Chandra and XMM-Newton observations of RDCS 1252.9-2927, A massive cluster at z = 1.24¹

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_-0.5 ^-0.7 keV, Z = 0.36_-0.10^+0.12 Z _⊙. On the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M₅₀₀ = (1.9 ± 0.3) × 10 ¹⁴ h₇₀^-1 M_⊙ 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 σ_g = 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.