Average Metallicity and Star Formation Rate of Lyα Emitters Probed by a Triple Narrowband Survey

We present the average metallicity and star formation rate (SFR) of Lyα emitters (LAEs) measured from our large-area survey with three narrowband (NB) filters covering the Lyα, [OII]3727, and Hα+[NII] lines of LAEs at z=2.2. We select 919 z=2.2 LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. By stacking the near-infrared images of 105 LAEs observed with KPNO/NEWFIRM, we obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by NB imaging or deep spectroscopy. The stacked object has an Hα luminosity of 1.7x10^{42} erg/s corresponding to an SFR of 14 Msun/yr. We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z>0.09 Zsun (2σ) based on the [OII]/(Hα+[NII]) index together with photoionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at z∼2, are extremely metal-poor (Z<0.02 Zsun) galaxies at the 4σ level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of z∼2 UV-selected galaxies toward lower masses (5x10^8 Msun), but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account.

In order to test these NIR NB analysis results, we carry out NIR spectroscopy for bright z=2.2 LAEs with Keck/NIRSPEC and Magellan/MMIRS. We successfully detect Hα emission from seven LAEs, two out of which have [OII] and [OIII] line detections. These two LAEs with Hα and multiple oxygen lines provide oxygen abundance estimates of LAEs that complement the previous spectroscopic constraints on metallicity of LAEs from the [NII]/Hα index.