Multi-wavelength study of XMMU J2235.3-2557: the most massive galaxy cluster at z > 1

Abstract

Context: The galaxy cluster XMMU J2235.3-2557 (hereafter XMM2235), spectroscopically confirmed at z = 1.39, is one of the most distant X-ray selected galaxy clusters. It has been at the center of a multi-wavelength observing campaign with ground and space facilities.
Aims: We characterize the galaxy populations of passive members, the thermodynamical properties and metal abundance of the hot gas, and the total mass of the system using imaging data with HST/ACS (i₇₇₅ and z₈₅₀ bands) and VLT/ISAAC (J and K_S bands), extensive spectroscopic data obtained with VLT/FORS2, and deep (196 ks) Chandra observations.
Methods: Chandra data allow temperature and metallicity to be measured with good accuracy and the X-ray surface brightness profile to be traced out to 1´ (or 500 kpc), thus allowing the mass to be reliably estimated. Out of a total sample of 34 spectroscopically confirmed cluster members, we selected 16 passive galaxies (without detectable [OII]) within the central 2´ (or 1 Mpc) with ACS coverage, and inferred star formation histories for subsamples of galaxies inside and outside the core by modeling their spectro-photometric data with spectral synthesis models.
Results: Chandra data show a regular elongated morphology, closely resembling the distribution of core galaxies, with a significant cool core. We measure a global X-ray temperature of kT = 8.6_-1.2^+1.3 keV (68% confidence), which we find to be robust against several systematics involved in the X-ray spectral analysis. By detecting the rest frame 6.7 keV Iron K line in the Chandra spectrum, we measure a metallicity Z = 0.26^+0.20_-0.16 Z_⊙. In the likely hypothesis of hydrostatic equilibrium, we obtain a total mass of M_tot(<1 Mpc)= (5.9±1.3) × 10¹⁴ M_⊙. By modeling both the composite spectral energy distributions and spectra of the passive galaxies in and outside the core, we find a strong mean age radial gradient. Core galaxies, with stellar masses in excess of 10¹¹ M_⊙, appear to have formed at an earlier epoch with a relatively short star formation phase (z = 5-6), whereas passive galaxies outside the core show spectral signatures suggesting a prolonged star formation phase to redshifts as low as z ≈ 2.
Conclusions: Overall, our analysis implies that XMM2235 is the hottest and most massive bona-fide cluster discovered to date at z > 1, with a baryonic content, both its galaxy population and intracluster gas, in a significantly advanced evolutionary stage at 1/3 of the current age of the Universe.

Based on observations carried out using the

Advanced Camera for Surveys at the Hubble Space Telescope under

Program ID 10698; the Very Large Telescope at the ESO Paranal

Observatory under Program IDs 274.A-5024(B), 077.A-0177(A, B),

074.A-0023(A), 077.A-0110(A, B).