The HETDEX Pilot Survey. II. The Evolution Of The Ly Alpha Escape Fraction From The Ultraviolet Slope And Luminosity Function Of 1.9 < Z < 3.8 Laes

Abstract

We study the escape of Ly alpha photons from Ly alpha emitting galaxies (LAEs) and the overall galaxy population using a sample of 99 LAEs at 1.9 < z < 3.8 detected through integral-field spectroscopy of blank fields by The Hobby Eberly Telescope Dark Energy Experiment Pilot Survey. For 89 LAEs with broadband counterparts we measure ultraviolet (UV) luminosities and UV slopes, and estimate E(B - V) under the assumption of a constant intrinsic UV slope for LAEs. These quantities are used to estimate dust-corrected star formation rates (SFRs). Comparison between the observed Ly alpha luminosity and that predicted by the dust-corrected SFR yields the Ly alpha escape fraction. We also measure the Ly alpha luminosity function and luminosity density (rho(Ly alpha)) at 2 < z < 4. Using this and other measurements from the literature at 0.3 < z < 7.7 we trace the redshift evolution of rho(Ly alpha). We compare it to the expectations from the star formation history of the universe and characterize the evolution of the Ly alpha escape fraction of galaxies. LAEs at 2 < z < 4 selected down to a luminosity limit of L(Ly alpha) > (3-6) x 10(42) erg s(-1) (0.25-0.5 L*), have a mean < E(B - V)> = 0.13 +/- 0.01, implying an attenuation of similar to 70% in the UV. They show a median UV uncorrected SFR = 11 M-circle dot yr(-1), dust-corrected SFR = 34 M-circle dot yr(-1), and Ly alpha equivalent widths (EWs) which are consistent with normal stellar populations. We measure a median Ly alpha escape fraction of 29%, with a large scatter and values ranging from a few percent to 100%. The Ly alpha escape fraction in LAEs correlates with E(B - V) in a way that is expected if Ly alpha photons suffer from similar amounts of dust extinction as UV continuum photons. This result implies that a strong enhancement of the Ly alpha EW with dust, due to a clumpy multi-phase interstellar medium (ISM), is not a common process in LAEs at these redshifts. It also suggests that while in other galaxies Ly alpha can be preferentially quenched by dust due to its scattering nature, this is not the case in LAEs. We find no evolution in the average dust content and Ly alpha escape fraction of LAEs from z similar to 4 to 2. We see hints of a drop in the number density of LAEs from z similar to 4 to 2 in the redshift distribution and the Ly alpha luminosity function, although larger samples are required to confirm this. The mean Ly alpha escape fraction of the overall galaxy population decreases significantly from z similar to 6 to z similar to 2, in agreement with recent results. Our results point toward a scenario in which star-forming galaxies build up significant amounts of dust in their ISM between z similar to 6 and 2, reducing their Ly alpha escape fraction, with LAE selection preferentially detecting galaxies which have the highest escape fractions given their dust content. The fact that a large escape of Ly alpha photons is reached by z similar to 6 implies that better constraints on this quantity at higher redshifts might detect re-ionization in a way that is uncoupled from the effects of dust.