X-ray evolution of active galactic nuclei and hierarchical galaxy formation

We have incorporated the description of the X-ray properties of active galactic nuclei (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 10(43.5) < L-X/ergs s(-1) < 10(44..5), with 50% of the total specific intensity produced at z < 2. The predicted number density of X-ray luminous AGNs (L-X > 10(44.5) ergs s(-1) in the 2-10 keV band) peaks at z approximate to 2, with a decline of around 3 dex to z = 0; for the low-luminosity sources (10(43) < L-X = ergs s(-1) < 10(44)), it has a broader and less pronounced maximum around z approximate to 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 similar to 1.5, with the discrepancy progressively extending to intermediate-luminosity objects at higher redshifts; we discuss possible origins of 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.