Searching for galaxy clusters using the aperture mass statistics in 50 VLT fields

¹ Institut für Astrophysik und Extraterrestrische Forschung (IAEF), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: mhetter@astro.uni-bonn.de
² Department of Physics, University “La Sapienza”, P.le A. Moro 2, 00185, Rome, Italy
³ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁴ Department of Physics and Astronomy, University of British Columbia, Agricultural Road 6224, Vancouver, V6T 1Z1, B.C., Canada
⁵ Observatoire de Paris, LERMA, 61 avenue de l'Observatoire, 75014 Paris, France

Received: 29 April 2005
Accepted: 30 June 2005

Abstract

Application of the aperture mass (M_ap-) statistics provides a weak lensing method for the detection of cluster-sized dark matter halos. We present a new aperture filter function and maximise the effectiveness of the M_ap-statistics to detect cluster-sized halos using analytical models. We then use weak lensing mock catalogues generated from ray-tracing through N-body simulations, to analyse the effect of image treatment on the expected number density of halos. Using the M_ap-statistics, the aperture radius is typically several arcminutes, hence the aperture often lies partly outside a data field, consequently the signal-to-noise ratio of a halo detection decreases. We study these border effects analytically and by using mock catalogues. We find that the expected number density of halos decreases by a factor of two if the size of a field is comparable to the diameter of the aperture used. We finally report on the results of a weak lensing cluster search applying the M_ap-statistics to 50 randomly selected fields which were observed with FORS1 at the VLT. Altogether the 50 VLT fields cover an area of 0.64 square degrees. The I-band images were taken under excellent seeing conditions (average seeing ) which results in a high number density of galaxies used for the weak lensing analysis (). In five of the VLT fields, we detect a significant M_ap-signal which coincides with an overdensity of the light distribution. These detections are thus excellent candidates for shear-selected clusters.