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We discuss the rest-frame ultraviolet emission line spectra of a large (~100)sample of low luminosity redshift z~3.1 Lyman alpha emitters (LAEs) drawn froma Subaru imaging survey in the SSA22 survey field. Our earlier work based onsmaller samples indicated that such sources have high [OIII]/[OII] line ratiospossibly arising from a hard ionising spectrum that may be typical of similarsources in the reionisation era. With optical spectra secured from VLT/VIMOS,we re-examine the nature of the ionising radiation in a larger sample using thestrength of the high ionisation diagnostic emission lines of CIII]1909,CIV1549, HeII1640, and OIII]1661,1666 in various stacked subsets. Our analysisconfirms earlier suggestions of a correlation between the strength of Ly-alphaand CIII] emission and we find similar trends with broad band UV luminosity andrest-frame UV colour. Using various diagnostic line ratios and our stellarphotoionisation models, we determine both the gas phase metallicity andhardness of the ionisation spectrum characterised by xi_ion - the number ofLyman continuum photons per UV luminosity. We confirm our earlier suggestionthat xi_ion is significantly larger for LAEs than for continuum-selected Lymanbreak galaxies, particularly for those LAEs with the faintest UV luminosities.We briefly discuss the implications for cosmic reionisation if the metal-poorintensely star-forming systems studied here are representative examples ofthose at much higher redshift.

The Mean Ultraviolet Spectrum of a Representative Sample of Faint z~3 Lyman Alpha Emitters