X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation

We have incorporated the description of the X-ray properties of aActive gGalactic nNuclei (AGNs) into a semi-analytic model of galaxy formation, adopting physically motivated scaling laws for accretion triggered by galaxy encounters. Our model reproduces the level of the cosmic X-ray background at 30 keV; we predict that the largest contribution (around two-thirds) comes from sources with intermediate X-ray luminosity 1043.5<LX/ergss-1<1044.5, with 50% of the total specific intensity produced at z<2. The predicted number density of luminous X-ray-luminous AGNs (LX>1044.5 ergs s-1 in the 2-10 keV band) peaks at z~2, with a decline of around 3 dex to z=0; for the low- luminosity sources (1043<LX/ergss-1<1044), it has a broader and less pronounced maximum around z~1.5. The comparison with the data shows a generally good agreement. The model predictions slightly exceed the observed number of low-luminosity AGNs at z~1.5, with the discrepancy progressively extending to intermediate-luminosity objects at higher redshifts; we discuss possible origins offor the mismatch. Finally, we predict the source counts and the flux distribution at different redshifts in the hard (20--100 keV) X-ray band for the sources contributing to the X- ray background.