Abstract

We present deep Chandra and XMM-Newton observations of the galaxy cluster RDCS 1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS) and confirmed by extensive spectroscopy with the Very Large Telescope at redshift z=1.237. With the Chandra data, the X-ray emission from the intracluster medium is well resolved and traced out to 500 kpc, thus allowing a measurement of the physical properties of the gas with unprecedented accuracy at this redshift. We detect a clear 6.7 keV iron K line in the Chandra spectrum providing a redshift within 1% of the spectroscopic one. By augmenting our spectroscopic analysis with the XMM-Newton data (MOS detectors only), we significantly narrow down the 1 sigma error bar to 10% for the temperature and 30% for the metallicity, with best-fit values kT=6.0(-0.5)(+0.7) keV, Z=0.36(-0.10)(+0.12) Z(circle dot). On the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M-500=(1.9+/-0.3)10(14) h(70)(-1) M-circle dot within R(Delta=500)similar or equal to536 kpc. Overall, these observations imply that RDCS 1252.9-2927 is the most X-ray luminous and likely the most massive bona fide cluster discovered to date at z>1. When combined with current samples of distant clusters, these data lend further support to a mild evolution of the cluster scaling relations, as well the metallicity of the intracluster gas. Inspection of the cluster mass function in the current cosmological concordance model (h, Omega(m), Omega(Lambda))=(0.7, 0.3, 0.7) and sigma(8)=0.7-0.8 shows that RDCS 1252.9-2927 is an M* cluster at z=1.24, in keeping with number density expectations in the RDCS survey volume.