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

We present models of the evolution of the gaseous and stellar content ofgalaxies incorporating the formation of H_2 out of HI gas as part of such amodel. We do so by formulating a subgrid model for gas clouds that useswell-known cloud scaling relations and solves for the HI-H_2 balance set by theH_2 formation on dust grains and its FUV-induced photodissociation by thetemporally and spatially varying interstellar radiation field. This allows theseamless tracking of the evolution of the H_2 gas phase, its precursor ColdNeutral Medium (CNM) HI gas, simultaneously with the star formation. Our mostimportant findings are: a) a significant dependence of the HI-H_2 transitionand the resultant H_2 gas mass on the ambient metallicity and the H_2 formationrate, b) the important influence of the characteristic star formation timescale(regulating the ambient FUV radiation field) on the equilibrium H_2 gas massand c) the possibility of a diffuse H_2 gas phase. Finally, we implement andbriefly explore a novel approach of using the ambient H_2 gas mass fraction asa criterion for the onset of star formation. (abridged)Comment: 46 pages, 14 figures (AASTEX); accepted for Ap