Measurements of the z=4-10 X-ray Luminosity Function: the high space density of moderate-luminosity, obscured AGN

Abstract: SMBHs are theorised to undergo significant growth in the early Universe, however, the X-ray Luminosity Function (XLF), used as a principal tracer of the SMBH accretion density, lacks observational constraints at z>6, until now. We present new measurements of the z=4-10 XLF at intermediate luminosities, taking advantage of recent deep near-IR imaging from UltraVISTA that enables us to identify galaxies and AGN at high redshifts within which we identify X-ray sources using Chandra COSMOS data. We first performed a cross-match to a deep Chandra source list, for which the X-ray sensitivity can be accurately quantified, before exploiting available X-ray data further through direct extraction of X-ray counts at the positions of COSMOS2020 galaxies. With the resulting z=4-10 X-ray AGN sample, comprised of 21 blind detections and 11 directly extracted detections, we have measured the early space density of AGN, at moderate-luminosities where the majority of early SMBH growth occurred. These measurements reveal higher space-densities than expected, based on the extrapolation of XLF models from lower redshifts. Whilst our measured space densities at z=4-5 are consistent with model predictions, at z=5-7 we find space densities of the order of 10$\times$ the extrapolated model predictions and could be as high as 220$\times$ the model extrapolations at z=7-10. In addition, we find evidence that a large fraction of the early AGN population are heavily obscured, with an obscured fraction of 0.982$^{+0.007}_{-0.008}$; correcting for this obscuration further increases the measured space densities. Comparing to recent JWST results, these measurements begin to bridge the gap between the bright-end of the quasar luminosity function and the latest JWST observations of very early, low-luminosity AGN, indicating a larger fraction of the first galaxies play host to rapidly growing SMBH than previously thought.