio Study of the ISO Lockman Hole Survey Sources
M. Yun, D. Sanders, K. Kawara, Y. Taniguchi, & H. Okuda
One of the most exciting discoveries made by the Infrared Space Observatory (ISO) is the detection of a substantial excess in the number density of faint far-infrared (90micron and 175micron) sources, suggesting strong cosmological evolution in the population of luminous infrared galaxies. The number density of 175micron sources above the 150mJy level, derived from two deep fields (each 44x44arcmin) in the Lockman Hole region, is nearly 10 times larger than predicted from no evolution models. Several lines of reasoning suggest that the majority of the faint ISOPHOT sources may be at modest redshift (z<=1), and the ISOPHOT sources appear to bridge the gap between the starburst galaxies in the local universe and z>=e2 obscured starbursts detected by the SCUBA surveys. Therefore the study of these ISOPHOT sources may offer the best chance to understand the more distant (and optically faint) submm galaxy phenomenon, which is thought to dominate the cosmic star formation activity in the early epochs.

We have carried out deep, high resolution 1.4 GHz observations of the ISO survey fields using the VLA as part of a multi-wavelength follow-up study. In concert with the deep optical and near infrared (B,V,R,I,K) imaging, individual ISO sources are identified using the high resolution radio images. Spectroscopic study to determine the redshift distribution and the nature of the powering source is also currently underway. As Carilli & Yun (1999) have demonstrated, the radio-to-submillimeter flux ratio is a sensitive function of redshift, and the detection of radio continuum emission alone can immediately offer a handle on the redshift distribution. Combined with the ongoing SCUBA observations to determine their submm SED, photometric redshifts may be obtained even for sources that are too obscured to allow any optical spectroscopy. We are also investigating the nature of a radio selected galaxy sample using the same multi-wavelength database.

Zhao, Jun-Hui (CFA)
A High Velocity Ionized Component in the Nuclear Region of NGC 253
Jun-Hui Zhao, W.M. Goss, J. Ulvestad, & K.R. Anantharamaiah
We report on new VLA H53-alpha line observations at 7 mm of the nuclear region of the starburst galaxy NGC 253 with an angular resolution of 1''x0.5'' (PA=0 degrees) and a velocity resolution of 44 km/sec. At a distance of 3 Mpc, 1'' corresponds to ~15 pc. Fig. 1 shows the H53-alpha line distribution in the nuclear region of NGC 253 (90 pc x 90 pc in linear extent). The ratio of line-to-continuum varies from 40 percent at the position SP-4 to 20 percent at the position (SP-0) of the strongest free-free emitting source (Ulvestad and Antonucci 1997). A H53-alpha component with a large peculiar velocity (400 km/sec) is detected at the position 0.3'' NE to the continuum peak. This velocity is offset by 200 km/sec from the systematic velocity. The large velocity offset suggests that a total mass of 4x10^7 M_sun is included within a radius of 4.5 pc, assuming this component is bounded by the nuclear gravity. The mass density of 6x10^5 M_sin/pc^3 is inferred. The presence of this component is confirmed in both the H66-alpha line observations with the VLA and an early SEST observation of H41-alpha at 3mm. Based on these observations, we infer that the 7 mm recombination line of the peculiar velocity component arises from a high density population of HII regions with an average density n_e>1x10^5 cm^{-3}. In addition, the velocity contours of the H53-alpha line (see Fig. 1) clearly shows a counterrotation in the nucleus, confirming the earlier H92-alpha result of Anantharamaiah & Goss (1996).

Ziegler, B.L. (University Goettingen)
Galaxy Transformation in Poor Clusters at z=0.25
B.L. Ziegler, R.L. Davies, R.G. Bower, I.R. Smail, & M.L. Balogh
We have undertaken a large-scale observational campaign to study galaxy evolution in clusters at 0.2<z<0.3 selected to have low X-ray luminosity (<0.8e+44erg/s). Optical (BVRI) and near-infrared (K') photometry was obtained with the Hale and Calar Alto 2.2m telescope to select galaxies for spectroscopy. Spectra of more than 400 galaxies in the 8 clusters were gained with the Calar Alto 3.5m telescope and the WHT to assess membership and classify galaxies according to starformation activity. HST WFPC2 images of the 8 clusters are scheduled between January and July 2000, as well as XMM observations of 3 of the clusters. All gathered information on galaxy evolution in these low density environments will be compared to rich clusters at the same epoch, which will let us study the role of different mechanisms like ram-pressure stripping, merging and harassment suggested to explain the observed galaxy transformation in clusters.

Zwaan, Martin (Kapteyn Astronomical Institute)
Deep HI Imaging in Galaxy Clusters at z=0.2
M.A. Zwaan, P.G. van Dokkum, M.A.W. Verheijen, & F. Briggs
We present results of HI surveys of galaxy clusters Abell 1689 and 2218 at z~0.2 with the VLA and the WSRT. Current theories require clusters to accrete a substantial number of gas rich galaxies over the last few Gyrs, but the processes governing the fate of the gas in the infalling galaxies are still unclear. The observations help to establish how the evolution of gas disks determines the morphological content of clusters at z=0. Large multicolor HST mosaics are available for both clusters, for Abell 2218 the mosaic consists of 22 pointings. The WSRT survey took ~400 hours to complete and is currently the deepest HI survey at z~0.2.