KECK SPECTROSCOPY OF GRAVITATIONALLY LENSED z ≃ 4 GALAXIES: IMPROVED CONSTRAINTS ON THE ESCAPE FRACTION OF IONIZING PHOTONS

The fraction of ionizing photons that escape from young star-forming galaxies is one of the largest uncertainties in determining the role of galaxies in cosmic reionization. Yet traditional techniques for measuring this fraction are inapplicable at the redshifts of interest due to foreground screening by the Ly alpha forest. In an earlier study, we demonstrated a reduction in the equivalent width of low-ionization absorption lines in composite spectra of Lyman break galaxies at z similar or equal to 4 compared to similarmeasures at z similar or equal to 3. This might imply a lower covering fraction of neutral gas and hence an increase with redshift in the escape fraction of ionizing photons. However, our spectral resolution was inadequate to differentiate between several alternative explanations, including changes with redshift in the outflow kinematics. Here we present higher quality spectra of three gravitationally lensed Lyman break galaxies at z similar or equal to 4 with a spectral resolution sufficient to break this degeneracy of interpretation. We present a method for deriving the covering fraction of low-ionization gas as a function of outflow velocity and compare the results with similar quality data taken for galaxies at lower redshift. We find an interesting but tentative trend of lower covering fractions of low-ionization gas for galaxies with strong Ly alpha emission. In combination with the demographic trends of Ly alpha emission with redshift from our earlier work, our results provide new evidence for a reduction in the average Hi covering fraction, and hence an increase in the escape fraction of ionizing radiation from Lyman break galaxies, with redshift.