The Isotropic gamma-ray Emission above 100 GeV: Where Do Very High-energy gamma-rays Come From?

Astrophysical sources of very high energy (VHE; >100 GeV) gamma-rays are rare, since GeV and TeV photons can be only emitted in extreme circumstances involving interactions of relativistic particles with local radiation and magnetic fields. In the context of the Fermi Large Area Telescope (LAT), only a few sources are known to be VHE emitters, where the largest fraction belongs to the rarest class of active galactic nuclei: the blazars. In this work, we explore Fermi-LAT data for energies >100 GeV and Galactic latitudes b > vertical bar 50 degrees vertical bar in order to probe the origin of the extragalactic isotropic gamma-ray emission. Since the production of such VHE photons requires very specific astrophysical conditions, we would expect that the majority of the VHE photons from the isotropic gamma-ray emission originate from blazars or other extreme objects like star-forming galaxies, gamma-ray bursts, and radio galaxies, and that the detection of a single VHE photon at the adopted Galactic latitudes would be enough to unambiguously trace the presence of such a counterpart. Our results suggest that blazars are, by far, the dominant class of sources above 100 GeV, although they account for only 22.8(-4.1)(+4.5)% of the extragalactic VHE photons. The remaining 77(-4.5)(+4.1)% of the VHE photons still have an unknown origin.