New composition dependent cooling and heating curves for galaxy
  evolution simulations
  In this paper, we present a new calculation of composition-dependent
radiative cooling and heating curves of low-density gas, intended primarily for
use in numerical simulations of galaxy formation and evolution. These curves
depend on only five parameters: temperature, density, redshift, [Fe/H], and
[Mg/Fe]. They are easily tabulated and can be efficiently interpolated during a
simulation.
  The ionization equilibrium of 14 key elements is determined for temperatures
between 10K and 10^9K and densities up to 100 amu/cm^3 taking into account
collisional and radiative ionization, by the cosmic UV background and an
interstellar radiation field, and by charge-transfer reactions. These elements,
ranging from H to Ni, are the ones most abundantly produced and/or released by
SNIa, SNII, and intermediate-mass stars. Self-shielding of the gas at high
densities by neutral Hydrogen is taken into account in an approximate way by
exponentially suppressing the H-ionizing part of the cosmic UV background for
HI densities above a threshold density of n_HI,crit=0.007 cm^-3. We discuss how
the ionization equilibrium, and the cooling and heating curves depend on the
physical properties of the gas.
  The main advantage of the work presented here is that, within the confines of
a well-defined chemical evolution model and adopting the ionization equilibrium
approximation, it provides accurate cooling and heating curves for a wide range
of physical and chemical gas properties, including the effects of
self-shielding. The latter is key to resolving the formation of cold, neutral,
high-density clouds suitable for star formation in galaxy simulations.
