A Comprehensive Study of Ly$\hat{I}\pm$ Emission in the High-redshift Galaxy Population

Abstract

We present an exhaustive census of Lyman alpha (Ly$\hat{I}\pm$) 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${}_{\ast }$) 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$\hat{I}\pm$ equivalent widths ($W_{Ly\hat{I}\pm }$) and escape fractions ($f_{esc}$) for our galaxies. We find both quantities to anti-correlate with M${}_{\ast }$, star-formation rate (SFR), UV luminosity, and UV slope (${\hat{I}}^2$). We then model the $W_{Ly\hat{I}\pm }$ distribution as a function of M${}_{UV}$ and ${\hat{I}}^2$ using a Bayesian approach. Based on our model and matching the properties of typical Lyman break galaxy (LBG) selections, we conclude that the $W_{Ly\hat{I}\pm }$ distribution in such samples is heavily dependent on the limiting M${}_{UV}$ of the survey. Regarding narrowband surveys, we find their $W_{Ly\hat{I}\pm }$ selections to bias samples toward low M${}_{\ast }$, while their line-flux limitations preferentially leave out low-SFR galaxies. We can also use our model to predict the fraction of Ly$\hat{I}\pm$-emitting LBGs at $4⩽z⩽7$. We show that reported drops in the Ly$\hat{I}\pm$ fraction at $z⩾6$, 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 $z\sim 7$, but highlights that current data is not inconsistent with this process taking place at $z>7$.

Accepted for publication in ApJ; Figure 3 updated