Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies
Alison Crocker
University of Oxford
Molecular gas and star formation in early-type galaxies
Elliptical and lenticular galaxies have long been thought to be 'red and dead' stellar systems without ongoing star formation or cold gas. Varied observations now firmly indicate that this is not true - E/S0s have both cold gas (HI, H$_{2}$, dust) and recent star formation (detected in UV, absorption linestrengths and ionized emission lines). However, little is yet known about the details of star formation in these galaxies. Having obtained both molecular gas maps (using mm interferometers) and optical integral field unit (IFU) data that give spatially resolved information on the stellar populations, stellar kinematics and ionised gas, we are now at a stage where the process of star formation in E/S0 galaxies may be studied. We find ionised gas always present where there is molecular gas (with shared kinematics), however star formation is surprisingly not always the cause of this ionisation. Moreover, while most E/S0s with molecular gas host young stellar populations, about a third do not according to both absorption linestrength analysis and UV-optical colours. Based on all these observations, a very tentative evolutionary sequence can be made, with pre-star formation, star-forming and dwindling-star formation categories. Comparing the E/S0s with molecular gas to the star-forming correlations between the FIR, radio and H$\beta$ emission, we find that the E/S0s are H$\beta$ deficient and FIR-strong. Despite this FIR-strength, the FIR is well correlated with the mass of molecular gas detected and we find that using the standard FIR-SFR conversion, our galaxies are consistent with both a constant star formation efficiency and/or a Schmidt-Kennicutt law. Further investigations into the physical conditions of the molecular gas in E/S0s are underway with observations in HCN, 13CO and HCO$^{+}$. Preliminary results show different 13CO/12CO and HCN/HCO$^{+}$ ratios than observed in spiral, starburst or Seyfert galaxies, indicating a difference in the state or chemistry of the molecular gas.
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