The effects of metallicity, UV radiation and non-equilibrium chemistry in high-resolution simulations of galaxies

Abstract

We present a series of hydrodynamic simulations of isolated galaxies with stellar mass of $10^{9} \, \rm{M}_{\odot}$. The models use a resolution of $750 \, \rm{M}_{\odot}$ per particle and include a treatment for the full non-equilibrium chemical evolution of ions and molecules (157 species in total), along with gas cooling rates computed self-consistently using the non-equilibrium abundances. We compare these to simulations evolved using cooling rates calculated assuming chemical (including ionisation) equilibrium, and we consider a wide range of metallicities and UV radiation fields, including a local prescription for self-shielding by gas and dust. We find higher star formation rates and stronger outflows at higher metallicity and for weaker radiation fields, as gas can more easily cool to a cold (few hundred Kelvin) star forming phase under such conditions. Contrary to variations in the metallicity and the radiation field, non-equilibrium chemistry generally has no strong effect on the total star formation rates or outflow properties. However, it is important for modelling molecular outflows. For example, the mass of H$_{2}$ outflowing with velocities $> 50 \, \rm{km} \, \rm{s}^{-1}$ is enhanced by a factor $\sim 20$ in non-equilibrium. We also compute the observable line emission from CII and CO. Both are stronger at higher metallicity, while CII and CO emission are higher for stronger and weaker radiation fields respectively. We find that CII is generally unaffected by non-equilibrium chemistry. However, emission from CO varies by a factor of $\sim 2 - 4$. This has implications for the mean $X_{\rm{CO}}$ conversion factor between CO emission and H$_{2}$ column density, which we find is lowered by up to a factor $\sim 2.3$ in non-equilibrium, and for the fraction of CO-dark molecular gas.