Polytropic state of the intracluster medium in the X-COP cluster sample

¹ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
e-mail: vittorio.ghirardini2@unibo.it
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Dipartimento di Fisica e Astronomia Università di Bologna, Via Piero Gobetti, 93/2, 40129 Bologna, Italy
⁴ INFN, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁷ INAF – IASF Milano, Via Bassini 15, 20133 Milano, Italy

Received: 14 December 2018
Accepted: 1 June 2019

Abstract

Aims. In this work, we have investigated the relation between the radially resolved thermodynamic quantities of the intracluster medium in the X-COP cluster sample, aiming to assess the stratification properties of the ICM.

Methods. We modeled the relations between radius, gas temperature, density, and pressure using a combination of power-laws, also evaluating the intrinsic scatter in these relations.

Results. We show that the gas pressure is remarkably well correlated to the density, with very small scatter. Also the temperature correlates with gas density with similar scatter. The slopes of these relations have values that show a clear transition from the inner cluster regions to the outskirts. This transition occurs at the radius r_t = 0.19(±0.04) R₅₀₀ and electron density n_t = (1.91 ± 0.21) × 10⁻³ cm⁻³ E²(z). We find that above 0.2 R₅₀₀ the radial thermodynamic profiles are accurately reproduced by a well defined and physically motivated framework, where the dark matter follows the NFW potential and the gas is represented by a polytropic equation of state. By modeling the gas temperature dependence upon both the gas density and radius, we propose a new method to reconstruct the hydrostatic mass profile based only on the relatively inexpensive measurement of the gas density profile.