Mission: Impossible (Escape from the Lyman Limit)

We investigate the intrinsic opacity of high-redshift galaxies to outgoing ionising photons using high-quality photometry of a sample of 27 spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep Field. Our measurement is based on maximum-likelihood fitting of model galaxy spectral energy distributions-including the effects of intrinsic Lyman-limit absorption and random realizations of intervening Lyman-series and Lyman-limit absorption-to photometry of galaxies from space- and ground-based broad-band images. Our method provides several important advantages over the methods used by previous groups, including most importantly that two-dimensional sky subtraction of faint-galaxy images is more robust than one-dimensional sky subtraction of faint-galaxy spectra. We find at the 3sigma statistical confidence level that on average no more than 4% of the ionising photons escape galaxies of redshift 1.9<z<3.5. This result is consistent with observations of low- and moderate-redshift galaxies but is in direct contradiction to a recent result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies. Dividing our sample in subsamples according to luminosity, intrinsic ultraviolet colour, and redshift, we find no evidence for selection effects that could explain such discrepancy. Even when all systematic effects are included, the data could not realistically accomodate any escape fraction value larger than ~15%.