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 (similar to 100) sample of low luminosity redshift z similar to 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]lambda 1909, CIV lambda 1549, HeII lambda 1640, and O III]lambda lambda 1661, 1666 angstrom in various stacked subsets. Our analysis confirms earlier suggestions of a correlation between the strength of Ly alpha 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 xi(ion) - the number of Lyman continuum photons per UV luminosity. We confirm our earlier suggestion that. 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.