Interstellar gamma-ray emission from cosmic rays in star-forming galaxies

Abstract

Fermi/LAT observations of star-forming galaxies in the ~0.1-100GeV range have made possible a first population study. Evidence was found for a correlation between gamma-ray luminosity and tracers of the star formation activity. Studying galactic cosmic rays (CRs) in various global conditions can yield information about their origin and transport in the interstellar medium (ISM). This work addresses the question of the scaling laws that can be expected for the interstellar gamma-ray emission as a function of global galactic properties, with the goal of establishing whether the current experimental data in the GeV range can be constraining. I developed a 2D model for the non-thermal emissions from steady-state CR populations interacting with the ISM in star-forming galaxies. Most CR-related parameters were taken from Milky Way studies, and a large number of galaxies were then simulated with sizes from 4 to 40kpc, several gas distributions, and star formation rates (SFR) covering six orders of magnitude. The evolution of the gamma-ray luminosity over the 100keV-100TeV range is presented, with emphasis on the contribution of the different emission processes and particle populations, and on the transition between transport regimes. The model can reproduce the normalisation and trend inferred from the Fermi/LAT population study over most of the SFR range. This is obtained with a plain diffusion scheme, a single diffusion coefficient, and the assumption that CRs experience large-scale volume-averaged interstellar conditions. There is, however, no universal relation between high-energy gamma-ray luminosity and star formation activity, as illustrated by the scatter introduced by different galactic global properties and the downturn in gamma-ray emission at the low end (abridged).