Photoionization of Intergalactic Gas and Cooling Flows in Clusters of Galaxies

This dissertation examines the effect of photoionization on the gas left over from galaxy formation, namely, intergalactic and intracluster gas. We examine the environmental impact of quasars on the intergalactic medium (IGM) and derive analytic and numerical results for ionization-front expansion as a function of redshift z in an expanding Friedmann cosmology. We find that in order for the IGM to be ionized by quasars, the closure parameter of intergalactic gas, Omega _I <.03. We present new photoionization models (using updated Opacity Project photoionization cross sections for C, N, O, and Si) of optically thin, low-density intergalactic gas clouds at constant pressure, photoionized by QSOs. We model all ion stages of H, He, C, N, O, Si and iron, and predict the column density ratios of clouds at specified values of ionization parameter U = n_gamma /n_H and metallicity. We also examine whether H_2 cooling or photoelectron escape can explain recent line-width observations by Pettini et al. (1990) that suggest low temperatures (T < 10^4 K) in some Lyalpha clouds. In order to discover cooling gas in the dominant galaxy in X-ray emitting clusters of galaxies, we have optically observed a complete, X-ray selected sample of 23 distant clusters of galaxies extracted from the Einstein Extended Medium Sensitivity Survey. We find luminous ( >=10⁴¹ erg s^{-1}) extended H_alpha emission in 11 of these clusters. Since this cooling gas is a signature of a massive "cooling flow," we have discovered new cooling flow clusters in a distant sample of previously unknown clusters. We present the detailed predictions of a photoionization model introduced by Voit and Donahue (1990) explaining the source of optical line emission associated with cooling flows in clusters of galaxies. We discuss the energetics and consequences of such a model, as compared to other models put forth in the literature. The individual column densities of the clouds may generate the range of line ratios observed in cooling flows.