Consistent analysis of the AGN LF in X-ray and MIR in the XMM-LSS field

TL;DR

This study presents a comprehensive, bias-mitigated analysis of AGN luminosity functions in X-ray and MIR in the XMM-LSS field, revealing new insights into black hole growth and obscured AGN contributions across cosmic time.

Contribution

It provides the largest combined census of AGN in X-ray and MIR, with the first observation of the IRLF knee at z>2 and a detailed comparison of luminosity densities.

Findings

The IRLF knee is observed at z>2.

The AGN luminosity density peaks at z≈2.25.

Obscured AGN activity dominates cosmic black hole growth.

Abstract

The luminosity function (LF) of active galactic nuclei (AGN) probes the history of supermassive black hole assembly and growth across cosmic time. To mitigate selection biases, we present a consistent analysis of the AGN LFs derived for both X-ray and mid-infrared (MIR) selected AGN in the XMM-Large Scale Structure (XMM-LSS) field. There are 4268 AGN used to construct the MIR luminosity function (IRLF) and 3427 AGN used to construct the X-ray luminosity function (XLF), providing the largest census of the AGN population out to $z=4$ in both bands with significant reduction in uncertainties. We are able for the first time to see the knee of the IRLF at $z>2$ and observe a flattening of the faint-end slope as redshift increases. The bolometric luminosity density, a proxy for the cosmic black hole accretion history, computed from our LFs shows a peak at $z\approx2.25$ consistent with recent estimates of the peak in the star formation rate density (SFRD). However, at earlier epochs, the AGN luminosity density is flatter than the SFRD. If confirmed, this result suggests that the build up of black hole mass outpaces the growth of stellar mass in high mass systems at $z\gtrsim 2.5$. This is consistent with observations of redshift $z\sim 6$ quasars which lie above the local $M-\sigma$ relationship. The luminosity density derived from the IRLF is higher than that from the XLF at all redshifts. This is consistent with the dominant role of obscured AGN activity in the cosmic growth of supermassive black holes.