FIRST SPECTROSCOPIC EVIDENCE FOR HIGH IONIZATION STATE AND LOW OXYGEN ABUNDANCE IN Lyα EMITTERS,

Abstract

We present results from Keck/NIRSPEC and Magellan/MMIRS follow-up\nspectroscopy of Lya emitters (LAEs) at z=2.2 identified in our Subaru\nnarrowband survey. We successfully detect Ha emission from seven LAEs, and\nperform a detailed analysis of six LAEs free from AGN activity, two out of\nwhich, CDFS-3865 and COSMOS-30679, have [OII] and [OIII] line detections. They\nare the first [OII]-detected LAEs at high-z, and their [OIII]/[OII] ratios and\nR23-indices provide the first simultaneous determinations of ionization\nparameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionization\nparameter (q_{ion}=2.5^{+1.7}_{-0.8}x10^8 cm s^{-1}) and a low oxygen abundance\n(12+log(O/H)=7.84^{+0.24}_{-0.25}) in contrast with moderate values of other\nhigh-z galaxies such as Lyman-break galaxies (LBGs). COSMOS-30679 also\npossesses a relatively high ionization parameter (q_{ion}=8^{+10}_{-4}x10^7 cm\ns^{-1}) and a low oxygen abundance (12+log(O/H)=8.18^{+0.28}_{-0.28}). Both\nLAEs appear to fall below the mass-metallicity relation of z~2 LBGs. Similarly,\na low metallicity of 12+log(O/H)<8.4 is independently indicated for typical\nLAEs from a composite spectrum and the [NII]/Ha index. Such high ionization\nparameters and low oxygen abundances can be found in local star-forming\ngalaxies, but this extreme local population occupies only ~0.06% of the SDSS\nspectroscopic galaxy sample with a number density ~100 times smaller than that\nof LAEs. With their high ionization parameters and low oxygen abundances, LAEs\nwould represent an early stage of galaxy formation dominated by massive stars\nin compact star-forming regions. High-q_{ion} galaxies like LAEs would produce\nionizing photons efficiently with a high escape fraction achieved by\ndensity-bounded HII regions, which would significantly contribute to cosmic\nreionization at z>6.\n