The mean ultraviolet spectrum of a representative sample of faint z ~ 3 Lyman alpha emitters

We discuss the rest-frame ultraviolet (UV) emission line spectra of a large (~100) sample of low luminosity redshift z ~ 3.1 Lyman alpha emitters (LAEs) drawn from a Subaru imaging survey in the SSA22 survey field. Our earlier work based on smaller samples indicated that such sources have high [O III]/[O II] line ratios possibly arising from a hard ionizing spectrum that may be typical of similar sources in the reionization era.With optical spectra secured from VLT/VIMOS, we re-examine the nature of the ionizing radiation in a larger sample using the strength of the high ionization diagnostic emission lines of CIII]λ1909, CIVλ1549, HeIIλ1640, and OIII]λλ1661, 1666Å in various stacked subsets. Our analysis confirms earlier suggestions of a correlation between the strength of Ly α and CIII] emission and we find similar trends with broad-band UV luminosity and rest-frame UV colour. Using various diagnostic line ratios and our stellar photoionization models, we determine both the gas phase metallicity and hardness of the ionization spectrum characterized by ξ_ion - the number of Lyman continuum photons per UV luminosity. We confirm our earlier suggestion that ξ_ion is significantly larger for LAEs than for continuum-selected Lyman break galaxies, particularly for those LAEs with the faintest UV luminosities.We briefly discuss the implications for cosmic reionization if the metal-poor intensely star-forming systems studied here are representative examples of those at much higher redshift.