Molecular Hydrogen in Galaxy Simulations

Abstract

Contemporary galaxy simulations are currently capable of resolving the dense molecular phase of the interstellar medium. The behaviour of this molecular gas is complicated by the ability for molecular hydrogen to self-shield, protecting the deeper layers of clouds from dissociating radiation that would otherwise break the molecules into constituent parts and heat the gas. An accurate model of molecular hydrogen needs to couple to the local radiation field of the galaxy while also accounting for the effects of self-shielding. I present a self-consistent chemical network to model the formation and destruction of molecular hydrogen and related primordial gas species (H, He, and their ions). The model is designed to couple to a realistic UV radiation field modeled using discrete bands. It is intended for use in the GASOLINE N-body hydrodynamics code for galaxy simulations alongside the TREVR/TREVR2 ray-tracing radiative transfer routines. When combined with these routines, my model offers a correct treatment for shielding that allows for radiation from multiple sources to be shielded independently. I include several tests to ensure the fidelity of this model, including simulated HII regions, photodissociation regions, and the evolution of primordial gas prior to galaxy formation. This model is applicable in the simulation of a realistic interstellar medium in isolated disk galaxies and the evolution of dwarf galaxies. A proper model for molecular hydrogen with radiation in a galaxy enables simulations to produce observable quantities that can be used to evaluate the quality of our simulated galaxies. This model will provide opportunities to explore the connection between molecular hydrogen and models of star formation.