A HARD IONIZING SPECTRUM IN z=3-4 Lyα EMITTERS WITH INTENSE [O III] EMISSION: ANALOGS OF GALAXIES IN THE REIONIZATION ERA?

We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O III]lambda lambda 5007, 4959, [O II]lambda 3727, and H beta for a sample of 15 redshift z similar or equal to 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 [O III]/[O II] line ratio than is seen in similar redshift LBGs. By comparing their distributions on a [O III]/[O II] 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 [O III]/[O II] and R23 and conclude such a hard spectrum can only partially explain such intense line emission. The additional possibility is that such a large [O III]/[O II] 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 [O III] emission represent a promising analog of those z > 7 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.