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 Ly alpha emitters (LAEs) measured from our large-area survey with three narrowband (NB) filters covering the Ly alpha, [O II]lambda 3727, and H alpha+[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 H alpha 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 H alpha luminosity of 1.7 x 10(42) erg s(-1) corresponding to an SFR of 14 M-circle dot yr(-1). We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z greater than or similar to 0.09 Z(circle dot) (2 sigma) based on the [O II]/(H alpha+[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 similar to 2, are extremely metal-poor ( Z < 2 x 10(-2) Z(circle dot)) galaxies at the 4 sigma level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of z similar to 2 UV-selected galaxies toward lower masses (5 x 10(8) M-circle dot), but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The H alpha and Ly alpha luminosities of our NB-selected LAEs indicate that the escape fraction of Ly alpha photons is similar to 12%-30%, much higher than the values derived for other galaxy populations at z similar to 2.