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.