Non-thermal emission from star-forming galaxies

Abstract

Star-forming galaxies are a copious source of non-thermal radiation. This emission spans 20 orders of magnitude in photon energy, from ultrahigh energy gamma-rays at PeV energies to radio emission at mueV energies. Most of this emission is produced by radiative losses of charged relativistic particles, or cosmic rays, which were accel- erated in the shock waves of supernova remnants, amongst other sources. Cosmic rays are dynamically important for galaxy evolution in that they provide an impor- tant feedback mechanism for star-formation. While we can observe the local cosmic ray spectrum directly, to study their effects in regions of the Galaxy well beyond the Solar system or in external galaxies, we need to observe their non-thermal emission. This work shows how a galaxy model that requires only a few input parameters that are easily observed for high redshift objects, such as the star formation rate, stellar mass, half-light radius and redshift, is sufficient to reproduce their non-thermal spec- tra. The model achieves this by considering the energy losses cosmic rays suffer as they diffuse through the interstellar medium within the galactic disc and halo. This knowledge is then used to derive the contribution star-forming galaxies make to the diffuse isotropic gamma-ray background, explain the origin of the far-infrared radio and other empirical correlations, and simultaneously obtain radio spectral indices that match observation. This is achieved self-consistently and making 'standard' assump- tions for quantities like, e.g., the fraction of supernova mechanical energy that goes to CRs and the state of the interstellar medium, and without having to resort to fine- tuning of the model. The code developed in this work, CONGRuENTS (COsmic-ray, Neutrino, Gamma-ray and Radio Non-Thermal Spectra), is open source and available at github.com/astromatt42/CONGRuENTS.