CRexit: How different cosmic ray transport modes affect thermal instability in the circumgalactic medium

¹ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
² Institute for Physics and Astronomy, University Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
³ Max-Planck Institute for Astrophysics (MPA), Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany

^⋆ Corresponding author: maweber@aip.de

Received: 29 January 2025
Accepted: 15 April 2025

Abstract

The circumgalactic medium (CGM) plays a critical role in galaxy evolution, influencing gas flows, feedback processes, and galactic dynamics. Observations have shown a substantial cold gas reservoir in the CGM, but the mechanisms driving its formation and evolution remain unclear. Cosmic rays (CRs), as a source of non-thermal pressure, are increasingly recognised as key regulators of cold gas dynamics. This study explores how CRs affect cold clouds that condense from the hot CGM through thermal instability (TI). Using three-dimensional CR magnetohydrodynamics simulations with the moving-mesh code Arepo, we assessed the impact of various CR transport models on cold gas evolution. Under purely advective CR transport, CR pressure significantly suppressed the collapse of thermally unstable regions, altering the CGM's structure. In contrast, our realistic CR transport models revealed that CRs escape collapsing regions via anisotropic streaming and diffusion along magnetic fields, reducing their ability to prevent collapse and diminishing their impact on the thermal structure of the cold CGM. The ratio of the CR escape timescale to the cloud collapse timescale emerged as a critical factor in determining the influence of CRs on TI. The CRs remained confined within cold clouds when effective CR diffusion was slow, thereby maximising their pressure support and inhibiting collapse. The fast and effective CR diffusion realised in our two-moment CR-magnetohydrodynamics model facilitated rapid CR escape, diminishing their stabilising effect. This realistic CR transport model shows a wide dynamic range of the effective CR diffusion coefficient; its CR-energy-weighted median ranges from 10²⁹ to 10³⁰ cm² s⁻¹ for thermally to CR-dominated atmospheres, respectively. In addition to these CR transport-related effects, we demonstrated that a high numerical resolution is crucial, as it is necessary to avoid spuriously large clouds formed in low-resolution simulations, which would result in overly long CR escape times and artificially amplified CR pressure support.