Radiative accretion flow onto giant galaxies in clusters

Steady accretion flows toward the center of a cluster of galaxies are modeled with the inclusion of radiation losses by line emission appropriate for standard cosmic abundances. All inertial terms are retained, allowing the flows to become supersonic near the cluster. An upper limit to the accretion rate is determined on the basis of both the computed flows which pass through the sonic point and observed X-ray fluxes from clusters of galaxies. It is found that the calculated steady accretion flows cannot account for the existence of optical emission at large distances from the nucleus, as observed in M87 and NGC 1275, but that the observations can be understood if the inflow is unsteady or thermally unstable. A simple analysis of comoving isobaric thermal instabilities is developed. A possible model for the unresolved X-ray point source at the center of M87 is outlined whereby kinetic energy from infalling gas clouds is converted to X-ray emission as the clouds collide with the central H II region in the galaxy.