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