2011A&A...530A..27C

We investigate the properties of cluster ClG 0332-2747 at z=0.734 in the GOODS-South field, which was undetected in the Chandra 2Ms observations. We explore possible scenarios to explain the discrepancy between its low X-ray emission and that expected from the M-L_X relation. We use the large public spectroscopic database available in the GOODS-South field to estimate the dynamical mass and the virialization status of ClG 0332-2747. Cluster members selected from their photometric redshift are used with spectroscopic ones to analyse the galaxy population of the cluster. In the newly released Chandra 4Ms observations we detect a faint extended X-ray emission associated to the cluster. Finally, we compare the optical and X-ray properties of ClG 0332-2747 with the predictions of a well-tested semianalytic model. We estimate the velocity dispersion and the virial mass considering all 44 spectroscopic members, or 20 red-sequence members only. We obtain σ_v=634±105km/s, M₂₀₀=3.07^+1.57_–1.16x10¹⁴M_☉ in the former case, and slightly lower values in the latter case. The cluster appears to have reached the virial equilibrium: it shows a perfectly Gaussian velocity distribution and no evidence for substructures. ClG 0332-2747 contains a high fraction of bright red galaxies and is dominated by a very massive (1.1x10¹²M_☉) old brightest cluster galaxy (BCG), suggesting that it formed at an early epoch. We detect a faint extended X-ray source centred on the BCG, with a total X-ray luminosity of L_X∼2x10⁴²erg/s (0.1-2.4keV). This L_X is lower by a factor of ∼10-20 than expected according to the M-L_X relation. We provide a possible explanation of this discrepancy in assigning it to the effects of AGN feedback on the ICM: the semianalytic model reproduces the M-L_X relation measured from ``X-ray bright'' clusters, and it predicts a high scatter at low masses owing to heating and expulsion of the cluster gas. Interestingly, the model clusters with an evolved galaxy population like ClG 0332-2747 present the largest scatter in X-ray luminosity. However, the low X-ray emission of ClG 0332-2747 is just marginally compatible with predictions, which indicates that additional feedback effects should be included in the model. We propose a scenario where ``X-ray underluminous'' clusters are explained by the strong feedback effect on the ICM in highly evolved clusters. Our hypothesis can be tested by the combined analysis of the galaxy population and of the X-ray emission in large cluster samples.