2013ApJ...769....3N

We present results from Keck/NIRSPEC and Magellan/MMIRS follow-up spectroscopy of Lyα emitters (LAEs) at z = 2.2 identified in our Subaru narrowband survey. We successfully detect Hα emission from seven LAEs, and perform a detailed analysis of six LAEs free from active galactic nucleus activity, two out of which, CDFS-3865 and COSMOS-30679, have [O II] and [O III] line detections. They are the first [O II]-detected LAEs at high-z, and their [O III]/[O II] ratios and R23-indices provide the first simultaneous determinations of ionization parameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionization parameter (q_ion = 2.5^+1.7_–0.8 x 10⁸ cm/s) and a low oxygen abundance (12 + log(O/H) = 7.84^+0.24_–0.25) in contrast with moderate values of other high-z galaxies such as Lyman break galaxies (LBGs). COSMOS-30679 also possesses a relatively high ionization parameter (q_ion = 8⁺¹⁰_–4 x 10⁷ cm/s) and a low oxygen abundance (12 + log(O/H) = 8.18 ^+0.28_–0.28). Both LAEs 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 typical LAEs from a composite spectrum and the [N II]/Hα index. Such high ionization parameters and low oxygen abundances can be found in local star-forming galaxies, but this extreme local population occupies only ∼0.06% of the Sloan Digital Sky Survey spectroscopic galaxy sample with a number density ∼100 times smaller than that of LAEs. With their high ionization parameters and low oxygen abundances, LAEs would represent an early stage of galaxy formation dominated by massive stars in compact star-forming regions. High-q_ion galaxies like LAEs would produce ionizing photons efficiently with a high escape fraction achieved by density-bounded H II regions, which would significantly contribute to cosmic reionization at z > 6.