Physical cool-core condensation radius in massive galaxy clusters^⋆

¹ Shanghai Science and Technology Museum, Shanghai 200127, PR China
e-mail: wanglei@mail.bnu.edu.cn
² Department of Astronomy, Beijing Normal University, Beijing 100875, PR China
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50122 Firenze, Italy
⁴ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544-1001, USA
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, 40129 Bologna, Italy
⁶ INFN, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

Received: 29 May 2022
Accepted: 9 March 2023

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 (t_cool), free-fall (t_ff), and turbulence (t_eddy) timescales as a function of radius.

Methods. By requiring the profile-crossing condition, t_cool/t_eddy = 1, we measured the cool-core condensation radius, R_ccc, 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, R_qcf, obtained via the condition t_cool = 25 × t_ff, 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 R_ccc 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 R_qcf is typically three times larger than R_ccc, with a wider distribution, and growing more slowly along R_ccc, according to an average relation R_qcf^∝ R_ccc^0.46, 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 R_7.7 Gyr, we propose that R_qcf is an indicator of the size of global cool cores tied to the long-term macro weather, with the inner R_ccc closely tracing the effective condensation rain and chaotic cold accretion (CCA) zone that feeds the central supermassive black hole (SMBH).