The Spitzer mid-infrared AGN survey. II-the demographics and cosmic evolution of the AGN population

TL;DR

This study uses mid-infrared data from Spitzer to analyze the demographics and evolution of active galactic nuclei (AGN), revealing differences in obscured and unobscured populations and their contribution to cosmic radiation.

Contribution

It provides the first detailed luminosity functions of AGN selected via mid-infrared, showing their evolution and obscuration properties across redshifts, which was less accessible with optical or X-ray data.

Findings

AGN luminosity function follows a broken power-law with redshift-dependent break luminosity.

Obscured AGN fraction decreases with luminosity but remains around 50% at high redshift.

AGN contribute approximately 12% to the Universe's total radiation, with high radiative efficiency estimates.

Abstract

We present luminosity functions derived from a spectroscopic survey of AGN selected from Spitzer Space Telescope imaging surveys. Selection in the mid-infrared is significantly less affected by dust obscuration. We can thus compare the luminosity functions of the obscured and unobscured AGN in a more reliable fashion than by using optical or X-ray data alone. We find that the AGN luminosity function can be well described by a broken power-law model in which the break luminosity decreases with redshift. At high redshifts ($z>1.6$), we find significantly more AGN at a given bolometric luminosity than found by either optical quasar surveys or hard X-ray surveys. The fraction of obscured AGN decreases rapidly with increasing AGN luminosity, but, at least at high redshifts, appears to remain at $\approx 50$\% even at bolometric luminosities $\sim 10^{14}L_{\odot}$. The data support a picture in which the obscured and unobscured populations evolve differently, with some evidence that high luminosity obscured quasars peak in space density at a higher redshift than their unobscured counterparts. The amount of accretion energy in the Universe estimated from this work suggests that AGN contribute about 12\% to the total radiation intensity of the Universe, and a high radiative accretion efficiency $\approx 0.18^{+0.12}_{-0.07}$ is required to match current estimates of the local mass density in black holes.