The XMM/2dF survey III: Comparison between optical and X-ray cluster detection methods

We directly compare X-ray and optical techniques of cluster detection by combining SDSS photometric data with a wide-field ($\sim 1.8$ deg${}^2$) XMM-{\em Newton} survey in the North Galactic Pole region. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey. The most probable cause of the small number of optical cluster candidates detected in our X-ray survey is that they are relatively poor clusters, fainter than the X-ray flux limit (for extended sources) of our survey $f_x(0.3-2\mathrm{k}\mathrm{e}\mathrm{V})\simeq 2\times {10}^{-14}ergc{m}^{-2}{s}^{-1}$.