Lyman-continuum emission from galaxies at z ≃ 3.4

We report the detection of significant Lyman-continuum flux in the composite spectrum of 29 Lyman-break galaxies (LBGs) with redshifts [z] = 3.40 +/- 0.09. After correction for opacity due to intervening absorption using a new composite QSO spectrum evaluated at the same redshift, the ratio of emergent flux density at 1500 Angstrom in the rest frame to that in the Lyman continuum is L (1500)/L (900) = 4.6 +/- 1.0. If the relative intensity of the inferred escaping Lyman-continuum radiation is typical of LBGs at z similar to 3 (the galaxies in this sample are drawn from the bluest quartile of LBG spectral energy distributions due to known selection effects), then observed LBGs produce about 5 times more H-ionizing photons per unit comoving volume than QSOs at z similar to 3. The associated contribution to the metagalactic ionizing radiation field is J(v)(912 Angstrom) approximate to 1.2 +/- 0.3 x 10(-21) ergs s(-1) cm(-2) Hz(-1) sr(-1) at z similar to3, very close to most estimates of the radiation field background based on the "proximity effect." A preliminary analysis of the density of faint QSOs in our Lyman-break galaxy survey indicates that the standard extrapolated QSO luminosity function may slightly overpredict the QSO contribution to J(v)(912) ata z similar to3. We briefly discuss the implications of a galaxy-dominated UV background at high redshifts.