Average metallicity and star formation rate of Ly$\alpha$ emitters probed by a triple narrowband survey*

Abstract

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]λ3727, 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. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted [OII] and Hα nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully 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 1.7×1042ergs–1 corresponding to an SFR of 14M⊙yr6–1. We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z≳0.09Z⊙(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<2×10–2Z⊙) 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 (5×108M⊙), but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The Hα and Lyα luminosities of our NB-selected LAEs indicate that the escape fraction of Lyα photons is ∼12, much higher than the values derived for other galaxy populations at z∼2.