Where Do Cooling Flows Cool?

Abstract

Although only about 5 percent of the total baryonic mass in luminous elliptical galaxies is in the form of cooled interstellar gas, it is concentrated within the optical effective radius r_e where it influences the local dynamical mass. The mass of cooled gas must be spatially distributed since it greatly exceeds the masses of central black holes. We explore here the proposition that a population of low mass, optically dark stars is created from the cooled gas. We consider a wide variety of radial distributions for the interstellar cooling, but only a few are consistent with observed X-ray surface brightness profiles. In a region of concentrated interstellar cooling, the X-ray emission can exceed that observed, suggesting the presence of additional support by magnetic stresses or non-thermal pressure. In general we find that the mass of cooled gas contributes significantly to stellar dynamical mass to light ratios which vary with galactic radius. If the stars formed from cooled interstellar gas are optically luminous, their influence on the the mass to light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to the rate that old stars lose mass, which is nearly independent of the initial mass function of the old stellar population.