On the Absence of High-Redshift AGNs: Little Growth in the Supermassive
  Black Hole Population at High Redshifts
  We search for high-redshift (z>4.5) X-ray 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 pre-selected samples with high spatial
resolution NIR/MIR (HST 1.6 micron and Spitzer 4.5 micron) and submillimeter
(ALMA 850 micron) observations. The combination of the NIR/MIR and
submillimeter pre-selections 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, and three are ALMA 850 micron sources. We find that
(i) the number density of X-ray AGNs is dropping rapidly at high redshifts,
(ii) the detected AGNs do not contribute significantly to the 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.
