The origin of X - ray emission of two distant ( z > 1 ) cluster candidates with XMM - Newton

We present here a study of XMM-Newton data of two distant galaxy cluster candidates. One of these was discovered serendipitously in near infrared data, CL J0533-2411, the other one corresponds to the cluster EIS J0533-2412 part of the EIS cluster survey. The estimated redshift of CL J0533-2411 is z = 1.2 − 1.7. EIS J0533-2412 is a rich system (Λcl = 299), with a spectroscopically confirmed redshift of z = 1.3. Both galaxy concentrations show firm X-ray detections, located within 30′′ of their optical center. However, we cannot resolve the sources with XMM-Newton . If the X-ray emission originates from the X-ray emitting intra-cluster medium (ICM) it would be extremely concentrated which is rather unlikely (core radii below 14 h 65 kpc and 40 h 65 kpc, respectively). We argue that the X–ray sources are more likely AGNmembers of the galaxy concentrations. We set an upper limit for the bolometric luminosity of a hot ICM in the range ∼ 0.7−2.1×10 h 65 erg/s for CL J0533-2411 , depending on the exact redshift. For EIS J0533-2412 the limit is Lbol = (6.2±1.4)×10 43 h 65 erg/s. We interpret our result in the following way: EIS J0533-2412 (and possibly CL J0533-2411 ) are proto-clusters and show matter overdensities before collapse, which explains the low significance of extended X-ray emission.