2016ApJ...830...52E

Star-forming galaxies form a sequence in the [O iii] λ5007/\rmHβversus [N ii] λ6584/\rmHαdiagnostic diagram, with low-metallicity, highly ionized galaxies falling in the upper left corner. Drawing from a large sample of UV-selected star-forming galaxies at z∼2 with rest-frame optical nebular emission line measurements from Keck-MOSFIRE, we select the extreme ∼5% of the galaxies lying in this upper left corner, requiring log([N ii]/\rmHα) \leqslant -1.1 and log([O iii]/\rmHβ) \geqslant  0.75. These cuts identify galaxies with 12+log(\rmO/H)\lesssim 8.0, when oxygen abundances are measured via the O3N2 diagnostic. We study the Lyαproperties of the resulting sample of 14 galaxies. The mean (median) rest-frame Lyαequivalent width is 39 (36) Å, and 11 of the 14 objects (79%) are Lyαemitters (LAEs) with W_Lyα\gt 20 \mathring\rmA . We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at 2.0\lt z\lt 2.6 identified without regard to their optical line ratios; this sample has mean (median) Lyαequivalent width -1 (-4) Å, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at Lyαthan those in the comparison sample and have ∼50% lower median oxygen abundances. Both factors are likely to facilitate the escape of Lyα: in less dusty galaxies Lyαphotons are less likely to be absorbed during multiple scatterings, while the harder ionizing spectrum and higher ionization parameter associated with strong, low-metallicity star formation may reduce the covering fraction or column density of neutral hydrogen, further easing Lyαescape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to Lyαphotons across a range of redshifts.