The deepest X-ray view of high-redshift galaxies: constraints on
  low-rate black-hole accretion
  We exploit the 7 Ms \textit{Chandra} observations in the \chandra\,Deep
Field-South (\mbox{CDF-S}), the deepest X-ray survey to date, coupled with
CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076
galaxies at $3.5\leq z < 6.5$. This aim is achieved by stacking the
\textit{Chandra} data at the positions of optically selected galaxies, reaching
effective exposure times of $\geq10^9\mathrm{s}$. We detect significant
($>3.7\sigma$) X-ray emission from massive galaxies at $z\approx4$. We also
report the detection of massive galaxies at $z\approx5$ at a $99.7\%$
confidence level ($2.7\sigma$), the highest significance ever obtained for
X-ray emission from galaxies at such high redshifts. No significant signal is
detected from galaxies at even higher redshifts. The stacking results place
constraints on the BHAD associated with the known high-redshift galaxy samples,
as well as on the SFRD at high redshift, assuming a range of prescriptions for
X-ray emission due to X- ray binaries. We find that the X-ray emission from our
sample is likely dominated by processes related to star formation. Our results
show that low-rate mass accretion onto SMBHs in individually X-ray-undetected
galaxies is negligible, compared with the BHAD measured for samples of X-ray
detected AGN, for cosmic SMBH mass assembly at high redshift. We also place,
for the first time, constraints on the faint-end of the AGN X-ray luminosity
function ($\mathrm{logL_X\sim42}$) at $z>4$, with evidence for fairly flat
slopes. The implications of all of these findings are discussed in the context
of the evolution of the AGN population at high redshift.
