Tomographic Imaging of the Fermi-LAT γ-Ray Sky through Cross-correlations: A Wider and Deeper Look

We investigate the nature of the extragalactic unresolved γ-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. First, the use of a larger γ-ray data set allows us to investigate the energy dependence of the cross-correlation in more detail, using up to eight energy bins over a wide energy range of [0.25,500] GeV. Second, we consider larger and deeper catalogs (2MASS Photometric Redshift catalog, 2MPZ; WISE × SuperCOSMOS, WI×SC; and SDSS DR12 photometric redshift data set) in addition to the ones employed in the previous studies (NVSS and SDSS QSOs). Third, 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 signals between the UGRB maps and all the catalogs considered, on angular scales smaller than 1°. Significances range from 16.3σ for NVSS 7σ, for SDSS DR12 and WI×SC, to 5σ for 2MPZ and for 4σ SDSS QSOs. Furthermore, including redshift tomography, the significance of the SDSS DR12 signal strikingly rises up to ∼12σ and that of WI×SC to ∼10.6σ. 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 γ-ray sources contributing to the UGRB.