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HST Emission Line Galaxies at z ~ 2: The Ly-alpha Escape Fraction

We compare the H-beta line strengths of 1.90 < z < 2.35 star-forming galaxies observed with the near-IR grism of the Hubble Space Telescope with ground-based measurements of Ly-alpha from the HETDEX Pilot Survey and narrow-band imaging. By examining the line ratios of 73 galaxies, we show that most star-forming systems at this epoch have a Ly-alpha escape fraction below ~6%. We confirm this result by using stellar reddening to estimate the effective logarithmic extinction of the H-beta emission line (c_Hbeta = 0.5) and measuring both the H-beta and Ly-alpha luminosity functions in a ~ 100,000 cubic Mpc volume of space. We show that in our redshift window, the volumetric Ly-alpha escape fraction is at most 4.4+/-2.1(1.2)%, with an additional systematic ~25% uncertainty associated with our estimate of extinction. Finally, we demonstrate that the bulk of the epoch's star-forming galaxies have Ly-alpha emission line optical depths that are significantly greater than that for the underlying UV continuum. In our predominantly [O~III] 5007-selected sample of galaxies, resonant scattering must be important for the escape of Ly-alpha photons.