An entropy threshold for strong Hα and radio emission in the cores of galaxy clusters

Our Chandra X-Ray Observatory archival study of intracluster entropy in a sample of 222 galaxy clusters shows that Ha and radio emission from the brightest cluster galaxy are much more pronounced when the cluster's core gas entropy is <= 30 keV cm(2). The prevalence of Ha emission below this threshold indicates that it marks a dichotomy between clusters that can harbor multiphase gas and star formation in their cores and those that cannot. The fact that strong central radio emission also appears below this boundary suggests that AGN feedback turns on when the intracluster medium starts to condense, strengthening the case for AGN feedback as the mechanism that limits star formation in the universe's most luminous galaxies.