HI To H₂ Transition In Early-type Galaxies

Prior studies conducted on disk galaxies provide a nearly linear empirical relationship between the surface density ratio of molecular hydrogen to atomic hydrogen (Rmol) and the hydrostatic midplane pressure (HSMP) of the galaxy. This empirical relationship is currently being used as input into semi-analytic models of galaxy evolution. The goal of this project is to stress-test these disk galaxy models using early-type galaxies; a good model should have applicability and predictive power beyond the ranges of conditions that are found in nearby spirals today. 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-type’s central stellar densities are typically larger than that observed in disk galaxies, which translates to larger central pressures. For all these reasons, early-type galaxies provide complementary tests of HI/H₂ physics to spirals. For this project, cold gas (HI and H₂) surface densities are obtained from interferometric observations from the Very Large Array and the Combined Array for Research in Millimeter-wave Astronomy. The derivation of the HSMP also requires optical maps from the SPITZER space telescope. Preliminary results indicate that the empirical relationship exists for early-types and has a significantly shallower (non-linear) slope, which indicates that the relationship between Rmol and HSMP found from disk galaxies cannot be applied to early-type galaxies. Additionally, obtaining a more accurate empirical value of this relationship broadens our understanding of how gas behaves in galaxies.