The HETDEX Survey: The Ly$\alpha$ Escape Fraction from 3D-HST Emission Line Galaxies at $z \sim 2$

TL;DR

This study measures the Ly$ ext{alpha}$ escape fraction in 935 emission-line galaxies at redshift around 2, revealing its dependence on galaxy properties and primarily on stellar mass and internal extinction.

Contribution

It provides the first large sample measurement of Ly$ ext{alpha}$ escape fraction at $z\, extasciitilde 2$ using HST and ground-based spectroscopy, highlighting key controlling factors.

Findings

Ly$ ext{alpha}$ escape fraction decreases with increasing stellar mass.

Most Ly$ ext{alpha}$ escape variation is driven by stellar mass and internal extinction.

Mean escape fraction is approximately 6%.

Abstract

We measure the Ly$\alpha$ escape fraction of 935 [OIII]-emitting galaxies between $1.9 < z < 2.35$ by comparing stacked spectra from the Hubble Space Telescope/WFC3's near-IR grism to corresponding stacks from the Hobby Eberly Telescope Dark Energy Experiment's Internal Data Release 2. By measuring the stacks' H$\beta$ to Ly$\alpha$ ratios, we determine the Ly$\alpha$ escape fraction as a function of stellar mass, star formation rate, internal reddening, size, and [OIII]/H$\beta$ ratio. We show that the escape fraction of Ly$\alpha$ correlates with a number of parameters, such as galaxy size, star formation rate, and nebular excitation. However, we also demonstrate that most of these relations are indirect, and the primary variables that control the escape of Ly$\alpha$ are likely stellar mass and internal extinction. Overall, the escape of Ly$\alpha$ declines from $\gtrsim 18\%$ in galaxies with $\log M/M_{\odot} \lesssim 9$ to $\lesssim 1\%$ for systems with $\log M/M_{\odot} \gtrsim 10$, with the sample's mean escape fraction being $6.0^{+0.6\%}_{-0.5\%}$.