The evolution of early-type galaxies

We use the Tully-Fisher relation (TFR) to compare the behavior of S0 and late-type spiral galaxies. We determine circular velocities based on stellar kinematics derived from stellar absorption line spectroscopy for ten S0s in the Coma Cluster and eight S0s in the Virgo Cluster. We combine these results with similar measurements of thirteen Coma S0 galaxies obtained previously. We find that there is only a small offset, Δm(H) ∼ 0.2, in the H-band luminosity at a given circular velocity, υc ∼ 200 km s⁻¹, between late-type spirals and the S0 data presented here. This result implies a similar total H -band mass-to-light ratio (within an effective radius) among disk galaxies of different Hubble type. As the older stellar population in S0s is dimmer, this suggests a somewhat larger fraction of stellar mass in these S0s than in late-type spirals. These results do not seem to agree with the proposal that star formation in S0s was switched off suddenly a few Gigayears ago due to outside mechanisms and that a large ∼2 mag offset is expected. We also find that the relation between (I- and H-band) luminosity and upsilonc for the S0 galaxies is at best poorly defined and has a scatter of ∼1 mag, significantly larger than the Tully-Fisher relation (TFR) for late-type spirals in clusters, where the observed I- and H-band scatter is σ ∼ 0.3 mag. This substantial scatter is similar to that found in a study of 18 nearby S0 galaxies in the field (Neistein et al. 1999) where σ(I) ∼ 0.7 mag, implying that no tight TFR holds for field S0 galaxies. Our scatter is also much larger than that found by Mathieu et al. (2002) (σ(I) ∼ 0.3 mag) for six nearby S0s. Our results suggest that differing formation histories can lead to S0s with diverse properties and that S0s are more likely to be the outcomes of minor mergers or some "pre-processing" in groups of galaxies falling into clusters, rather than simply late-type spirals that have been stripped of their gas but are kinematically preserved. We suggest that it is likely that many mechanisms, such as slow encounters, tidal interactions, and gas stripping, may have occurred in the lifetimes of the galaxies and produced the heterogeneous class of S0s that we observe today. We also present a study of the dynamics and content of a sample of ten nearby ultraluminous infrared galaxies (ULIRGs) based on 2.3 μm CO absorption line spectroscopy and near-infrared H- and K s-band imaging. Using velocity dispersions derived from the spectroscopy, disk scale-lengths obtained from the imaging, and a set of likely model density profiles, dynamical masses for each ULIRG are calculated and compared to molecular gas mass estimates derived from millimeter interferometric observations and from a standard conversion between ¹²CO emission and H₂ mass. For a majority of the ULIRGs in the sample, we cannot reconcile the large amounts of nuclear molecular gas mass predicted by the standard conversion with our estimates of dynamical masses for the galaxies. Indeed, for several of the galaxies, the calculated molecular gas mass exceeds or completely fills the total dynamical mass budget for the system. Molecular gas masses are found to be up to ten times the dynamical masses. (Abstract shortened by UMI.