HUBBLE SPACE TELESCOPE EMISSION LINE GALAXIES AT z similar to 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 similar to 6%. We confirm this result by using stellar reddening to estimate the effective logarithmic extinction of the H beta emission line (c(H beta) = 0.5) and measuring both the H beta and Ly alpha luminosity functions in a similar to 100,000 Mpc(3) volume of space. We show that in our redshift window, the volumetric Ly alpha escape fraction is at most 4.4(-1.2)(+2.1)%, with an additional systematic similar to 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] lambda 5007- selected sample of galaxies, resonant scattering must be important for the escape of Ly alpha photons.