Stress Testing Empirical and Theoretical Models of Molecule Formation using Cold Gas Rich Early-Type Galaxies

We present an update on work which aims to use high resolution observations of CO (CARMA) and HI (JVLA) in early-type galaxies to stress test existing empirical and first principle physical models of molecule formation. These existing models are being used as input into semi-analytic models of galaxy evolution. A good model should have applicability and predictive power beyond the ranges of conditions that are found in nearby spirals. Compared to disk galaxies, early-type galaxies have different gas contents, stellar populations, and evolutionary histories. Early-type galaxies also have a much wider range of HI/H2 mass ratios than spirals. Additionally, early-types’ central stellar densities are typically larger than those observed in disk galaxies, which translates to larger central pressures. For all of these reasons, the early-type galaxies provide complementary tests of HI/H2 physics to that done for disk galaxies. Our previous work, using low resolution HI data (15”) for a small sample of 11 CO-rich early-type galaxies, indicates that an empirical relationship between the surface density ratio of molecular hydrogen to atomic hydrogen (Rmol) and the hydrostatic midplane pressure (HSMP) exists for early-type galaxies, but has a much more shallow slope than that observed for disk galaxies. We also found that the first principle physical models tend to over-predict the molecular fraction in the inner regions of our sample galaxies. The reliability of these initial results is limited by low number statistics, insufficient spatial resolution, and sample bias toward CO-rich objects. To remedy this, we have increased our sample to 32 early-type galaxies, broadened the parameter space to include galaxies rich in HI but deficient in CO, and are in the process of collecting new HI maps at 5” for all targets. We present examples of this recent high resolution imaging. We also highlight the effect that spatial resolution has on the derived Rmol-HSMP empirical relation as well as compare the molecular fraction derived from this new data with the predictions from the first principle physical models.