Incorporating the Molecular Gas Phase in Galaxy-sized Numerical Simulations: First Applications in Dwarf Galaxies

We present models of the coupled evolution of the gaseous and stellar content of galaxies incorporating the formation of H2 out of H gas. We do so by formulating a subgrid model for gas clouds that uses observed cloud scaling relations and tracks the formation of H2 on dust grains and its destruction by UV irradiation in the CNM phase, including the effects of shielding by dust and H2 self-shielding, as well as its collisional destruction in the WNM phase. We then apply our model to the evolution of a typical quiescent dwarf galaxy. Apart from their importance in galaxy evolution, their small size allows our simulations to track the thermal and dynamic evolution of gas as dense as n ~ 100 cm-3 and as cold as Tk ~ 40 K, where most of the H → H2 transition (and star formation) takes place. Our findings include (1) a strong dependence of the resulting H2 gas mass on the ambient metallicity and the adopted H2 formation rate, (2) constraints on the star formation parameters from the effects of stellar feedback on H2 formation, and (3) the possibility of a diffuse H2 gas phase outside star-forming regions. We expect these results to be valid in other types of galaxies for which the H → H2 phase transition is more difficult to resolve by high-resolution numerical studies (e.g., large spirals). Finally, we briefly examine using an H2 fraction threshold as a new, more realistic, star formation criterion for use in galaxy simulations.