Contribution from Star-Forming Galaxies to the Cosmic Gamma-Ray Background Radiation

TL;DR

This paper models the contribution of star-forming galaxies to the cosmic gamma-ray background using hierarchical galaxy formation, predicting their share and comparing it with Fermi observations, accounting for uncertainties and absorption effects.

Contribution

It introduces a new theoretical model linking gamma-ray luminosity to galaxy properties and estimates the contribution of star-forming galaxies to the EGRB, aligning with recent Fermi data.

Findings

Star-forming galaxies contribute about 4-7% to the EGRB.

Predicted galaxy detections are consistent with Fermi observations.

Intergalactic absorption modestly reduces EGRB flux at high energies.

Abstract

We present a new theoretical calculation of the contribution to the extragalactic gamma-ray background radiation (EGRB) from star-forming galaxies, based on a state-of-the-art model of hierarchical galaxy formation that is in quantitative agreement with a variety of observations of local and high-redshift galaxies. Gamma-ray luminosity ($L_\gamma$) and spectrum of galaxies are related to star formation rate ($\psi$), gas mass ($M_gas$), and star formation mode (quiescent or starburst) of model galaxies using latest observed data of nearby galaxies. We try the two limiting cases about gamma-ray production: the escape limit ($L_\gamma \propto \psi M_gas$) and the calorimetric limit ($L_\gamma \propto \psi$), and our standard model predicts 7 and 4% contribution from star-forming galaxies to the total EGRB flux (including bright resolved sources) recently reported by the Fermi Gamma-Ray Space Telescope. Systematic uncertainties do not allow us to determine the EGRB flux better than by a factor of ~2. The predicted number of nearby galaxies detectable by Fermi is consistent with the observation. Intergalactic absorption by pair-production attenuates the EGRB flux only by a modest factor of ~1.3 at the highest Fermi energy band, and the reprocessed cascade emission does not significantly alter EGRB at lower photon energies. The sum of the known contributions from AGNs and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.