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

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 $\approx 0\hbox{$.\!\!^{\prime\prime}$}6$) which results in a high number density of galaxies used for the weak lensing analysis ($n\approx 26\,{\rm arcmin}^{-2}$). 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.