Galaxies at a Cosmic-Ray Eddington Limit

TL;DR

This paper investigates the cosmic ray Eddington limit in galaxies, finding that cosmic rays are unlikely to be the primary factor limiting starburst galaxy luminosity, through a new framework applied to five galaxies.

Contribution

It introduces a general framework for the cosmic ray Eddington limit considering the circumgalactic environment and applies it to real galaxies with different cosmic ray transport models.

Findings

The cosmic ray Eddington limit exists but is not reached in studied galaxies.

Limit values are higher than actual star formation rates and gas densities.

Cosmic ray pressure is unlikely to be the main limiting factor in starburst galaxies.

Abstract

Cosmic rays have been shown to be extremely important in the dynamics of diffuse gas in galaxies, helping to maintain hydrostatic equilibrium, and serving as a regulating force in star formation. In this paper, we address the influence of cosmic rays on galaxies by re-examining the theory of a cosmic ray Eddington limit, first proposed by Socrates et al. (2008) and elaborated upon by Crocker et al. (2021a) and Huang & Davis (2022). A cosmic ray Eddington limit represents a maximum cosmic ray energy density above which the interstellar gas cannot be in hydrostatic equilibrium, resulting in a wind. In this paper, we continue to explore the idea of a cosmic ray Eddington limit by introducing a general framework that accounts for the circumgalactic environment and applying it to five galaxies that we believe to be a good representative sample of the star forming galaxy population, using different cosmic ray transport models to determine what gives each galaxy the best chance to reach this limit. We show that while an Eddington limit for cosmic rays does exist, for our five galaxies, the limit either falls at star formation rates that are much larger or gas densities that are much lower than each galaxy's measured values. This suggests that cosmic ray pressure is not the main factor limiting the luminosity of starburst galaxies.