Astrophysical interpretation of the anisotropies in the unresolved
  gamma-ray background
  Recently, a new measurement of the auto- and cross-correlation angular power
spectrum (APS) of the isotropic gamma-ray background was performed, based on 81
months of data of the Fermi Large-Area Telescope (LAT). Here, we fit, for the
first time, the new APS data with a model describing the emission of unresolved
blazars. These sources are expected to dominate the anisotropy signal. The
model we employ in our analysis reproduces well the blazars resolved by Fermi
LAT. When considering the APS obtained by masking the sources in the 3FGL
catalogue, we find that unresolved blazars under-produce the measured APS below
$\sim$1 GeV. Contrary to past results, this suggests the presence of a new
contribution to the low-energy APS, with a significance of, at least,
5$\sigma$. The excess can be ascribed to a new class of faint gamma-ray
emitters. If we consider the APS obtained by masking the sources in the 2FGL
catalogue, there is no under-production of the APS below 1 GeV, but the new
source class is still preferred over the blazars-only scenario (with a
significance larger than 10$\sigma$). The properties of the new source class
and the level of anisotropies induced in the isotropic gamma-ray background are
the same, independent of the APS data used. In particular, the new gamma-ray
emitters must have a soft energy spectrum, with a spectral index ranging,
approximately, from 2.7 to 3.2. This complicates their interpretation in terms
of known sources, since, normally, star-forming and radio galaxies are observed
with a harder spectrum. The new source class identified here is also expected
to contribute significantly to the intensity of the isotropic gamma-ray
background.
