The Impact Of Cosmic Rays on thermal instability in the cgm
I worked on this project with Drummond Fielding and Chris Hayward during my time as a Pre-Doctoral Fellow at the Flatiron Institute’s Center for Computational Astrophysics. Check out the full text on the arXiv!
Cosmic rays change the morphology and accretion rates of cold gas
Large reservoirs of cold (T ~ 10^4 K ) gas are observed out to and beyond the virial radius in the circumgalactic medium (CGM). Cold CGM gas makes up a substantial fraction of galactic baryons (30-50%) and accretes onto the galaxy to fuel star formation. Photoionization modeling suggests that the density of cold CGM gas is significantly lower than expected for it to be in thermal pressure equilibrium with hot CGM gas. Although the density of the hot CGM gas is itself poorly constrained, the observations are consistent with the presence of non-thermal pressure support in the CGM.
One theory for the abundance of cold gas is that it forms directly in the CGM through thermal instability — a process that shares many similarities with the condensation and precipitation of rain in the atmosphere. In this work, we run the first 3D simulations of thermal instability in a stratified medium including cosmic ray physics. Cosmic rays are an important source of energy in the galactic disk, and are likely present in the CGM of galaxies. Our simulations demonstrate how cosmic rays change the morphology of the cold gas that forms through thermal instability: cold gas has lower densities and larger radii. Cosmic rays can also prevent cold from accreting onto its galaxy, possibly explaining observations of quenched galaxies with massive reservoirs of cold gas in their CGM.
Figure 1: 2D slice of the 3D density in simulations with different ratios of cosmic ray pressure (increasing left to right). In all simulations, cold, dense gas forms through thermal instability. With increased cosmic ray pressure, the cold gas clouds become larger and less dense.