A mature cluster with X-ray emission at z = 2.07

¹ Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS–Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
e-mail: raphael.gobat@cea.fr
² Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-strasse 27, 8093 Zürich, Switzerland
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo, Japan
⁶ Graduate University for Advanced Studies, Osawa 2-21-1, Mitaka, Tokyo, Japan
⁷ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁸ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
¹⁰ National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726, USA
¹¹ Center for Astrophysics, University of Science and Technology of China, Hefei 230026, PR China
¹² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, I 40127 Bologna, Italy

Received: 5 November 2010
Accepted: 16 November 2010

Abstract

We report evidence of a fully established galaxy cluster at z = 2.07, consisting of a  ~20σ overdensity of red, compact spheroidal galaxies spatially coinciding with extended X-ray emission detected with XMM-Newton. We use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to estimate the redshift of the structure from a prominent z = 2.07 spectroscopic redshift spike of emission-line galaxies, concordant with the accurate 12-band photometric redshifts of the red galaxies. Using NICMOS and Keck AO observations, we find that the red galaxies have elliptical morphologies and compact cores. While they do not form a tight red sequence, their colours are consistent with that of a  ≳1.3 Gyr population observed at z ~ 2.1. From an X-ray luminosity of 7.2 × 10⁴³ erg s^-1 and the stellar mass content of the red galaxy population, we estimate a halo mass of 5.3–8 × 10¹³ M_⊙, comparable to the nearby Virgo cluster. These properties imply that this structure could be the most distant, mature cluster known to date and that X-ray luminous, elliptical-dominated clusters are already forming at substantially earlier epochs than previously known.