Exploring the selection of galaxy clusters and groups

Abstract

Data from a new, wide field, coincident optical and X-ray survey, the X-ray Dark Cluster Survey (XDCS) are presented. The aim was to conduct simultaneous and independent searches for clusters of galaxies in the optical and X-ray passbands. Optical cluster detection algorithms implemented on the data are detailed. This resulted in catalogues of 185 I-band selected, 290 colour selected and 15 X-ray selected systems, residing in of optical + X-ray imaging. The relationship between optical (L(_E)) and X-ray luminosity (L(_x) ) was examined and found to exhibit significant scatter. This study highlights the higher efficiency and resolution of optical colour selection compared with other cluster detection methods. Spectroscopic redshifts confirmed the reality of a subsample of systems which were found with the optical algorithms, but required to have no detectable X-ray emission. These systems show comparable optical luminosity to the most X-ray luminous clusters, but orders of magnitude lower X-ray emission. This is consistent with the large scatter seen in the L(_x)-L(_e) relation. A near-infrared multicolour technique was explored and extended to search for high redshift (z>l) clusters. Finally, application of such techniques to forthcoming wide field near-infrared surveys was discussed and predictions for cluster finding in such surveys made.