It is Feasible to Directly Measure Black Hole Masses in the First
  Galaxies
  In the local universe, black hole masses have been inferred from the observed
increase in the velocities of stars at the centres of their host galaxies. So
far, masses of supermassive black holes in the early universe have only been
inferred indirectly, using relationships calibrated to their locally observed
counterparts. Here, we use the latest observational constraints on the
evolution of stellar masses in galaxies to predict, for the first time, that
the region of influence of a central supermassive black hole at the epochs
where the first galaxies were formed is $\mathit{directly \ resolvable}$ by
current and upcoming telescopes. We show that the existence of the black hole
can be inferred from observations of the gas or stellar disc out to $> 0.5$ kpc
from the host halo at redshifts $z \gtrsim 6$. Such measurements will usher in
a new era of discoveries unraveling the formation of the first supermassive
black holes based on subarcsecond-scale spectroscopy with the $JWST$, $ALMA$,
and the $SKA$. The measured mass distribution of black holes will allow
forecasting of the future detection of gravitational waves from the earliest
black hole mergers.
