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

Understand

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.