Affiliation: The University of Edinburgh

Contribution: Oral

Title: Constraints on the X-ray Luminosity Function of AGN at z=4-10

Abstract: AGN play an important role within their host galaxies, influencing their evolution and growth. Little is known about the initial formation and evolution of AGN in the early Universe. However, new observations by JWST are unveiling larger populations of these sources than previously expected, indicating lower luminosity AGN may be more common in the z>6 Universe, when significant galaxy build up is occurring. In order to obtain robust and extensive constraints on this early population of AGN we need measurements of the AGN X-ray Luminosity Function (XLF), which traces the growth of the AGN with cosmic time, particularly for the intermediate luminosities that dominate the blackhole mass assembly at z>6. I will present new measurements of the high-redshift XLF using Chandra-selected AGN within the COSMOS2020 galaxy survey. We identified counterparts to X-ray sources and pushed the limit of Chandras sensitivity to identify sources below the normal sensitivity and obtain X-ray constraints in the very high redshift regime. We are thus able to place constraints on the space density of moderate-luminosity AGN out to z~10. Our measurements reveal higher space-densities than expected, based on the extrapolation of XLF models from lower redshifts. Furthermore, we find evidence that a large fraction of the early AGN population are heavily obscured; correcting for this obscuration further increases our measured space densities at moderate X-ray luminosities. Our constraints begin to bridge the gap between the bright-end of the QLF and the latest JWST observations of very early, low-luminosity AGN, indicating that a larger fraction of the first galaxies likely play host to a rapidly growing SMBH than previously thought.

This contribution can be found here (pdf).