A High Fraction of Lyα-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2

Abstract

Star-forming galaxies form a sequence in the [O III] λ5007/Hβ versus [N II] λ6584/Hɑ 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 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([NII]/Hβ) ⩽ -1.1 and log([O III]/Hβ) ⩾/0.75. These cuts identify galaxies with 12 + log(O/H ≾ 8.0, when oxygen abundances are measured via the O3N2 diagnostic. We study the Lyα properties of the resulting sample of 14 galaxies. The mean (median) rest-frame Lyα equivalent width is 39 (36) Å, and 11 of the 14 objects (79%) are Lyα emitters (LAEs) with W_(Lyα) > 20 Å. We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at 2.0 < z < 2.6 identified without regard to their optical line ratios; this sample has mean (median) Lyα equivalent width −1 (−4) Å, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at Lyα than 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 Lyα 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 Lyα escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to Lyα photons across a range of redshifts.

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