An XMM-Newton observation of the massive, relaxed galaxy cluster ClJ1226.9+3332 at z= 0.89

A detailed X-ray analysis of an XMM-Newton observation of the high-redshift (z= 0.89) galaxy cluster ClJ1226.9+3332 is presented. After careful consideration of background subtraction issues, the X-ray temperature is found to be 11.5^+1.1_-0.9 keV, the highest X-ray temperature of any cluster at z > 0.6. The temperature is consistent with the observed velocity dispersion. In contrast to MS 1054-0321, the only other very hot cluster currently known at z > 0.8, ClJ1226.9+3332, features a relaxed X-ray morphology, and its high overall gas temperature is not caused by one or several hotspots. The system thus constitutes a unique example of a high-redshift (z > 0.8), high-temperature (T > 10 keV), relaxed cluster, for which the usual hydrostatic equilibrium assumption and the X-ray mass are most reliable.

A temperature profile is constructed (for the first time at this redshift) and is consistent with the cluster being isothermal out to 45 per cent of the virial radius. Within the virial radius (corresponding to a measured overdensity of a factor of 200), a total mass of 1.4 +/- 0.5 × 10¹⁵ M_solar is derived, with a gas mass fraction of 12 +/- 5 per cent (for a Λ cold dark matter cosmology and H₀= 70 km s^-1 Mpc^-1). This total mass is similar to that of the Coma cluster. The bolometric X-ray luminosity is 5.3^+0.2_-0.2× 10⁴⁵ erg s^-1. Analysis of a short Chandra observation confirms the lack of significant point-source contamination, the temperature, and the luminosity, albeit with lower precision. The probabilities of finding a cluster of this mass within the volume of the discovery X-ray survey are ~8 × 10^-5 for Ω_M= 1 and 0.64 for Ω_M= 0.3, making Ω_M= 1 highly unlikely.

The entropy profile suggests that entropy evolution is being observed. The metal abundance (of Z= 0.33^+0.14_-0.10 Z_solar), gas mass fraction and gas distribution are consistent with those of local clusters; thus the bulk of the metals were in place by z= 0.89.