A Comprehensive Study of Ly$\alpha$ Emission in the High-redshift Galaxy
  Population
  We present an exhaustive census of Lyman alpha (Ly$\alpha$) emission in the
general galaxy population at $3<z<4.6$. We use the Michigan/Magellan Fiber
System (M2FS) spectrograph to study a stellar mass (M$_*$) selected sample of
625 galaxies homogeneously distributed in the range
$7.6<\log{\mbox{M$_*$/M$_{\odot}$}}<10.6$. Our sample is selected from the
3D-HST/CANDELS survey, which provides the complementary data to estimate
Ly$\alpha$ equivalent widths ($W_{Ly\alpha}$) and escape fractions ($f_{esc}$)
for our galaxies. We find both quantities to anti-correlate with M$_*$,
star-formation rate (SFR), UV luminosity, and UV slope ($\beta$). We then model
the $W_{Ly\alpha}$ distribution as a function of M$_{UV}$ and $\beta$ using a
Bayesian approach. Based on our model and matching the properties of typical
Lyman break galaxy (LBG) selections, we conclude that the $W_{Ly\alpha}$
distribution in such samples is heavily dependent on the limiting M$_{UV}$ of
the survey. Regarding narrowband surveys, we find their $W_{Ly\alpha}$
selections to bias samples toward low M$_*$, while their line-flux limitations
preferentially leave out low-SFR galaxies. We can also use our model to predict
the fraction of Ly$\alpha$-emitting LBGs at $4\leqslant z\leqslant 7$. We show
that reported drops in the Ly$\alpha$ fraction at $z\geqslant6$, usually
attributed to the rapidly increasing neutral gas fraction of the universe, can
also be explained by survey M$_{UV}$ incompleteness. This result does not
dismiss reionization occurring at $z\sim7$, but highlights that current data is
not inconsistent with this process taking place at $z>7$.
