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 H-2 out of H (I) gas. We do so by formulating a subgrid model for gas clouds that uses observed cloud scaling relations and tracks the formation of H-2 on dust grains and its destruction by UV irradiation in the CNM phase, including the effects of shielding by dust and H-2 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 similar to 100 cm(-3) and as cold as T-k similar to 40 K, where most of the H (I) -> H-2 transition (and star formation) takes place. Our findings include (1) a strong dependence of the resulting H-2 gas mass on the ambient metallicity and the adopted H-2 formation rate, (2) constraints on the star formation parameters from the effects of stellar feedback on H-2 formation, and (3) the possibility of a diffuse H-2 gas phase outside star-forming regions. We expect these results to be valid in other types of galaxies for which the H (I) -> H-2 phase transition is more difficult to resolve by high-resolution numerical studies (e. g., large spirals). Finally, we briefly examine using an H-2 fraction threshold as a new, more realistic, star formation criterion for use in galaxy simulations.