Tomography of the Fermi-LAT gamma-ray diffuse extragalactic signal via
  cross-correlations with galaxy catalogs
  Building on our previous cross-correlation analysis (Xia et al. 2011) between
the isotropic gamma-ray background (IGRB) and different tracers of the
large-scale structure of the universe, we update our results using 60-months of
data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space
Telescope. We perform a cross-correlation analysis between the IGRB and objects
that may trace the astrophysical sources of the IGRB: SDSS-DR6 QSOs, the
SDSS-DR8 Main Galaxy Sample, Luminous Red Galaxies (LRGs) in the SDSS catalog,
2MASS galaxies, and radio NVSS galaxies. The benefit of correlating the
Fermi-LAT signal with catalogs of objects at various redshifts is to provide
tomographic information on the IGRB which is crucial to separate the various
contributions and to clarify its origin. We observe a significant (>3.5 sigma)
cross-correlation signal on angular scales smaller than 1 deg in the NVSS,
2MASS and QSO cases and, at lower statistical significance (~3.0 sigma), with
SDSS galaxies. These results are robust against the choice of the statistical
estimator, estimate of errors, map cleaning procedure and instrumental effects.
Finally, we test the hypothesis that the IGRB observed by Fermi-LAT originates
from the summed contributions of three types of unresolved extragalactic
sources: BL Lacs, FSRQs and Star-Forming Galaxies (SFGs). We find that a model
in which the IGRB is mainly produced by SFGs ($72^{+23}_{-37}$% with 2 sigma
errors), with BL Lacs and FSRQs giving a minor contribution, provides a good
fit to the data. We also consider a possible contribution from Misaligned
Active Galactic Nuclei, and we find that, depending on the details of the model
and its uncertainty, they can also provide a substantial contribution, partly
degenerate with the SFG one. (abridged)
