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

C. L. Barlow-Hall, J. Aird

First listed 2025-06-19 | Last updated 2025-06-19

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.

Short digest

The authors measure the z=4–10 X-ray luminosity function at intermediate luminosities by combining UltraVISTA-selected COSMOS2020 galaxies with Chandra COSMOS imaging, using both blind detections and forced X-ray extraction (21 blind + 11 extracted). They find AGN space densities that exceed low‑z XLF extrapolations: consistent at z=4–5, but ~10× higher at z=5–7 and potentially up to ~220× at z=7–10. The sample indicates a dominantly obscured early AGN population with an obscured fraction of 0.982(+0.007/−0.008), and correcting for this further boosts the inferred densities. These results bridge bright-end quasar counts and JWST detections of faint AGN, implying a larger fraction of early galaxies hosted rapidly growing SMBHs than previously thought.

Key figures to inspect

• Figure 1: Inspect the hard/full-band area curves to see the survey selection function—how sky coverage versus count rate (and the flux conversion assumptions) sets the accessible Lx range that underpins the z=4–10 XLF bins.
• Figure 2: Compare false-probability histograms for blind versus extracted sources and the high‑z subset to understand contamination control, the effect of masking around blind detections, and where the adopted thresholds place the high‑z candidates.
• Figure 3: Follow how the false fraction changes with the chosen probability threshold for the combined catalog; this justifies the final threshold that targets ~5% false positives among high‑z sources and sets the reliability of the extracted detections.
• Figure 4: Examine rest‑frame 2–10 keV Lx versus redshift with both galaxy- and AGN-template photo‑z fits; note which points would drop out of the high‑z sample and how conservative vs optimistic selections map onto the moderate‑Lx regime used for the XLF.