2004AJ....127..230R

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 σ error bar to 10% for the temperature and 30% for the metallicity, with best-fit values kT=6.0^+0.7_–0.5keV, Z=0.36^+0.12_–0.10Z_☉. On the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M₅₀₀=(1.9±0.3)x10¹⁴h^–1₇₀M_☉ within R_Δ=500≃536 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,Ω_m,Ω_Λ)=(0.7,0.3,0.7) and σ₈=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.