A Hard Ionizing Spectrum in z=3-4 Ly-alpha Emitters with Intense [OIII]
  Emission: Analogs of Galaxies in the Reionization Era?
  We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines
[OIII]5007,4959, [OII]3727, and H-beta for a sample of 15 redshift z=3.1-3.7
Ly-alpha emitters (LAEs) and Lyman break galaxies (LBGs). In conjunction with
spectra from other surveys, we confirm earlier indications that LAEs have a
much higher [OIII]/[OII] line ratio than is seen in similar redshift LBGs. By
comparing their distributions on a [OIII]/[OII] versus R23 diagram, we
demonstrate that this difference cannot arise solely because of their lower
metallicities but most likely is due to a harder ionizing spectrum. Using
measures of H-beta and recombination theory, we demonstrate, for a subset of
our LAEs, that xi_ion - the number of Lyman continuum photons per UV luminosity
- is indeed 0.2-0.5 dex larger than for typical LBGs at similar redshifts.
Using photoionization models we estimate the effect this would have on both
[OIII]/[OII] and R23 and conclude such a hard spectrum can only partially
explain such intense line emission. The additional possibility is that such a
large [OIII]/[OII] ratio is in part due to density rather than ionization bound
nebular regions, which would imply a high escape fraction of ionizing photons.
We discuss how further observations could confirm this possibility. Clearly
LAEs with intense [OIII] emission represent a promising analog of those z>7
sources with similarly strong lines which are thought to be an important
contributor to cosmic reionization.
