Lyman-continuum galaxies and the escape fraction of Lyman-break galaxies

Lyman-break galaxies (LBGs) at z ̃ 3-4 are targeted to measure the fraction of Lyman-continuum (LyC) flux that escapes from high-redshift galaxies. However, z ̃ 3-4 LBGs are identified using the Lyman-break technique which preferentially selects galaxies with little or no LyC. We re-examine the standard LBG selection criteria by performing spectrophotometry on composite spectra constructed from 794 UnGR-selected z ̃ 3 LBGs from the literature while adding LyC flux of varying strengths. The modified composite spectra accurately predict the range of redshifts, properties, and LyC flux of LBGs in the literature that have spectroscopic LyC measurements while predicting the existence of a significant fraction of galaxies outside the standard selection region. These galaxies, termed Lyman-continuum galaxies (LCGs), are expected to have high levels of LyC flux and are estimated to have a number density ̃30-50 per cent that of the LBG population. We define Robs(Un) as the relative fraction of observed LyC flux, integrated from 912 Å to the shortest rest-frame wavelength probed by the Un filter, to the observed non-ionizing flux (here measured at 1500 Å). We use the 794 spectra as a statistical sample for the full z ̃ 3 LBG population, and find Robs(Un)=5.0-0.4 +1.0(4.1-0.3 +0.5) per cent, which corresponds to an intrinsic LyC escape fraction of fesc LyC=10.5-0.8 +2.0(8.6-0.6 +1.0) per cent (contamination corrected). From the composite spectral distributions we estimate Robs(Un)̃16±3, fesc LyC̃33±7 per cent for LCGs and Robs(Un)̃8±3, fesc LyC̃16±4 per cent for the combined LBG+LCG z ̃ 3 sample. All values are measured in apertures defined by the ultraviolet continuum and do not include extended and/or offset LyC flux. A complete galaxy census and the total emergent LyC flux at high redshift are essential to quantify the contribution of galaxies to the ionizing photon budget of the Universe, particularly during the epoch of reionization.