2012MNRAS.425.3058C

Motivated by the observed connection between molecular hydrogen (H₂) and star formation, we present a method for tracking the non-equilibrium abundance and cooling processes of H₂ and H₂-based star formation in smoothed particle hydrodynamic simulations. The local abundances of H₂ are calculated by integrating over the hydrogen chemical network. This calculation includes the gas phase and dust grain formation of H₂, shielding of H₂ and photodissociation of H₂ by Lyman-Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low-metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H₂ abundance. This link limits star formation to 'star-forming regions', represented by areas with abundant H₂. We use simulations of isolated disc galaxies to verify that the transition from atomic to molecular hydrogen occurs at realistic densities and surface densities. Using these same isolated galaxies, we establish that gas particles of 10⁴ M_☉ or less are necessary to follow the molecular gas in this implementation.