On the Connection between Supermassive Black Hole and Galaxy Growth in
  the Reionization Epoch
  The correlation between the mass of supermassive black holes (SMBHs;
$\mathcal{M}_{\rm BH}$) and their host galaxies ($\mathcal{M}_\star$) in the
reionization epoch provides valuable constraints on how their growth are
jointly assembled in the early universe. High-redshift quasars typically have a
$\mathcal{M}_{\rm BH}$/$\mathcal{M}_\star$ ratio significantly elevated in
comparison to the local value. However, it is unclear by how much this apparent
offset is driven by observational biases for the most distant quasars. To
address this issue, we model the sample selection and measurement biases for a
compilation of 20 quasars at $z\sim6$ with host properties based on ALMA
observations. We find that the observed distribution of quasars in the
$\mathcal{M}_{\rm BH} - \mathcal{M}_\star$ plane could be fully reproduced by
assuming that the underlying SMBH populations at $z\sim6$ follow the
relationship in the local universe. However, a positive or even a negative
evolution in $\mathcal{M}_{\rm BH}$/$\mathcal{M}_\star$ can also explain the
data, depending on how the intrinsic scatter evolves and the strength of
various systematic uncertainties. This imposes critical requirements on
improving the accuracy of mass measurements and expanding the current sample to
lower $\mathcal{M}_{\rm BH}$ limits to break degeneracies. Interestingly,
quasars that are significant outliers in $\mathcal{M}_{\rm
BH}$/$\mathcal{M}_\star$ tend to move towards the local relation given their
instantaneous BH accretion rate and star formation rate. This may provide
tentative evidence that a self-regulated SMBH-galaxy coevolution scenario is
already in place at $z\sim6$, with AGN feedback being a possible driver.
