The role of Cerenkov radiation in the pressure balance of cool core clusters of galaxies

TL;DR

This paper proposes that Cerenkov radiation within galaxy cluster cores provides a pressure mechanism that prevents catastrophic cooling and collapse, offering a natural solution to the cooling flow problem in galaxy clusters.

Contribution

It introduces a novel mechanism where Cerenkov radiation traps energy and maintains pressure in cluster cores, explaining how cooling is balanced without requiring additional heating.

Findings

Cerenkov radiation dominates electron cooling at the cluster core

Radiation trapping provides pressure to prevent core collapse

Estimated cooling flow rate aligns with observations

Abstract

Despite the substantial progress made recently in understanding the role of AGN feedback and associated non-thermal effects, the precise mechanism that prevents the core of some clusters of galaxies from collapsing catastrophically by radiative cooling remains unidentified. In this paper we demonstrate that the evolution of a cluster's cooling core, in terms of its density, temperature, and magnetic field strength, inevitably enables the plasma electrons there to quickly become Cerenkov loss dominated, with emission at the radio frequency of $\lesssim$ 350 Hz, and with a rate considerably exceeding free-free continuum and line emission. However, the same does not apply to the plasmas at the cluster's outskirts, which lacks such radiation. Owing to its low frequency, the radiation cannot escape, but because over the relevant scale size of a Cerenkov wavelength the energy of an electron in the gas cannot follow the Boltzmann distribution to the requisite precision to ensure reabsorption always occurs slower than stimulated emission, the emitting gas cools before it reheats. This leaves behind the radiation itself, trapped by the overlying reflective plasma, yet providing enough pressure to maintain quasi-hydrostatic equilibrium. The mass condensation then happens by Rayleigh-Taylor instability, at a rate determined by the outermost radius where Cerenkov radiation can occur. In this way, it is possible to estimate the rate at $\ap 2 M_\odot$~year$^{-1}$, consistent with observational inference. Thus the process appears to provide a natural solution to the long standing problem of `cooling flow' in clusters; at least it offers another line of defense against cooling and collapse, should gas heating by AGN feedback be inadequate in some clusters