Abstract
We present physical properties of spectroscopically confirmed Ly$\alpha$
emitters (LAEs) with very large rest-frame Ly$\alpha$ equivalent widths
EW$_0$(Ly$\alpha$). Although the definition of large EW$_\rm
0$(Ly$\alpha$) LAEs is usually difficult due to limited statistical and
systematic uncertainties, we identify six LAEs selected from $3000$ LAEs
at $z2$ with reliable measurements of EW$_0$ (Ly$\alpha$) $\simeq
200-400$ \AA\ given by careful continuum determinations with our deep
photometric and spectroscopic data. These large EW$_0$(Ly$\alpha$) LAEs
do not have signatures of AGN, but notably small stellar masses of $M_* =
10^7-8$ $M_ødot$ 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 $L(Ly\alpha)=3.7\times
10^42$ erg s$^-1$ and $M_UV=-18.0$ as well as the blue UV continuum
slope of $= -2.5\pm0.2$ and the low dust extinction $E(B-V)_* =
0.02^+0.04_-0.02$, which indicate a high median Ly$\alpha$ escape fraction
of $f_esc^Ly\alpha=0.68\pm0.30$. This large $f_esc^\rm
Ly\alpha$ value is explained by the low Hi column density in the ISM
that is consistent with FWHM of the Ly$\alpha$ line, $FWHM
(Ly\alpha)=212\pm32$ km s$^-1$, significantly narrower than those of small
EW$_0$(Ly$\alpha$) LAEs. Based on the stellar evolution models, our
observational constraints of the large EW$_0$ (Ly$\alpha$), the small
$\beta$, and the rest-frame Heii equivalent width imply that at least a
half of our large EW$_0$(Ly$\alpha$) LAEs would have young stellar ages
of $20$ Myr and very low metallicities of $Z<0.02 Z_ødot$ regardless
of the star-formation history.
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