The effects of local stellar radiation and dust depletion on non-equilibrium interstellar chemistry

Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionizes and heats the gas, and (2) the depletion of metals on to dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M-circle dot per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Way-mass, using the FIRE galaxy formation models together with the CHIMES non-equilibrium chemistry and cooling module. In our fiducial model, we couple the chemistry to the stellar fluxes calculated from star particles using an approximate radiative transfer scheme; and we implement an empirical density-dependent prescription for metal depletion. For comparison, we also run simulations with a spatially uniform radiation field, and without metal depletion. Our fiducial model broadly reproduces observed trends in H I and H 2 mass with stellar mass, and in line luminosity versus star formation rate for [C II](158 mu m), [O I](63 mu m), [ O III](88 mu m), [N II](122 mu m), and H alpha(6563 angstrom). Our simulations with a uniform radiation field predict fainter luminosities, by up to an order of magnitude for [O III] 88 mu m and H a6563A, while ignoring metal depletion increases the luminosity of carbon and oxygen lines by a factor approximate to 2. However, the overall evolution of the galaxy is not strongly affected by local stellar fluxes or metal depletion, except in dwarf galaxies where the inclusion of local fluxes leads to weaker outflows and hence higher gas fractions.