Cosmic rays and thermal instability in self-regulating cooling flows of massive galaxy clusters

¹ Institut d’Astrophysique de Paris, CNRS, Sorbonne Université, UMR7095, 98bis bd Arago, 75014 Paris, France
e-mail: ricarda.beckmann@iap.fr
² Institute of Astronomy and Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, UMR7293, Boulevard de l’Observatoire, CS 34229, 06304 Nice, France
⁴ LERMA, Observatoire de Paris, PSL Research Univ., CNRS, Sorbonne Université, UMR8112, 75014 Paris, France
⁵ Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
⁶ SOFIA Science Center, USRA, NASA Ames Research Center, M.S. N232-12, Moffett Field, CA 94035, USA
⁷ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁸ Department of Mathematics, University of Vienna, Oskar-Morgenstern-Platz 1, 1090 Vienna, Austria
⁹ Institut Universitaire de France, Ministère de l’Education Nationale, de l’Enseignement Supérieur et de la Recherche, 1 rue Descartes, 75231 Paris Cedex 05, France
¹⁰ Centre de Recherche Astrophysique de Lyon, ENS de Lyon, Université Lyon 1, CNRS, UMR5574, 69230 Saint-Genis-Laval, France

Received: 26 October 2021
Accepted: 23 May 2022

Abstract

One of the key physical processes that helps prevent strong cooling flows in galaxy clusters is the continued energy input from the central active galactic nucleus (AGN) of the cluster. However, it remains unclear how this energy is thermalised so that it can effectively prevent global thermal instability. One possible option is that a fraction of the AGN energy is converted into cosmic rays (CRs), which provide non-thermal pressure support, and can retain energy even as thermal energy is radiated away. By means of magneto-hydrodynamical simulations, we investigate how CR injected by the AGN jet influence cooling flows of a massive galaxy cluster. We conclude that converting a fraction of the AGN luminosity as low as 10% into CR energy prevents cooling flows on timescales of billion years, without significant changes in the structure of the multi-phase intra-cluster medium. CR-dominated jets, by contrast, lead to the formation of an extended, warm central nebula that is supported by CR pressure. We report that the presence of CRs is not able to suppress the onset of thermal instability in massive galaxy clusters, but CR-dominated jets do significantly change the continued evolution of gas as it continues to cool from isobaric to isochoric. The CR redistribution in the cluster is dominated by advection rather than diffusion or streaming, but the heating by CR streaming helps maintain gas in the hot and warm phase. Observationally, self-regulating, CR-dominated jets produce a γ-ray flux in excess of current observational limits, but low CR fractions in the jet are not ruled out.