The Impact of Cosmic Rays on the Kinematics of the Circumgalactic Medium

TL;DR

This study uses hydrodynamical simulations to show that cosmic-ray pressure significantly influences the kinematics and thermal state of the circumgalactic medium, aligning simulation results more closely with observations.

Contribution

It demonstrates the impact of cosmic-ray pressure on CGM kinematics and thermal properties, providing a more realistic simulation framework compared to cosmic-ray-free models.

Findings

Cosmic rays dominate thermal pressure within 50 kpc of the CGM.

Cosmic-ray presence results in narrower OVI and broader SiIII absorption features.

Simulated gas velocities align with observational data when cosmic rays are included.

Abstract

We use hydrodynamical simulations of two Milky Way-mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sightlines of the simulated galaxies' CGM and use Voigt profile fitting methods to extract ion column densities, Doppler-b parameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky-Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower OVI absorption features and broader SiIII absorption features, a quality which is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM.