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

TL;DR

This study uses high-resolution Keck spectroscopy of gravitationally lensed z=4 galaxies to better constrain the escape fraction of ionizing photons, revealing a trend of increasing escape fraction with redshift.

Contribution

It introduces a new method to measure the covering fraction of low-ionization gas and applies it to high-redshift galaxies, improving understanding of ionizing photon escape.

Findings

Lower covering fractions of low-ionization gas in galaxies with strong Lyman alpha emission.

Evidence for increased escape fraction of ionizing radiation at higher redshifts.

Spectral resolution enabled differentiation between outflow kinematics and gas covering fraction.

Abstract

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 Lyman 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=4 compared to similar measures at z=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 3 gravitationally lensed Lyman break galaxies at z=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 a significant trend of lower covering fractions of low-ionization gas for galaxies with strong \Lya emission. In combination with the demographic trends of \Lya emission with redshift from our earlier work, our results provide new evidence for a reduction in the average H I covering fraction, and hence an increase in the escape fraction of ionizing radiation from Lyman break galaxies, with redshift.