High-energy emission from star-forming galaxies

TL;DR

This study models high-energy emissions from starburst galaxies M82 and NGC253, showing that cosmic-ray interactions produce gamma-ray fluxes consistent with observations and revealing elevated cosmic-ray energy densities compared to typical galaxies.

Contribution

It introduces a detailed numerical model for cosmic-ray propagation and emission in starburst galaxies, linking radio and gamma-ray data to cosmic-ray energy densities and star formation rates.

Findings

Predicted gamma-ray fluxes match Fermi, VERITAS, and HESS observations.

Cosmic-ray energy densities are about ten times higher than in the Milky Way.

Gamma-ray luminosity scales with star formation rate as SFR^1.4.

Abstract

Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, the steady-state energy distributions of these particles in the starburst galaxies M82 and NGC253 can be determined with a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region, and a radial profile is assumed for the magnetic field. The resulting radiative yields of electrons and protons are calculated: the predicted >100MeV and >100GeV fluxes are in agreement with the corresponding quantities measured with the orbiting Fermi telescope and the ground-based VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in central regions of starburst galaxies, as inferred from the radio and gamma-ray measurements of (respectively) non-thermal synchrotron and neutral-pion-decay emission, are U=O(100) eV/cm3, i.e. at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. These very different energy density levels reflect a similar disparity in the respective supernova rates in the two environments. A L(gamma) ~ SFR^(1.4) relationship is then predicted, in agreement with preliminary observational evidence.