Exploring the selection of galaxy clusters and groups: an optical survey for X-ray dark clusters

Data from a new, wide-field, coincident optical and X-ray survey, the X-ray Dark Cluster Survey (XDCS), are presented. This survey comprises 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. Two distinct optically selected catalogues are constructed, one based on I-band overdensity, the other on overdensities of colour-selected galaxies. The superior accuracy of the colour-selection technique over that of the single-passband method is demonstrated, via internal consistency checks and comparison with external spectroscopic redshift information. This is compared with an X-ray-selected cluster catalogue. In terms of gross numbers, the survey yields 185 I-band-selected, 290 colour-selected and 15 X-ray-selected systems, residing in similar to11 deg(2) of optical + X-ray imaging. The relationship between optical richness/luminosity and X-ray luminosity is examined, by measuring X-ray luminosities at the positions of our 290 colour-selected systems. Power-law correlations between the optical richness/luminosity and X-ray luminosity are fitted, both exhibiting approximately 0.2 dex of intrinsic scatter. Interesting outliers in these correlations are discussed in greater detail. Spectroscopic follow-up of a subsample of X-ray underluminous systems confirms their reality.