Lyα Emitters with Very Large Lyα Equivalent Widths, EW_ 0(Lyα) ≃ 200-400 Å, at z∼ 2

We present physical properties of spectroscopically confirmed Lyα emitters (LAEs) with very large rest-frame Lyα equivalent widths EW_ 0(Lyα). Although the definition of large EW_ 0(Lyα) LAEs is usually difficult due to limited statistical and systematic uncertainties, we identify six LAEs selected from ∼ 3000 LAEs at z∼ 2 with reliable measurements of EW_ 0 (Lyα) ≃ 200-400 Å given by careful continuum determinations with our deep photometric and spectroscopic data. These large EW_ 0(Lyα) LAEs do not have signatures of AGN, but notably small stellar masses of M_ * = 10^7-8 M_⊙ and high specific star-formation rates (star formation rate per unit galaxy stellar mass) of ∼ 100 Gyr^-1. These LAEs are characterized by the median values of L( Lyα)=3.7× 10^42 erg s^-1 and M_ UV=-18.0 as well as the blue UV continuum slope of β = -2.5±0.2 and the low dust extinction E(B-V)_ * = 0.02^+0.04_-0.02, which indicate a high median Lyα escape fraction of f_ esc^ Lyα=0.68±0.30. This large f_ esc^ Lyα value is explained by the low Hi column density in the ISM that is consistent with FWHM of the Lyα line, FWHM (Lyα)=212±32 km s^-1, significantly narrower than those of small EW_ 0(Lyα) LAEs. Based on the stellar evolution models, our observational constraints of the large EW_ 0 (Lyα), the small β, and the rest-frame Heii equivalent width imply that at least a half of our large EW_ 0(Lyα) LAEs would have young stellar ages of ≲ 20 Myr and very low metallicities of Z<0.02 Z_⊙ regardless of the star-formation history.