Dartmouth Events

Abstract: At the center of essentially every massive galaxy is a monstrous black hole producing luminous radiation driven by the accretion of gas. By observing these active galactic nuclei (AGN) we may trace the growth of black holes across cosmic time. However, our knowledge of the full underlying AGN population is hindered by complex observational biases. The optical waveband, in particular, can be strongly influenced by selection effects and dilution from host galaxy star formation. Using a novel approach to simulate the impact of selection effects on multi-wavelength observations, we find the Eddington ratio distribution for optically-selected AGN is consistent with a broad power-law, as seen in the X-rays. This suggests that a universal Eddington ratio distribution may be enough to describe the full multi-wavelength AGN population independent of host galaxy type or age.

Building on these results, we have expanded a semi-numerical galaxy formation simulation to include this straightforward prescription for AGN accretion.  This method describes the full AGN population by independently connecting galaxy and AGN growth to the evolution of the host dark matter halos, which allows us to look for observational evidence of co-evolution and the characteristics of host galaxies that facilitate AGN activity. We find that a simple model for AGN accretion can broadly reproduce the host galaxies and dark matter halos of X-ray AGN. Furthermore, we find that the Eddington ratio distribution evolves with redshift, consistent with the behavior predicted by hydrodynamic simulations, and that different AGN selection techniques yield samples with very different host galaxy properties.

From our successful model of the full population of AGN, we have built synthetic X-ray spectral energy distributions to investigate the origin of the cosmic X-ray background. With complete information of the underlying AGN, galaxy, and dark matter halos from this simulation, we probe the CXB as a function of observables to uncover properties of the growing black hole population contributing to this background emission.