The dynamical impact of cosmic rays in the Rhea magnetohydrodynamics simulations

TL;DR

This paper investigates how cosmic rays influence galaxy evolution by supporting winds, reducing star formation, and enriching the circumgalactic medium, using advanced Rhea simulations that incorporate multi-phase ISM and magnetic fields.

Contribution

It introduces a comprehensive simulation framework that models cosmic ray transport and interactions in Milky Way-like galaxies, revealing their significant dynamical effects.

Findings

CRs reduce star formation rates

CR-driven outflows have mass loading factors of ~0.2

CRs enrich the circumgalactic medium with magnetic fields

Abstract

This study explores the dynamical impact of cosmic rays (CRs) in Milky Way-like galaxies using the Rhea simulation suite. Cosmic rays, with their substantial energy density, influence the interstellar medium (ISM) by supporting galactic winds, modulating star formation, and shaping ISM energetics. The simulations incorporate a multi-phase ISM, self-consistent CR transport in the advection-diffusion approximation, and interactions with magnetic fields to study their effect on galaxy evolution. Key findings reveal that CRs reduce star formation rates, and drive weak but sustained outflows with mass loading factors of $\sim0.2$, transporting a substantial fraction (20%-60%) of the injected CR energy. These CR-driven outflows are launched not just from the galactic center but across the entire disk, illustrating their pervasive dynamical influence. Galactic disks supported by CRs exhibit broader vertical structures compared to magnetic-field-dominated setups, though the scale heights are similar. CR feedback enhances magnetic flux transport to the circumgalactic medium (CGM), leading to a magnetically enriched CGM with field strengths of $\sim0.5\mu\mathrm{G}$ while reducing gas temperatures to $\lesssim10^5\,\mathrm{K}$. The CR energy is relatively smoothly distributed in the disk, with gradient lengths exceeding the typical size of molecular clouds, indicating that the CR behavior is not adiabatic.