Cosmic ray calorimetry in star-forming galaxy populations and implications for their contribution to the extra-galactic $\gamma$-ray background

TL;DR

This paper investigates how cosmic ray-driven outflows in star-forming galaxies affect their ability to produce gamma rays and contribute to the extragalactic gamma-ray background, using self-consistent 1D numerical models.

Contribution

It introduces a self-consistent 1D modeling approach to assess cosmic ray calorimetry in star-forming galaxies considering outflows and their impact on gamma-ray production.

Findings

CR-driven outflows can significantly reduce galaxy calorimetry.

Certain galaxy properties enhance gamma-ray contributions.

Outflows modify the gamma-ray output and EGB contribution.

Abstract

Star-forming galaxies (SFGs) have been established as an important source population in the extra-galactic $\gamma$-ray background (EGB). Their intensive star-formation creates an abundance of environments able to accelerate particles, and these build-up a rich sea of cosmic rays (CRs). Above GeV energies, CR protons can undergo hadronic interactions with their environment to produce $\gamma$-rays. SFGs can operate as CR proton "calorimeters", where a large fraction of the CR energy is converted to $\gamma$-rays. However, CRs also deposit energy and momentum to modify the thermal and hydrodynamic conditions of the gas in SFGs, and can become a powerful driver of outflows. Such outflows are ubiquitous among some types of SFGs, and have the potential to severely degrade their CR proton calorimetry. This diminishes their contribution to the EGB. In this work, we adopt a self-consistent treatment of particle transport in outflows from SFGs to assess their calorimetry. We use 1D numerical treatments of galactic outflows driven by CRs and thermal gas pressure, accounting for the dynamical effects and interactions of CRs. We show the impact CR-driven flows have on the relative contribution of SFG populations to the EGB, and investigate the properties of SFGs that contribute most strongly.