A photoionization model for the optical line emission from cooling flows

Abstract

We present the detailed predictions of a photoionization model previously outlined in Voit and Donahue to explain the optical line emission associated with cooling flows in X-ray emitting clusters of galaxies. In this model, EUV/soft X-ray radiation from condensing gas photoionizes clouds that have already cooled. We discuss the energetics and specific consequences of such a model, as compared to other models put forth in the literature. We also discuss the consequences of magnetic fields and cloud-cloud shielding. Our results illustrate how varying the individual column densities of the ionized clouds can reproduce the range of line ratios observed and strongly suggest that the emission-line nebulae are self-irradiated condensing regions at the centers of cooling flows.