The Starburst Contribution to the Extragalactic γ‐Ray Background

Cosmic ray protons interacting with gas at the mean density of theinterstellar medium in starburst galaxies lose energy rapidly via inelasticcollisions with ambient nuclei. The resulting pions produce secondary electronsand positrons, high-energy neutrinos, and gamma-ray photons. We estimate thecumulative gamma-ray emission from starburst galaxies. We find a totalintegrated background above 100 MeV of F_gamma ~ 10^{-6} GeV/cm^2/s/sr and acorresponding specific intensity at GeV energies of nuI_nu ~ 10^{-7}GeV/cm^2/s/sr. Starbursts may thus account for a significant fraction of theextra-galactic $\gamma$-ray background. We show that the FIR-radio correlationprovides a strong constraint on the gamma-ray emission from starburst galaxiesbecause pions decay into both gamma-rays and radio-emitting electron/positronpairs. We identify several nearby systems where the potential for observinggamma-ray emission is the most favorable (M82, NGC 253, and IC 342), predicttheir fluxes, and predict a linear FIR-gamma-ray correlation for the denseststarbursts. If established, the FIR-gamma-ray correlation would provide strongevidence for the ``calorimeter'' theory of the FIR-radio correlation and wouldimply that cosmic rays in starburst galaxies interact with gas at approximatelythe mean density of the interstellar medium (ISM), thereby providing animportant constraint on the physics of the ISM in starbursts.Comment: text revised and updated in response to referee's comments, 7 pages, 1 table, 1 figure, emulateap