Statistical properties of SZ and X-ray cluster detections

¹ ITA, Zentrum für Astronomie, Universität Heidelberg, Albert Überle Str. 2, 69120 Heidelberg, Germany e-mail: francesco@ita.uni-heidelberg.de
² Max Planck Institut für Extraterrestrische Physik, 85478 Garching, Germany
³ Max Planck Institut für Astrophysik, 85478 Garching, Germany
⁴ INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁵ Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁶ INFN-National Institute for Nuclear Physics, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

Received: 8 February 2008
Accepted: 29 February 2008

Abstract

Aims. We calibrate the number density, completeness, reliability, and the lower mass limit of galaxy-cluster detections through their thermal SZ signal and compare them to X-ray cluster detections.

Methods. We simulate maps of the thermal SZ effect and the X-ray emission from light cones constructed in a large, hydrodynamical, cosmological simulation volume, including realistic noise contributions. The maps are convolved with linear, optimised, single- and multi-band filters to identify local peaks and their signal-to-noise ratios. The resulting peak catalogues are then compared to the halo population in the simulation volume to identify true and spurious detections.

Results. Multi-band filtering improves the statistics of SZ cluster detections considerably compared to single-band filtering. Observations with the characteristics of ACT detect clusters with masses M ≥ 6-910¹³ , quite independent of redshift, reach 50% completeness at ~10¹⁴  and 100% completeness at ~210¹⁴ . Samples are contaminated by a few spurious detections, but they are only a small percentage of all detections. This is broadly comparable to X-ray cluster detections with XMM-Newton with 100 ks exposure time in the soft band, except that the mass limit for X-ray detections increases much more steeply with redshift than for SZ detections. A comparison of true and filtered signals in the SZ and X-ray maps confirms that the filters introduce at most a negligible bias.