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Star formation in the universe's largest galaxies---the ones at the centersof galaxy clusters---depends critically on the thermodynamic state of their hotgaseous atmospheres. Central galaxies with low-entropy, high-densityatmospheres frequently contain multiphase star-forming gas, while those withhigh-entropy, low-density atmospheres never do. The dividing line between thesetwo populations in central entropy, and therefore central cooling time, isamazingly sharp. Two hypotheses have been proposed to explain the dichotomy.One points out that thermal conduction can prevent radiative cooling of clustercores above the dividing line. The other holds that cores below the dividingline are subject to thermal instability that fuels the central AGN through acold-feedback mechanism. Here we explore those hypotheses with an analysis ofthe H-alpha properties of ACCEPT galaxy clusters. We find that the twohypotheses are likely to be complementary. Our results support a picture inwhich cold clouds inevitably precipitate out of cluster cores in which coolingoutcompetes thermal conduction and rain down on the central black hole, causingAGN feedback that stabilizes the cluster core. In particular, the observeddistribution of the cooling-time to freefall-time ratio is nearly identical tothat seen in simulations of this cold-feedback process, implying thatcold-phase accretion, and not Bondi-like accretion of hot-phase gas, isresponsible for the AGN feedback that regulates star formation in largegalaxies.

Cooling Time, Freefall Time, and Precipitation in the Cores of ACCEPT Galaxy Clusters