The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Abstract

Large reservoirs of cold (∼10 4 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmicray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.

Stress-Testing Cosmic Ray Physics: The Impact of Cosmic Rays on the Surviving Disk of Ram Pressure Stripped Galaxies

“…In agreement with previous work, the Isolated runs (top row) indicate that including CR permits the presence of low-density, lowtemperature gas, as CR provide nonthermal pressure support (Ji et al 2020;Butsky et al 2020). Note the low temperature ridge at 𝑇 ∼ 10 2 K is weak in NoCR runs (supported only by magnetic pressure in those cases).…”

Virial shocks are suppressed in cosmic ray-dominated galaxy haloes

“…In contrast, cooler gas in CR+ can remain at intermediate temperatures with volume-filling densities. One might worry that our resolution is insufficient to accurately capture these phenomena (Hummels et al 2019), but recent idealized simulations of the thermal instability in a patch of the CGM with CRs (Butsky et al 2020) have demonstrated exactly the results above, when P cr P thermal .…”

“…Ji, Oh & McCourt 2018;van de Voort et al 2020) and cosmic rays (CRs; e.g. Salem, Bryan & Corlies 2016;Farber et al 2018;Ji et al 2020;Buck et al 2020;Butsky et al 2020), is under increasingly active investigation. Ji et al (2020) utilized FIRE-2 simulations 1 with CR physics incorporated (Chan et al 2019;Hopkins et al 2020c) to investigate the effects of CRs on the CGM properties.…”

Which AGN jets quench star formation in massive galaxies?

“…11. 11 CR-dominated jets can also help quench by modifying the nonlinear behaviour of the thermal instability, as suggested in Ji et al (2020) and shown rigorously in Butsky et al (2020). In brief: if CR pressure balances gravity and dominates over thermal pressure in a thermally unstable medium, then cooling gas follows total pressure equilibrium (not just thermal pressure equilibrium) and cooling gas can remain diffuse (rather than being compressed to high densities; i.e.…”