THE GROWTH OF COOL CORES AND EVOLUTION OF COOLING PROPERTIES IN A SAMPLE OF 83 GALAXY CLUSTERS AT 0.3

We present first results on the cooling properties derived from Chandra X-ray observations of 83 high-redshift (0.3 0.75, consistent with earlier reports for clusters at z > 0.5 using similar definitions. Our measurements indicate that cool cores have been steadily growing over the 8 Gyr spanned by our sample, consistent with a constant, 150 M yr-1 cooling flow that is unable to cool below entropies of 10 keV cm2 and, instead, accumulates in the cluster center. We estimate that cool cores began to assemble in these massive systems at , which represents the first constraints on the onset of cooling in galaxy cluster cores. At high redshift (z ≳ 0.75), galaxy clusters may be classified as "cooling flows" (low central entropy, cooling time) but not "cool cores" (cuspy surface brightness profile), meaning that care must be taken when classifying these high-z systems. We investigate several potential biases that could conspire to mimic this cool core evolution and are unable to find a bias that has a similar redshift dependence and a substantial amplitude. © 2013. The American Astronomical Society. All rights reserved.