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 sub(2) out of H super(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 sub(2) on dust grains and its destruction by UV irradiation in the CNM phase, including the effects of shielding by dust and H sub(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 6 100 cm super(-3) and as cold as T sub(k) 6 40 K, where most of the H super(I) 1 H sub(2) transition (and star formation) takes place. Our findings include (1) a strong dependence of the resulting H sub(2) gas mass on the ambient metallicity and the adopted H sub(2) formation rate, (2) constraints on the star formation parameters from the effects of stellar feedback on H sub(2) formation, and (3) the possibility of a diffuse H sub(2) gas phase outside star-forming regions. We expect these results to be valid in other types of galaxies for which the H super(I) 1 H sub(2) phase transition is more difficult to resolve by high-resolution numerical studies (e.g., large spirals). Finally, we briefly examine using an H sub(2) fraction threshold as a new, more realistic, star formation criterion for use in galaxy simulations.