Where Do Cooling Flows Cool?

Abstract

Typically, ~5% of the total baryonic mass in luminous elliptical galaxies is in the form of cooled interstellar gas. Although the mass contributed by cooled gas is small relative to the mass of the old stellar system in these galaxies, it is almost certainly concentrated within the optical effective radius where it can influence the local dynamical mass. However, the mass of cooled gas cannot be confined to very small galactic radii (r ≲ 0.01re) since its mass would greatly exceed that of known central mass concentrations in giant ellipticals, normally attributed to massive black holes. We explore the proposition that a population of very low mass, optically dark stars is created from the cooled gas. For a wide variety of assumed radial distributions for the interstellar cooling, we find that the mass of cooled gas contributes significantly (~30%) to stellar dynamical mass-to-light ratios which, as a result, are expected to vary with galactic radius. However, if the stars formed from cooled interstellar gas are optically luminous, their perturbation on the mass-to-light ratio of the old stellar population may be reduced. Cooling mass dropout also perturbs the local apparent X-ray surface brightness distribution, often in a positive sense for centrally concentrated cooling. In general, the computed X-ray surface brightness exceeds observed values within re, suggesting the presence of additional support by magnetic stresses or nonthermal pressure. The mass of cooled gas inside re is sensitive to the rate at which old stars lose mass *, but this rate is nearly independent of the initial mass function of the old stellar population.