Nonequilibrium ionization states and cooling rates of the photoionized enriched gas

Abstract: Nonequilibrium (time-dependent) cooling rates and ionization state calculations are presented for low-density gas enriched with heavy elements (metals) and photoionized by external ultraviolet/X-ray radiation. We consider a wide range of gas densities and metallicities and also two types of external radiation field: a power-law and the extragalactic background spectra. We have found that both cooling efficiencies and ionic composition of enriched photoionized gas depend significantly on the gas metallicity and density, the flux amplitude, and the shape of ionizing radiation spectrum. The cooling rates and ionic composition of gas in nonequilibrium photoionization models differ strongly (by a factor of several) from those in photoequilibrium due to overionization of the ionic states in the nonequilibrium case. The difference is maximal at low values of the ionization parameter and similar in magnitude to that between the equlibrium and nonequilibrium cooling rates in the collisionally controlled gas. In general, the nonequilibrium effects are notable at $T\simlt 10^6$ K. In this temperature range, the mismatch of the ionic states and their ratios between the photoequilibrium and the photo-nonequilibrium models reach a factor of several. The net result is that the time-dependent energy losses due to each chemical element (i.e. the contributions to the total cooling rate) differ singificantly from the photoequilibrium ones. We advocate the use of nonequilibrium cooling rates and ionic states for gas with near-solar (and above) metallicity exposed to an arbitrary ionizing radiation flux. We provide a parameter space (in terms of temperature, density, metallicity and ionizing radiation flux), where the nonequilibrium cooling rates are to be used. (abridged)