2020ApJ...891...69C

We search for high-redshift (z > 4.5) X-ray active galactic nuclei (AGNs) in the deep central (off-axis angle < 5.'7) region of the 7 Ms Chandra Deep Field-South X-ray image. We compile an initial candidate sample from direct X-ray detections. We then probe more deeply in the X-ray data by using preselected samples with high spatial resolution near-infrared (NIR)/mid-infrared (MIR) (Hubble Space Telescope (HST) 1.6 µm and Spitzer 4.5 µm) and submillimeter (ALMA 850 µm) observations. The combination of the NIR/MIR and submillimeter preselections allows us to find X-ray sources with a wide range of dust properties and spectral energy distributions (SEDs). We use the SEDs from the optical to the submillimeter to determine if previous photometric redshifts were plausible. Only five possible z > 5 X-ray AGNs are found, all of which might also lie at lower redshifts. If they do lie at high redshifts, then two are Compton-thick AGNs. Three of the five are ALMA 850 µm sources, including the two Compton-thick AGN candidates. We find that (i) the number density of X-ray AGN drops rapidly at high redshifts, (ii) the detected AGNs do not contribute significantly to photoionization at z > 5, and (iii) the measured X-ray light density over z = 5-10 implies a very low black hole accretion density with very little growth in the black hole mass density in this redshift range.