The Direct Detection of Lyman Continuum Emission from Star‐forming Galaxies atz∼3

We present the results of rest-frame ultraviolet spectroscopic observationsof a sample of 14 z~3 star-forming galaxies in the SSA22a field. These spectraare characterized by unprecedented depth in the Lyman-continuum region. For thefirst time, we have detected escaping ionizing radiation from individualgalaxies at high redshift, with two of the 14 objects showing significantemission below the Lyman limit. We also measured the ratio of emergent fluxdensity at 1500 AA to that in the Lyman-continuum region, for the individualdetections (C49 and D3) and the sample average. If a correction for the averageIGM opacity is applied to the spectra of the objects C49 and D3, we findf_1500/f_900,corr(C49)=4.5 and f_1500/f_900,corr(D3)=2.9. The average emergentflux-density ratio in our sample is =22, implying an escapefraction ~4.5 times lower than inferred from the composite spectrum in Steidelet al. (2001). If this new estimate is representative of LBGs, theircontribution to the metagalactic ionizing radiation field isJ_nu(900)~2.6x10^{-22} erg/s/cm^2/Hz/sr, comparable to the contribution ofoptically-selected quasars at the same redshift. The sum of the contributionsfrom galaxies and quasars is consistent with recent estimates of the level ofthe ionizing background at z~3, inferred from the HI Ly-alpha forest opticaldepth. There is significant variance among the emergent far-UV spectra in oursample, yet the factors controlling the detection or non-detection ofLyman-continuum emission from galaxies are not well-determined. Because we donot yet understand the source of this variance, significantly larger sampleswill be required to obtain robust constraints on the galaxy contribution to theionizing background at z~3 and beyond (abridged).