The XMM cluster survey: active galactic nuclei and starburst galaxies in XMMXCS J2215.9-1738 at z = 1.46

M. Hilton, E. Lloyd-Davies, S. A. Stanford, J. P. Stott, C. A. Collins, A. K. Romer, M. Hosmer, B. Hoyle, S. T. Kay, A. R. Liddle, N. Mehrtens, C. J. Miller, M. Sahlén, P. T. P. Viana

Abstract
We use Chandra X-ray and Spitzer infrared observations to explore the AGN and starburst populations of XMMXCS J2215.9-1738 at z = 1.46, one of the most distant spectroscopically confirmed galaxy clusters known. The high resolution X-ray imaging reveals that the cluster emission is contaminated by point sources that were not resolved in XMM-Newton observations of the system, and have the effect of hardening the spectrum, leading to the previously reported temperature for this system being overestimated. From a joint spectroscopic analysis of the Chandra and XMM-Newton data, the cluster is found to have temperature T = 4.1^+0:6_-0.9 keV and luminosity L_X = (2.92^+0.24 _-0.35)×10⁴⁴ erg s^-1, extrapolated to a radius of 2 Mpc. As a result of this revised analysis, the cluster is found to lie on the σ_v - T relation, but the cluster remains less luminous than would be expected from self-similar evolution of the local L_X - T relation. Two of the newly discovered X-ray AGN are cluster members, while a third object, which is also a prominent 24 µm source, is found to have properties consistent with it being a high redshift, highly obscured object in the background. We find a total of eight > 5σ 24 µm sources associated with cluster members (four spectroscopically confirmed, and four selected using photometric redshifts), and one additional 24 µm source with two possible optical/near-infrared counterparts that may be associated with the cluster. Examining the IRAC colors of these sources, we find one object is likely to be an AGN. Assuming that the other 24 µm sources are powered by star formation, their infrared luminosities imply star formation rates ∼ 100 M_☉ yr^-1. We find that three of these sources are located at projected distances of < 250 kpc from the cluster center, suggesting that a large amount of star formation may be taking place in the cluster core, in contrast to clusters at low redshift.

Keywords
galaxies: clusters - galaxies: clusters: individual (XMMXCS J2215.9-1738) - galaxies: star formation - X-rays: galaxie: clusters

The Astrophysical Journal
Volume 718, Page 133
July 2010

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