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§1 Introduction

Arp's ``Atlas of Peculiar Galaxies'' (Arp 1966) appeared forty years after the discovery that galaxies are independent stellar systems. It was motivated by some of the questions which form central themes of this conference: How are elliptical galaxies related to spirals? How are galaxies formed, and how do they evolve? These questions are just as compelling today as they were in 1966. It is certainly true that optical compilations of the bizarre objects collected by Arp and others (notably, Vorontsov-Vel'yaminov 1959, 1977; Arp & Madore 1987; Malin & Carter 1983) and the detailed studies that they motivated have brought us an appreciation that peculiar galaxies have an importance far in excess of their statistical occurrence in galaxy catalogs, where they comprise anywhere from 3-9% of all objects (Arp & Madore 1975; Struck 1999). Still, thirty-five years later these questions remain largely unanswered.

By the time atlases of peculiar galaxies appeared, there were several catalogs devoted to demonstrating the various forms of galaxy morphology. Most prominent among these is the Hubble Atlas. These catalogs are arranged in sequences based on the symmetry of the optical morphologies, and focus on single galaxies in isolation, with asymmetric forms relegated to catch-all categories and illustrated with but a few token examples. The compendiums of peculiar galaxies, on the other hand, highlight those forms which deviate from symmetry, with the thinking that such objects are transitional forms illustrating the most spectacular phases of galactic evolution, and perhaps formation.

Optical morphology alone provides an interesting yet incomplete view of galaxies. This is particularly true when considering peculiar systems. The starlight in galaxies rarely extends beyond a few optical scale-lengths, well within the dark matter halos that are widely believed to surround galaxies. Any tracer that extends to larger radii will probe the influence of a larger fraction of the mass of galaxies and, because the dynamical time scales with radius, will sample galactic influences further in the past. The 21cm line of neutral hydrogen forms just such a tracer. Since the early days of radio astronomy, it has been known that the neutral gas often extends much further than the optical light (Roberts & Rots 1973). Additionally, hydrogen is the raw material from which stars and therefore galaxies are ultimately formed, so if large reservoirs of it exist at the present time, we might hope to find signatures of on-going formation processes in the neutral gas. No picture of galaxy formation and evolution is complete without knowledge of how the gas is distributed at large radii.

It is somewhat surprising, then, that thirty-five years after the optical compilation of Arp, no such compilation of HI maps exist for peculiar galaxies. This is not for lack of such observations. HI mapping requires an extensive investment in telescope time and post-observation reduction, limiting most studies to a few to a dozen objects in any give observational program. Nonetheless, an impressive amount of data on HI in weird galaxies and weird HI in otherwise normal galaxies has been amassed over the years. By our count, more than 400 peculiar systems have been mapped in HI. This may be far below the number of objects typically assembled in optical catalogs, but it is enough to warrant an attempt to assemble and arrange them in some organized form.

This Gallery is intended to do just that, and we have taken the occasion of this meeting, celebrating the twentieth anniversary of the VLA, to make a first attempt at its compilation. We concentrate on HI maps of peculiar galaxies or peculiar HI in otherwise normal galaxies, rather than attempting to include all Hubble Types. We have further limited ourselves to very disturbed systems, by and large including only a few (or no) examples of systems with less extreme peculiarities. There are already relatively large compilations of the HI morphology of ``normal'' Hubble types, many of which exhibit both minor and major peculiarities, in (motsly Dutch) theses or journal supplements (e.g. Wevers, van der Kruit & Allen 1986; Bosma 1978; Warmels 1988; Cayatte et al. 1990; van Driel & van Woerden 1991; Oosterloo 1988; Broeils 1992; Swaters 1999; Verheijen & Sancisi 2001), and the growing database of the Westerbork HI Survey of Irregular and Spiral Galaxies (WHISP) will eventually contain more than 400 late-type galaxies (van der Hulst, van Albada & Sancisi, these proceedings, p.451). On the other hand, no large compilation of HI in peculiars exits anywhere. This ``Rogues Gallery'' is our attempt to remedy this situation.

This is a very subjective compilation, assembled because one of us noticed the HI map of a given object somewhere and considered it interesting, or had marked the galaxy as an optically interesting object and subsequently searched the literature or telescope archives for existing HI observations. This assemblage is definitely not complete, and may not even be representative of all the forms of peculiar HI morphologies - for that we would need an optically blind HI survey. But one has to start somewhere, and this is where we have decided to start.

Following the practice of Hubble, Sandage, Arp, and Madore, we have divided the Gallery into several morphological classes. These are described in detail in §3. Within each class we have attempted to arrange objects in a suggestive morphological sequence, where the principles behind those sequences are described in §3.1. The general theme is the connection of similar forms, related from a dyamical (as with the class of interacting galaxies) or a morphological viewpoint (as with the class of galaxies with warps and/or asymmetries). Sometimes there is no clear transition between objects, and the category is just a compilation of systems that fall within the broadly defined class (as with the class of galaxies with extended HI envelopes).

The Gallery is laid out as follows. We describe the subjective gathering of the images and the proper way to reference them in §2. In §3 we define the different classes used and explain how objects are ordered in the Gallery. The layout of the images is described in §4, while §5 describes the tables and their entries. The acknowledgements appear in §6, followed by the tables, and finally the Gallery itself. At the end of the Gallery we have collected abstracts for authors who have allowed us to reproduce images prior to their publication in the literature.

next up previous
Next: Sources of Images and Up: An Rogues Gallery Previous: An Rogues Gallery
John Hibbard 2001-10-08