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

Abstract

Star-forming galaxies (SFGs) have been established as an important source population in the extra-galactic 𝛾-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 𝛾-rays. SFGs can operate as CR proton "calorimeters", where a large fraction of the CR energy is converted to 𝛾-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.

ICRC 2023 Rapporteur: Status of the Gamma-ray Astronomy field

“…The origin of their emission is thought to be due to the strong winds from the stars contained in the galaxy. In this conference, we had an update on the M82 detection by VERITAS [134] and also the proposal of PWNe as the origin of the 𝛾-ray emission and their non-negligible contribution to the Extragalactic Gamma-Ray Background [135]…”