Cosmic Ray Driven Outflows in an Ultraluminous Galaxy

TL;DR

This study uses gas dynamical simulations to investigate the role of cosmic rays in galactic outflows from a massive ultraluminous galaxy, finding limited impact of CRs on mass-loading rates.

Contribution

It introduces a detailed simulation of CR effects in massive galaxy outflows, showing CRs have limited influence on mass-loading in such environments.

Findings

CRs create a low-density bubble outside the thermal-driven shell.

SN thermal pressure dominates the large-scale outflow formation.

CR diffusion into the disk does not significantly increase mass-loading.

Abstract

In models of galaxy formation, feedback driven both by supernova (SN) and active galactic nucleus (AGN) is not efficient enough to quench star formation in massive galaxies. Models of smaller galaxies have suggested that cosmic rays (CRs) play a major role in expelling material from the star forming regions by diffusing SN energy to the lower density outskirts. We therefore run gas dynamical simulations of galactic outflows from a galaxy contained in a halo with $5\times10^{12}$~M$_{\odot}$ that resembles a local ultraluminous galaxy, including both SN thermal energy and a treatment of CRs using the same diffusion approximation as Salem & Bryan. We find that CR pressure drives a low-density bubble beyond the edge of the shell swept up by thermal pressure, but the main bubble driven by SN thermal pressure overtakes it later, which creates a large-scale biconical outflow. CRs diffusing into the disk are unable to entrain its gas in the outflows, yielding a mass-loading rate of only $\sim0.1~\%$ with varied CR diffusion coefficients. We find no significant difference in mass-loading rates in SN driven outflows with or without CR pressure. Our simulations strongly suggest that it is hard to drive a heavily mass-loaded outflow with CRs from a massive halo potential, although more distributed star formation could lead to a different result.