2013ApJ...774...23M

We present first results on the cooling properties derived from Chandra X-ray observations of 83 high-redshift (0.3 < z < 1.2) massive galaxy clusters selected by their Sunyaev-Zel'dovich signature in the South Pole Telescope data. We measure each cluster's central cooling time, central entropy, and mass deposition rate, and compare these properties to those for local cluster samples. We find no significant evolution from z ∼ 0 to z ∼ 1 in the distribution of these properties, suggesting that cooling in cluster cores is stable over long periods of time. We also find that the average cool core entropy profile in the inner ∼100 kpc has not changed dramatically since z ∼ 1, implying that feedback must be providing nearly constant energy injection to maintain the observed "entropy floor" at ∼10 keV cm². While the cooling properties appear roughly constant over long periods of time, we observe strong evolution in the gas density profile, with the normalized central density (ρ_g, 0_/ρcrit_) increasing by an order of magnitude from z ∼ 1 to z ∼ 0. When using metrics defined by the inner surface brightness profile of clusters, we find an apparent lack of classical, cuspy, cool-core clusters at z > 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 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 z_cool = 1.0_–0.2^+1.0, 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.