Star-forming galaxies provide a larger contribution to the isotropic gamma-ray background than misaligned active galactic nuclei

The total extragalactic γ-ray flux provides a powerful probe into the origin and evolution of the highest energy processes in our universe. An important component of this emission is the isotropic γ-ray background (IGRB), composed of sources that cannot be individually resolved by current experiments. Previous studies have determined that the IGRB can be dominated by either misaligned active galactic nuclei (mAGN) or star-forming galaxies (SFGs). However, these analyses are limited, because they have utilized binary source classifications and examined only one source class at a time. We perform the first combined joint-likelihood analysis that simultaneously correlates the γ-ray luminosity of extragalactic objects with both star-formation and mAGN activity. We find that SFGs produce 48⁺³³_-20% of the total IGRB at 1 GeV and 56⁺⁴⁰_-23% of the total IGRB at 10 GeV. The contribution of mAGN is more uncertain, but can also be significant. Future work to quantify the radio and infrared properties of nearby galaxies could significantly improve these constraints.