Physical cool-core condensation radius in massive galaxy clusters

Abstract

Aims. We investigate the properties of cool cores in an optimally selected sample of 37 massive and X-ray-bright galaxy clusters, with regular morphologies, observed with Chandra. We started by measuring the density, temperature, and abundance radial profiles of their intracluster medium (ICM). From these independent quantities, we computed the cooling (tcool), free-fall (tff), and turbulence (teddy) timescales as a function of radius. Methods. By requiring the profile-crossing condition, tcool/teddy = 1, we measured the cool-core condensation radius, Rccc, within which the balancing feeding and feedback processes generate the turbulent condensation rain and related chaotic cold accretion (CCA). We also constrained the complementary (quenched) cooling flow radius, Rqcf, obtained via the condition tcool = 25 × tff, that encompasses the region of thermally unstable cooling. Results. We find that in our our massive cluster sample and in the limited redshift range considered (1.3 × 1014 < M500 < 16.6 × 1014 M⊙, 0.03 < z < 0.29), the distribution of Rccc peaks at ∼0.01 r500 and the entire range remains below ∼0.07 r500, with a very weak increase with redshift and no dependence on the cluster mass. We find that Rqcf is typically three times larger than Rccc, with a wider distribution, and growing more slowly along Rccc, according to an average relation Rqcf∝ Rccc0.46, with a large intrinsic scatter. Conclusions. We suggest that this sublinear relation can be understood as an effect of the micro rain of pockets of cooled gas flickering in the turbulent ICM, whose dynamical and thermodynamical properties are referred to as “macro weather”. Substituting the classical ad hoc cool-core radius R7.7 Gyr, we propose that Rqcf is an indicator of the size of global cool cores tied to the long-term macro weather, with the inner Rccc closely tracing the effective condensation rain and chaotic cold accretion (CCA) zone that feeds the central supermassive black hole (SMBH).