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