An X-ray underluminous cluster of galaxies in the 4Ms CDFS observations

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio (RM), Italy
e-mail: marco.castellano@oa-roma.inaf.it
² Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
³ Dipartimento di Fisica, Università di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy

Received: 28 January 2011
Accepted: 17 March 2011

Abstract

Aims. 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.

Methods. 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.

Results. 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 ± 105 km s^-1, ${\mathit{M}}_{\mathrm{200}}\mathrm{=}\mathrm{3.0}{\mathrm{7}}_{-1.16}^{\mathrm{+}\mathrm{1.57}}\mathrm{\times }{\mathrm{10}}^{\mathrm{14}} {\mathit{M}}_{\mathrm{\odot }}$ 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 (1.1 × 10¹²   M_⊙) 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 ~ 2 × 10⁴²   erg   s^-1 (0.1–2.4 keV). 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.

Conclusions. 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.