Evolution of the X-ray profiles of poor clusters from the XMM-LSS survey

Abstract

A sample consisting of 27 X-ray-selected galaxy clusters from the X-ray Multi-Mirror Large-Scale Structure (XMM–LSS) survey is used to study the evolution in the X-ray surface brightness profiles of the hot intracluster plasma. These systems are mostly groups and poor clusters, with temperatures 0.6–4.8 keV, spanning the redshift range 0.05 to 1.05. Comparing the profiles with a standard β-model motivated by studies of low-redshift groups, we find 54 per cent of our systems to possess a central excess, which we identify with a cuspy cool core. Fitting β-model profiles, allowing for blurring by the XMM point spread function, we investigate trends with both temperature and redshift in the outer slope (β) of the X-ray surface brightness, and in the incidence of cuspy cores. Fits to individual cluster profiles and to profiles stacked in bands of redshift and temperature indicate that the incidence of cuspy cores does not decline at high redshifts, as has been reported in rich clusters. Rather such cores become more prominent with increasing redshift. β shows a positive correlation with both redshift and temperature. Given the β–T trend seen in local systems, we assume that temperature is the primary driver for this trend. Our results then demonstrate that this correlation is still present at z≳ 0.3, where most of our clusters reside.