Two Clusters of Galaxies with Radio-quiet Cooling Cores

Radio lobes inflated by active galactic nuclei at the centers of clusters area promising candidate for halting condensation in clusters with short centralcooling times because they are common in such clusters. In order to test theAGN-heating hypothesis, we obtained Chandra observations of two clusters withshort central cooling times yet no evidence for AGN activity: Abell 1650 andAbell 2244. The cores of these clusters indeed appear systematically differentfrom cores with more prominent radio emission. They do not have significantcentral temperature gradients, and their central entropy levels are markedlyhigher than in clusters with stronger radio emission, corresponding to centralcooling times ~ 1 Gigayear. Also, there is no evidence for fossil X-raycavities produced by an earlier episode of AGN heating. We suggest that either(1) the central gas has not yet cooled to the point at which feedback isnecessary to prevent it from condensing, possibly because it is conductivelystabilized, or (2) the gas experienced a major heating event $\gtrsim 1$ Gyr inthe past and has not required feedback since then. The fact that these clusterswith no evident feedback have higher central entropy and therefore longercentral cooling times than clusters with obvious AGN feedback strongly suggeststhat AGNs supply the feedback necessary to suppress condensation in clusterswith short central cooling times.Comment: ApJ Letter, in pres