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Building on our previous cross-correlation analysis (Xia et al. 2011) betweenthe isotropic gamma-ray background (IGRB) and different tracers of thelarge-scale structure of the universe, we update our results using 60-months ofdata from the Large Area Telescope (LAT) on board the Fermi Gamma-ray SpaceTelescope. We perform a cross-correlation analysis between the IGRB and objectsthat may trace the astrophysical sources of the IGRB: SDSS-DR6 QSOs, theSDSS-DR8 Main Galaxy Sample, Luminous Red Galaxies (LRGs) in the SDSS catalog,2MASS galaxies, and radio NVSS galaxies. The benefit of correlating theFermi-LAT signal with catalogs of objects at various redshifts is to providetomographic information on the IGRB which is crucial to separate the variouscontributions 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), withSDSS galaxies. These results are robust against the choice of the statisticalestimator, estimate of errors, map cleaning procedure and instrumental effects.Finally, we test the hypothesis that the IGRB observed by Fermi-LAT originatesfrom the summed contributions of three types of unresolved extragalacticsources: BL Lacs, FSRQs and Star-Forming Galaxies (SFGs). We find that a modelin which the IGRB is mainly produced by SFGs ($72^{+23}_{-37}$% with 2 sigmaerrors), with BL Lacs and FSRQs giving a minor contribution, provides a goodfit to the data. We also consider a possible contribution from MisalignedActive Galactic Nuclei, and we find that, depending on the details of the modeland its uncertainty, they can also provide a substantial contribution, partlydegenerate with the SFG one. (abridged)

Tomography of the Fermi-LAT gamma-ray diffuse extragalactic signal via cross-correlations with galaxy catalogs