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We investigate the nature of the extragalactic unresolved gamma-raybackground (UGRB) by cross-correlating several galaxy catalogs with sky-maps ofthe UGRB built from 78 months of Pass 8 Fermi-Large Area Telescope data. Thisstudy updates and improves similar previous analyses in several aspects.Firstly, the use of a larger gamma-ray dataset allows us to investigate theenergy dependence of the cross-correlation in more detail, using up to 8 energybins over a wide energy range of [0.25-500] GeV. Secondly, we consider largerand deeper catalogs (2MASS Photometric-Redshift catalog, 2MPZ; WISE xSuperCOSMOS, WISC; and SDSS-DR12 photometric-redshift dataset) in addition tothe ones employed in the previous studies (NVSS and SDSS-QSOs). Thirdly, weexploit the redshift information available for the above catalogs to dividethem into redshift bins and perform the cross-correlation separately in each ofthem.  Our results confirm, with higher statistical significance, the detection ofcross-correlation signal between the UGRB maps and all the catalogs considered,on angular scales smaller than 1 degree. Significances range from 16.3 sigmafor NVSS, 7 sigma for SDSS-DR12 and WISC, 5 sigma for 2MPZ and 4 sigma forSDSS-QSOs. Furthermore, including redshift tomography, the significance of theSDSS-DR12 signal strikingly rises up to 12 sigma and the one of WISC to 10.6sigma. We offer a simple interpretation of the signal in the framework of thehalo model. The precise redshift and energy information allows us to clearlydetect a change over redshift in the spectral and clustering behavior of thegamma-ray sources contributing to the UGRB.

Tomographic imaging of the Fermi-LAT gamma-ray sky through cross-correlations: A wider and deeper look