Star-Forming Galaxies Significantly Contribute to the Isotropic Gamma-Ray Background

Abstract

The origin of the isotropic gamma-ray background (IGRB) --- the portion of the extragalactic gamma-ray sky that is not resolvable into individual point sources --- provides a powerful probe into the evolution of the high-energy universe. Star-forming galaxies (SFGs) are among the most likely contributors to the IGRB, though their contribution is difficult to constrain because their flux distribution is dominated by numerous faint sources. We produce a novel joint-likelihood analysis of the $\gamma$-ray emission from 584 SFGs, utilizing advanced statistical techniques to compare the distribution of low-significance excesses against the non-Poissonian $\gamma$-ray background fluctuations. We first examine the theoretically well-motivated relationship between the far-IR and $\gamma$-ray luminosities of SFGs, utilizing a model where the $\gamma$-ray luminosity is given by log${}_{10}$(L${}_{\gamma }$/(erg s${}^{-1}$)) = $\alpha$ log${}_{10}$(L${}_{IR}$/(10${}^{10}$L${}_{\odot }$)) + $\beta$. We calculate best-fit parameters $\alpha$ = 1.18 $\pm$ 0.15, $\beta$ = 38.49 $\pm$ 0.24, with a log-normal dispersion in this relationship given by $\sigma$ = 0.39 $\pm$ 0.12. The best-fit values of $\alpha$ and $\beta$ are consistent with previous studies. We find a larger dispersion in the far-IR to $\gamma$-ray correlation than previous studies. This dispersion is significant at the level of 5.7$\sigma$. These results imply that SFGs significantly contribute to the IGRB, producing between 61.0${}_{-18.3}^{+30.2}\%$ of the total IGRB intensity above an energy of 1 GeV. Along with recent works, this strongly indicates that multiple source classes provide comparable contributions to the IGRB intensity. We discuss the implication of these results for the interpretation of the IceCube neutrinos.

16 pages, 13 figures

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