Gamma-ray emission from spectrally resolved cosmic rays in galaxies

Abstract Cosmic rays (CRs) are ubiquitous in the interstellar medium (ISM) of nearby galaxies, but many their properties not well-constrained. Gamma-ray observations provide a powerful tool this respect, allowing us to constrain both interaction CR protons with ISM and transport properties. To help better understand link between observational signatures physics, we use series magneto-hydrodynamical (MHD) Arepo simulations isolated galaxies performed using spectrally-resolved every computational cell, subsequent gamma-ray emission calculated Crayon+ (Cosmic RAY emissiON) code. In each our simulated halos, modelling energy-dependent spatial diffusion CRs leads more extended distribution high-energy (~100 GeV) gamma compared that predicted by ‘grey’ steady-state model, which is especially visible corresponding maps radial profiles. Despite this, total spectra can often be well approximated although recovering same spectral index typically requires minor variation energy dependence coefficient. Our reproduce observed shapes star-forming also match recent far infrared–gamma-ray relation. We find, however, spectrally resolved model yields marginally smaller luminosities for lower star formation rates grey CRs. work highlights importance an accurate prediction spatially emission, as will probed upcoming Cherenkov Telescope Array observatory.