Astronomy & Astrophysics

Free access article

Issue		A&A Volume 428, Number 1, December II 2004
Page(s)		21 - 37
Section		Cosmology (including clusters of galaxies)
DOI		10.1051/0004-6361:20041326

A&A 428, 21-37 (2004)
DOI: 10.1051/0004-6361:20041326

The Brera Multi-scale Wavelet HRI Cluster Survey

I. Selection of the sample and number counts

A. Moretti¹, L. Guzzo¹, S. Campana¹, D. Lazzati², M. R. Panzera¹, G. Tagliaferri¹, S. Arena¹, F. Braglia¹, I. Dell'Antonio³ and M. Longhetti¹

¹ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, Merate (LC) 23807, Italy
e-mail: moretti@merate.mi.astro.it
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Brown University, Providence, RI 02912, USA

(Received 20 May 2004 / Accepted 4 August 2004)

Abstract
We describe the construction of the Brera Multi-scale Wavelet (BMW) HRI Cluster Survey, a deep sample of serendipitous X-ray selected clusters of galaxies based on the ROSAT HRI archive. This is the first cluster catalog exploiting the high angular resolution of this instrument. Cluster candidates are selected on the basis of their X-ray extension only, a parameter which is well measured by the BMW wavelet detection algorithm. The survey includes 154 candidates over a total solid angle of ~160 deg ² at 10 ^-12 erg s ^-1 cm ^-2 and ~80 deg ² at $1.8\times10^$ erg s ^-1 cm ^-2. At the same time, a fairly good sky coverage in the faintest flux bins ( $3{-}5 \times 10^$ erg s ^-1 cm ^-2) gives this survey the capability of detecting a few clusters with $z\sim 1{-}1.2$ , depending on evolution. We present the results of extensive Monte Carlo simulations, providing a complete statistical characterization of the survey selection function and contamination level. We also present a new estimate of the surface density of clusters of galaxies down to a flux of $3\times 10^$ erg s ^-1 cm ^-2, which is consistent with previous measurements from PSPC-based samples. Several clusters with redshifts up to z=0.92 have already been confirmed, either by cross-correlation with existing PSPC surveys or from early results of an ongoing follow-up campaign. Overall, these results indicate that the excellent HRI PSF (5´´ FWHM on axis) more than compensates for the negative effect of the higher instrumental background on the detection of high-redshift clusters. In addition, it allows us to detect compact clusters that could be lost at lower resolution, thus potentially providing an important new insight into cluster evolution.

Key words: X-rays: galaxies: clusters -- surveys -- cosmology: large-scale structure of Universe

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