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We investigate the contribution of star-forming galaxies to the ionizingbackground at z~3, building on previous work based on narrowband (NB3640)imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman breakgalaxies (LBGs) and narrowband-selected Lya emitters (LAEs) to measureredshifts for 16 LBGs and 87 LAEs at z>3.055, such that our NB3640 imagingprobes the Lyman-continuum (LyC) region. When we include the existing set ofspectroscopically-confirmed LBGs, our total sample with z>3.055 consists of 41LBGs and 91 LAEs, of which nine LBGs and 20 LAEs are detected in our NB3640image. With our combined imaging and spectroscopic data sets, we criticallyinvestigate the origin of NB3640 emission for detected LBGs and LAEs. We removefrom our samples 3 LBGs and 3 LAEs with spectroscopic evidence of contaminationof their NB3640 flux by foreground galaxies, and statistically model theeffects of additional, unidentified foreground contaminants. The resultingcontamination and LyC-detection rates, respectively, are 62 +/-13% and 8 +/-3%for our LBG sample, and 47 +/-10% and 12 +/-2% for our LAE sample. Thecorresponding ratios of non-ionizing UV to LyC flux-density, corrected forintergalactic medium (IGM) attenuation, are 18.0 +34.8/-7.4 for LBGs, and 3.7+2.5/-1.1 for LAEs. We use these ratios to estimate the total contribution ofstar-forming galaxies to the ionizing background and the hydrogenphotoionization rate in the IGM, finding values larger than, but consistentwith, those measured in the Lya forest. Finally, the measured UV to LyCflux-density ratios imply model-dependent LyC escape fractions of f_{esc}^{LyC}~ 5-7% for our LBG sample and f_{esc}^{LyC} ~ 10-30% for our fainter LAEsample.

A Refined Estimate of the Ionizing Emissivity from Galaxies at z~3: Spectroscopic Follow-up in the SSA22a Field