Friday, July 12, 2013

1307.3049 (Martin W. Sommer et al.)

A comparative study of radio halo occurrence in SZ and X-ray selected galaxy cluster samples    [PDF]

Martin W. Sommer, Kaustuv Basu
We aim to make an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio halos have been a result of selection biases. To attain this goal, we construct near-complete samples of galaxy clusters based on public X-ray and Sunyaev-Zel'dovich (SZ) effect cluster catalogs, and search for diffuse, extended (Mpc-scale) emission near their center by analyzing data from the NRAO Very Large Array Sky Survey. We remove point sources using a matched filtering algorithm, and model the diffuse emission by radial fitting and direct integration. We use an extensive suite of simulations to check for biases in our methods. We model the relation between mass and radio halo power with a power law, accounting for the possibility of a population hosting no radio halo emission. In this way we constrain the fraction of targets hosting an extended emission at their centers at 1.4 GHz. We find power law slopes consistent with previous works, though our findings suggest the fraction of targets hosting radio halos may have to be revised upward when considering clusters selected in the Sunyaev-Zel'dovich effect: while approximately 65% of the X-ray selected targets are found to contain no extended radio emission, the corresponding fraction in the SZ selected samples is roughly 20%. The "radio-quiet" population in the X-ray selected cluster samples is in agreement with previous findings. We propose a simple explanation for the observed selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selected samples.
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