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)