Lyman‐Continuum Emission from Galaxies atz≃ 3.4

We report the detection of significant Lyman continuum flux in the composite spectrum of 29 Lyman break galaxies (LBGs) with redshifts = 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 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 ~ 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 co-moving volume than QSOs at z ~ 3. The associated contribution to the metagalactic ionizing radiation field is J_ (912) = 1. 2+/-0. 3 x 10^-21 ergs s^-1 cm^-2 Hz^-1 sr^-1 at z ~ 3, 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 over-predict the QSO contribution to J_ (912) at z ~ 3. We briefly discuss the implications of a galaxy-dominated UV background at high redshifts.