Non-equilibirum ionization and cooling of metal-enriched gas in the presence of a photoionization background

Simulations of the formation of galaxies, as well as ionization models used to interpret observations of quasar absorption lines, generally either assume ionization equilibrium or ignore the presence of the extragalactic background (EGB) radiation. We introduce a method to compute the non-equilibrium ionization and cooling of diffuse gas exposed to the EGB. Our method iterates the ionization states of the 11 elements that dominate the cooling (H, He, C, N, O, Ne, Si, Mg, S, Ca and Fe) and uses tabulated ion-by-ion cooling and photo-heating efficiencies to update the temperature of the gas. Our reaction network includes radiative and di-electric recombination, collisional ionization, photoionization, Auger ionization and charge transfer. We verify that our method reproduces published results for collisional equilibrium, collisional non-equilibrium and photoionized equilibrium. Non-equilibrium effects can become very important in cooling gas, particularly below 10(6) K. Photoionization and non-equilibrium effects both tend to boost the degree of ionization and to reduce cooling efficiencies. The effect of the EGB is larger for lower densities (i.e. higher ionization parameters). Hence, photoionization affects (equilibrium and non-equilibrium) cooling more under isochoric than under isobaric conditions. Non-equilibrium effects are smaller in the presence of the EGB and are thus overestimated when using collisional-only processes. The inclusion of the EGB alters the observational diagnostics of diffuse, metal-enriched gas (e.g. metal absorption lines probed in quasar sight lines) even more significantly than the cooling efficiencies. We argue that the cooling efficiency should be considered if ionization models are used to infer physical conditions from observed line ratios, as the a priori probability of observing gas is lower if its cooling time is shorter. We provide online tables of ionization fractions and cooling efficiencies, as well as other data, for equilibrium and non-equilibrium scenarios, and both with and without an EGB. Cooling efficiencies and diagnostics of the physical state of diffuse gas can become highly inaccurate if ionization equilibrium is assumed or if the existence of the ionizing background is ignored.