The Galaxy UV Luminosity Function Before the Epoch of Reionization
  We present a model for the evolution of the galaxy ultraviolet (UV)
luminosity function (LF) across cosmic time where star formation is linked to
the assembly of dark matter halos under the assumption of a mass dependent, but
redshift independent, efficiency. We introduce a new self-consistent treatment
of the halo star formation history, which allows us to make predictions at
$z>10$ (lookback time $\lesssim500$ Myr), when growth is rapid. With a
calibration at a single redshift to set the stellar-to-halo mass ratio, and no
further degrees of freedom, our model captures the evolution of the UV LF over
all available observations ($0\lesssim z\lesssim10$). The significant drop in
luminosity density of currently detectable galaxies beyond $z\sim8$ is
explained by a shift of star formation toward less massive, fainter galaxies.
Assuming that star formation proceeds down to atomic cooling halos, we derive a
reionization optical depth $\tau = 0.056^{+0.007}_{-0.010}$, fully consistent
with the latest Planck measurement, implying that the universe is fully
reionized at $z=7.84^{+0.65}_{-0.98}$. In addition, our model naturally
produces smoothly rising star formation histories for galaxies with $L\lesssim
L_*$ in agreement with observations and hydrodynamical simulations. Before the
epoch of reionization at $z>10$ we predict the LF to remain well-described by a
Schechter function, but with an increasingly steep faint-end slope
($\alpha\sim-3.5$ at $z\sim16$). Finally, we construct forecasts for surveys
with \JWST~and \WFIRST and predict that galaxies out to $z\sim14$ will be
observed. Galaxies at $z>15$ will likely be accessible to JWST and WFIRST only
through the assistance of strong lensing magnification.
