On the nature of local instabilities in rotating galactic coronae and cool cores of galaxy clusters

TL;DR

This paper investigates the local instabilities in rotating, magnetized, stratified plasmas with radiative cooling, revealing conditions under which cold gas condensation occurs in galaxy coronae and cluster cores.

Contribution

It provides general equations for analyzing local instabilities in such plasmas and applies them to models of galaxy coronae and cluster cool cores, highlighting the roles of various instabilities.

Findings

Cold cloud condensation is suppressed in cluster cool cores.

Rotating galactic coronae can support cold cloud formation under certain conditions.

Magnetorotational instability dominates when magnetic fields are weak.

Abstract

A long-standing question is whether radiative cooling can lead to local condensations of cold gas in the hot atmospheres of galaxies and galaxy clusters. We address this problem by studying the nature of local instabilities in rotating, stratified, weakly magnetized, optically thin plasmas in the presence of radiative cooling and anisotropic thermal conduction. For both axisymmetric and non-axisymmetric linear perturbations we provide the general equations that can be applied locally to specific systems to establish whether they are unstable and, in case of instability, to determine the kind of evolution (monotonically growing or over-stable) and the growth rates of unstable modes. We present results for models of rotating plasmas representative of Milky Way-like galaxy coronae and cool-cores of galaxy clusters. It is shown that the unstable modes arise from a combination of thermal, magnetothermal, magnetorotational and heat-flux-driven buoyancy instabilities. Local condensation of cold clouds tends to be hampered in cluster cool cores, while it is possible under certain conditions in rotating galactic coronae. If the magnetic field is sufficiently weak the magnetorotational instability is dominant even in these pressure supported systems.