A Comprehensive Study of Ly$\alpha$ Emission in the High-redshift Galaxy Population

Abstract

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 $z>7$.