Average metallicity and star formation rate of Lyα 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, [O II]λ3727, and Hα+[N II] 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 [O II] 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 × 10 42ergs-1 corresponding to an SFR of 14 M yr-1. We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z ≳ 0.09 Z (2σ) based on the [O II]/(Hα+[N II]) 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 × 10-2 Z ) 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 × 108 M ), 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%-30%, much higher than the values derived for other galaxy populations at z 2. © 2012. The American Astronomical Society. All rights reserved.