The evolution of cool-core clusters

Abstract

Context. Cool-core clusters are characterized by strong surface brightness peaks in the X-ray emission from the Intra Cluster Medium (ICM). This phenomenon is associated with complex physics in the ICM and has been a subject of intense debate and investigation in recent years. The evolution of cool-cores is still poorly constrained because of the small sample statistics and the observational challenge of analysing high redshift clusters. Aims. In order to quantify the evolution in the cool-core cluster population, we robustly measure the cool-core strength in a local, representative cluster sample, and in the largest sample of high-redshift clusters available to date. Methods. We use high-resolution Chandra data of three representative cluster samples spanning different redshift ranges: (i) the low redshift sample from the 400 Square degree (SD) survey with median $\langle$z$\rangle$ = 0.08; (ii) the high redshift sample from the 400 SD Survey with median $\ langle$z$\rangle$ = 0.59; and (iii) 15 clusters drawn from the Rosat Distant Cluster Survey and the Wide Angle Rosat Pointed Survey, with median $\langle$z$\rangle$ = 0.83. Our analysis is based on the measurement of the surface brightness concentration, cSB (Santos et al. 2008, A&A, 483, 35), which allows us to characterize the cool-core strength in low signal-to-noise data. We also obtain gas density profiles to derive cluster central cooling times and entropy. In addition to the X-ray analysis, we search for radio counterparts associated with the cluster cores. Results. We find a statistically significant difference in the c SB distributions of the two high-z samples, pointing towards a lack of concentrated clusters in the 400 SD high-z sample. Taking this into account, we confirm a negative evolution in the fraction of cool-core clusters with redshift, in particular for very strong cool-cores. This result is validated by the central entropy and central cooling time, which show strong anti-correlations with cSB. However, the amount of evolution is significantly smaller than previously claimed, leaving room for a large population of well formed cool-cores at z ∼ 1. Finally, we explore the potential of the proposed X-ray mission Wide Field X-ray Telescope to detect and quantify cool-cores up to z = 1.5. © 2010 ESO.