HUBBLE SPACE TELESCOPE 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 sub(H)[beta] = 0.5) and measuring both the H[beta] and Ly[alpha] luminosity functions in a ~100,000 Mpc super(3) volume of space. We show that in our redshift window, the volumetric Ly[alpha] escape fraction is at most 4.4 super(+2.1) sub(-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 [OIII] [lambda]5007-selected sample of galaxies, resonant scattering must be important for the escape of Ly[alpha] photons.