The Fate of Matter in Cooling Flows

Abstract

X-ray observations imply that ~100 M_sun_ yr^-1^ has been deposited over the last ~5 Gyr by typical cooling flows in clusters of galaxies on the bright galaxies in their centers. No successful alternative to the standard x-ray interpretation has yet been proposed, and there is a growing body of x-ray and optical evidence consistent with this interpretation. The only viable repository for this material appears to be stars or smaller compact bodies. We consider the detectability of these "accretion populations" through their photometric effects in the 0.3-2 micron region and also in the vacuum ultraviolet. In typical cluster cooling flows, star formation with the Local IMF or Larson bimodal IMF would be detectable even if the cooling rates have been overestimated by factors of 10-500 (independent of the lifetime of the flows). The radial dependence of the mass dropout has a strong influence on observational tests. UBV color anomalies almost certainly produced by accretion populations are present in about 50% of the cases studied. If the cooling rates are accurate, however, the bulk of the inflow in all cases must be deposited by a highly efficient mechanism in objects with (m) <<0.7 M_sun_. Unfortunately, it will be very difficult to verify low-mass star formation directly. NGC 1275 remains the best candidate for a system with a verifiable high-mass truncation of the IMF.