A HIGH FRACTION OF Lyα EMITTERS AMONG GALAXIES WITH EXTREME EMISSION LINE RATIOS AT z ∼ 2

Star-forming galaxies form a sequence in the [O iii] λ5007/ versus [N ii] λ6584/ diagnostic 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 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]/ ) and log([O iii]/ ) . These cuts identify galaxies with , when oxygen abundances are measured via the O3N2 diagnostic. We study the properties of the resulting sample of 14 galaxies. The mean (median) rest-frame equivalent width is 39 (36) Å, and 11 of the 14 objects (79%) are emitters (LAEs) with . We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at identified without regard to their optical line ratios; this sample has mean (median) equivalent width −1 (−4) Å, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at than those in the comparison sample and have ∼50% lower median oxygen abundances. Both factors are likely to facilitate the escape of : in less dusty galaxies photons 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 escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to photons across a range of redshifts.