Lyα emitters with very large Lyα equivalent widths, EW₀(Lyα) ≃ 200–400 Å, at<i>z</i>∼ 2

Abstract

We present physical properties of spectroscopically confirmed Lyα emitters (LAEs) with very large rest-frame Lyα equivalent widths EW&lt;inf&gt;0&lt;/inf&gt;(Lyα). Although the definition of large EW&lt;inf&gt;0&lt;/inf&gt;(Lyα) LAEs is usually difficult due to limited statistical and systematic uncertainties, we identify six LAEs selected from ∼3000 LAEs at &lt;it&gt;z&lt;/it&gt; ∼ 2 with reliable measurements of EW&lt;inf&gt;0&lt;/inf&gt; (Lyα) &amp;sime; 200–400 &lt;a&gt;&lt;ac&gt;A&lt;/ac&gt;&lt;ac&gt;&amp;ring;&lt;/ac&gt;&lt;/a&gt; given by careful continuum determinations with our deep photometric and spectroscopic data. These large EW&lt;inf&gt;0&lt;/inf&gt;(Lyα) LAEs do not have signatures of AGN, but notably small stellar masses of &lt;it&gt;M&lt;/it&gt;&lt;inf&gt;&amp;starf;&lt;/inf&gt; = 107–8 M&lt;inf&gt;&amp;odot;&lt;/inf&gt; 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 &lt;it&gt;L&lt;/it&gt;(Lyα) = 3.7 × 1042 erg s−1 and &lt;it&gt;M&lt;/it&gt;&lt;inf&gt;UV&lt;/inf&gt; = −18.0 as well as the blue UV continuum slope of β = −2.5 ± 0.2 and the low dust extinction &lt;f&gt;$E(B-V)_{\\rm *} = 0.02^{+0.04}_{-0.02}$&lt;/f&gt;, which indicate a high median Lyα escape fraction of &lt;f&gt;$f_{\\rm esc}^{\\rm Ly\\alpha }=0.68\\pm 0.30$&lt;/f&gt;. This large &lt;f&gt;$f_{\\rm esc}^{\\rm Ly\\alpha }$&lt;/f&gt; value is explained by the low H &lt;scp&gt;i&lt;/scp&gt; column density in the interstellar medium which is consistent with full width at half-maximum (FWHM) of the Lyα line, FWHM(Lyα) = 212 ± 32 km s−1, significantly narrower than those of small EW&lt;inf&gt;0&lt;/inf&gt;(Lyα) LAEs. Based on the stellar evolution models, our observational constraints of the large EW&lt;inf&gt;0&lt;/inf&gt; (Lyα), the small β, and the rest-frame He &lt;scp&gt;ii&lt;/scp&gt; EW imply that at least a half of our large EW&lt;inf&gt;0&lt;/inf&gt;(Lyα) LAEs would have young stellar ages of &amp;lsim;20 Myr and very low metallicities of &lt;it&gt;Z&lt;/it&gt; &lt; 0.02 Z&lt;inf&gt;&amp;odot;&lt;/inf&gt; regardless of the star formation history.