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The fraction of ionizing photons that escape from young star-forming galaxiesis one of the largest uncertainties in determining the role of galaxies incosmic reionization. Yet traditional techniques for measuring this fraction areinapplicable at the redshifts of interest due to foreground screening by theLyman alpha forest. In an earlier study, we demonstrated a reduction in theequivalent width of low-ionization absorption lines in composite spectra ofLyman break galaxies at z=4 compared to similar measures at z=3. This mightimply a lower covering fraction of neutral gas and hence an increase withredshift in the escape fraction of ionizing photons. However, our spectralresolution was inadequate to differentiate between several alternativeexplanations, including changes with redshift in the outflow kinematics. Herewe present higher quality spectra of 3 gravitationally lensed Lyman breakgalaxies at z=4 with a spectral resolution sufficient to break this degeneracyof interpretation. We present a method for deriving the covering fraction oflow-ionization gas as a function of outflow velocity and compare the resultswith similar quality data taken for galaxies at lower redshift. We find asignificant trend of lower covering fractions of low-ionization gas forgalaxies with strong \Lya emission. In combination with the demographic trendsof \Lya emission with redshift from our earlier work, our results provide newevidence for a reduction in the average H I covering fraction, and hence anincrease in the escape fraction of ionizing radiation from Lyman breakgalaxies, with redshift.

Keck Spectroscopy of Gravitationally Lensed z=4 Galaxies: Improved Constraints on the Escape Fraction of Ionizing Photons