A Refined Estimate of the Ionizing Emissivity from Galaxies at z~3:
  Spectroscopic Follow-up in the SSA22a Field
  We investigate the contribution of star-forming galaxies to the ionizing
background at z~3, building on previous work based on narrowband (NB3640)
imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman break
galaxies (LBGs) and narrowband-selected Lya emitters (LAEs) to measure
redshifts for 16 LBGs and 87 LAEs at z>3.055, such that our NB3640 imaging
probes the Lyman-continuum (LyC) region. When we include the existing set of
spectroscopically-confirmed LBGs, our total sample with z>3.055 consists of 41
LBGs and 91 LAEs, of which nine LBGs and 20 LAEs are detected in our NB3640
image. With our combined imaging and spectroscopic data sets, we critically
investigate the origin of NB3640 emission for detected LBGs and LAEs. We remove
from our samples 3 LBGs and 3 LAEs with spectroscopic evidence of contamination
of their NB3640 flux by foreground galaxies, and statistically model the
effects of additional, unidentified foreground contaminants. The resulting
contamination 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. The
corresponding ratios of non-ionizing UV to LyC flux-density, corrected for
intergalactic 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 of
star-forming galaxies to the ionizing background and the hydrogen
photoionization rate in the IGM, finding values larger than, but consistent
with, those measured in the Lya forest. Finally, the measured UV to LyC
flux-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 LAE
sample.
