The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman-break Galaxies at 3.4 < z < 4.5

Abstract

We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1 narrowband imaging in the GOODS Southern field to derive limits on the distribution of the escape fraction (f _esc) of ionizing radiation for L >= L* _z=3 Lyman-break galaxies (LBGs) at redshift 3.4-4.5. Only one LBG, at redshift z = 3.795, is detected in its Lyman continuum (LyC; S/N ~= 5.5), the highest redshift galaxy currently known with a direct detection. Its ultraviolet morphology is quite compact (R _eff = 0.8 kpc physical). Three out of seven active galactic nuclei are also detected in their LyC, including one at redshift z = 3.951 and z ₈₅₀ = 26.1. From stacked data (LBGs), we set an upper limit to the average f _esc in the range 5%-20%, depending on how the data are selected (e.g., by magnitude and/or redshift). We undertake extensive Monte Carlo simulations that take into account intergalactic attenuation, stellar population synthesis models, dust extinction, and photometric noise in order to explore the moments of the distribution of the escaping radiation. Various distributions (exponential, log-normal, and Gaussian) are explored. We find that the median f _esc is lower than sime6% with an 84% percentile limit not larger than 20%. If this result remains valid for fainter LBGs down to current observational limits, then the LBG population might be not sufficient to account for the entire photoionization budget at the redshifts considered here, with the exact details dependent upon the assumed ionizing background and QSO contribution thereto. It is possible that f _esc depends on the UV luminosity of the galaxies, with fainter galaxies having higher f _esc, and estimates of f _esc from a sample of faint LBGs from HUDF (i ₇₇₅ <=28.5) are in broad quantitative agreement with such a scenario.

Based on observations made at the European Southern Observatory Very Large Telescope, Paranal, Chile (ESO program 170.A-0788 The Great Observatories Origins Deep Survey: ESO Public Observations of the SST Legacy HST Treasury Chandra Deep Field South). Also based on observations obtained with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555.