Hirschmann Michaela, Dolag Klaus, Saro Alexandro, Borgani Stefano, Burkert Andreas
In this study, we present a detailed, statistical analysis of black hole (BH) growth and the evolution of active galactic nuclei (AGN) using cosmological hydrodynamic simulations run down to z=0. The simulations self-consistently follow radiative cooling, star formation, metal enrichment, BH growth and associated feedback processes from both supernovae typeII/Ia and AGN. We consider two simulation runs, one with a large co-moving volume of (128 Mpc/h)^3 and one with a smaller volume of (48 Mpc/h)^3 but with a higher mass resolution. Consistently with previous results, our simulations are in reasonably good agreement with BH properties of the local Universe. Furthermore, they can successfully reproduce the evolution of the bolometric AGN luminosity function for both the low- and the high-luminosity end up to z=2.5. The smaller but higher resolution run can match the observational data of the low bolometric luminosity end even up to z=4-5. We also perform a direct comparison with the observed soft and hard X-ray luminosity functions of AGN, including an empirical correction for a torus-level obscuration, and find a similarly good agreement. These results show that our simulations can self-consistently predict the observed "downsizing" trend in the AGN number density evolution, i.e. the number densities of luminous AGN peak at higher redshifts than those of faint AGN. Implications of the downsizing behaviour on active BHs, their masses and Eddington-ratios are discussed. Overall, the downsizing behaviour in the AGN number density can be attributed to a combination of the gas density evolution in the resolved vicinity of a (massive) black hole (which is depleted with evolving time mainly as a consequence of the radio-mode feedback) and to the decreasing mean relative velocities between the (low mass) black holes and the surrounding gas with decreasing redshift.
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http://arxiv.org/abs/1308.0333
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