Batch discovery of nine z~ 1 clusters using X-ray and K or R, z' images

Abstract

We present the results of an initial search for clusters of galaxies at z~ 1 and above, using data from 2.9 square degrees of XMM-Newton images. By selecting weak potentially extended X-ray sources with faint or no identifications in deep, ground-based optical imaging, we have constructed a starting sample of 19 high-redshift cluster candidates. Near-IR and R, z' imaging of these fields identified nine of them as high-redshift systems. Six of these were confirmed spectroscopically, three at z~ 1.0 and the other three in the 0.8 < z < 0.92 range. The remaining three systems have solid photometric evidence to be at z_phot~ 0.8, 1.0 and 1.3. The present sample significantly increases the number of such clusters. The measured density of z>~ 1 clusters, after discarding `low'-redshift systems at z<~ 0.92 is about 1.7 deg^-2 (with 68 per cent confidence interval equal to [1.0, 2.9]) for f_X>~ 2.5 10^-15 erg cm^-2 s^-1 ([0.5-2] keV) and this is a lower limit, having screened not all potential z~ 1 candidate clusters. Coordinates, X-ray measures and evidence for nine X-ray-selected high-redshift clusters is given.