Witnessing the Formation of a Galaxy Cluster at z = 0.485: Optical and X-Ray Properties of RX J1117.4+0743 ([VMF 98] 097)

Abstract

We present a multiwavelength study of the poor cluster RX J1117.4+0743 ([VMF 98] 097) at z=0.485, based on GMOS/Gemini South g' and r' photometry and spectroscopy, and XMM-Newton observations. We examine its nature and surroundings by analyzing the projected galaxy distribution, the galaxy velocity distribution, the weak-lensing mass reconstruction, and the X-ray spectroscopy and imaging. The cluster is composed by at least two structures along the line of sight, with σ_V=592+/-82 and 391+/-85 km s^-1, respectively. Other structures are also detected in X-ray, in the galaxy projected number density map, and by weak lensing. The derived temperature and bolometric X-ray luminosity reveal that [VMF 98] 097 behaves like a normal cluster, in agreement with L_X-T_X correlation found for z=0 and at z~0.4 clusters. The mass determination inferred from weak lensing is on average 3-4.8 times higher than the X-ray mass. We have two possible explanations for this discrepancy: (1) the cluster is in nonequilibrium, then the deviation of the X-ray estimated mass from the true value can be as high as a factor of 2; (2) the intervening mass along the line of sight of the cluster is producing an overestimation of the weak-lensing mass. Based on the analysis, we conclude that [VMF 98] 097 is a perturbed cluster with at least two substructures in velocity space and with other nearby structures at projected distances of about 1 h^-1₇₀ Mpc. This is an example of a poor cluster caught in the process of accreting substructures to become a rich cluster.

Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina); the Gemini program ID is GS-2003A-SV-206.