Do current X-ray observations capture most of the black-hole accretion
  at high redshifts?
  The cosmic black hole accretion density (BHAD) is critical for our
understanding of the formation and evolution of supermassive black holes (BHs).
However, at high redshifts ($z>3$), X-ray observations report BHADs
significantly ($\sim 10$ times) lower than those predicted by cosmological
simulations. It is therefore paramount to constrain the high-$z$ BHAD using
independent methods other than direct X-ray detections. The recently
established relation between star formation rate and BH accretion rate among
bulge-dominated galaxies provides such a chance, as it enables an estimate of
the BHAD from the star-formation histories (SFHs) of lower-redshift objects.
Using the CANDELS Lyman-$\alpha$ Emission At Reionization (CLEAR) survey, we
model the SFHs for a sample of 108 bulge-dominated galaxies at $z=$0.7-1.5, and
further estimate the BHAD contributed by their high-$z$ progenitors. The
predicted BHAD at $z\approx 4$-5 is consistent with the simulation-predicted
values, but higher than the X-ray measurements (by $\approx$3-10 times at
$z=$4-5). Our result suggests that the current X-ray surveys could be missing
many heavily obscured Compton-thick active galactic nuclei (AGNs) at high
redshifts. However, this BHAD estimation assumes that the high-$z$ progenitors
of our $z=$0.7-1.5 sample remain bulge-dominated where star formation is
correlated with BH cold-gas accretion. Alternatively, our prediction could
signify a stark decline in the fraction of bulges in high-$z$ galaxies (with an
associated drop in BH accretion). JWST and Origins will resolve the discrepancy
between our predicted BHAD and the X-ray results by constraining Compton-thick
AGN and bulge evolution at high redshifts.
