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We present results from Keck/NIRSPEC and Magellan/MMIRS follow-upspectroscopy of Lya emitters (LAEs) at z=2.2 identified in our Subarunarrowband survey. We successfully detect Ha emission from seven LAEs, andperform a detailed analysis of six LAEs free from AGN activity, two out ofwhich, CDFS-3865 and COSMOS-30679, have [OII] and [OIII] line detections. Theyare the first [OII]-detected LAEs at high-z, and their [OIII]/[OII] ratios andR23-indices provide the first simultaneous determinations of ionizationparameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionizationparameter (q_{ion}=2.5^{+1.7}_{-0.8}x10^8 cm s^{-1}) and a low oxygen abundance(12+log(O/H)=7.84^{+0.24}_{-0.25}) in contrast with moderate values of otherhigh-z galaxies such as Lyman-break galaxies (LBGs). COSMOS-30679 alsopossesses a relatively high ionization parameter (q_{ion}=8^{+10}_{-4}x10^7 cms^{-1}) and a low oxygen abundance (12+log(O/H)=8.18^{+0.28}_{-0.28}). BothLAEs appear to fall below the mass-metallicity relation of z~2 LBGs. Similarly,a low metallicity of 12+log(O/H)<8.4 is independently indicated for typicalLAEs from a composite spectrum and the [NII]/Ha index. Such high ionizationparameters and low oxygen abundances can be found in local star-forminggalaxies, but this extreme local population occupies only ~0.06% of the SDSSspectroscopic galaxy sample with a number density ~100 times smaller than thatof LAEs. With their high ionization parameters and low oxygen abundances, LAEswould represent an early stage of galaxy formation dominated by massive starsin compact star-forming regions. High-q_{ion} galaxies like LAEs would produceionizing photons efficiently with a high escape fraction achieved bydensity-bounded HII regions, which would significantly contribute to cosmicreionization at z>6.

First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lya Emitters