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

Erb, Dawn K. and Pettini, Max and Steidel, Charles C. and Strom, Allison L. and Rudie, Gwen C. and Trainor, Ryan F. and Shapley, Alice E. and Reddy, Naveen A. (2016) A High Fraction of Lyα-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2. Astrophysical Journal, 830 (1). Art. No. 52. ISSN 0004-637X. doi:10.3847/0004-637X/830/1/52.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Erb, Dawn K.0000-0001-9714-2758
Pettini, Max0000-0002-5139-4359
Steidel, Charles C.0000-0002-4834-7260
Strom, Allison L.0000-0001-6369-1636
Rudie, Gwen C.0000-0002-8459-5413
Trainor, Ryan F.0000-0002-6967-7322
Shapley, Alice E.0000-0003-3509-4855
Reddy, Naveen A.0000-0001-9687-4973
Additional Information:© 2016 American Astronomical Society. Received 2016 March 8; revised 2016 May 16; accepted 2016 June 4; published 2016 October 10. We would like to thank the anonymous referee for a helpful and constructive report. D.K.E. is supported by the U.S. National Science Foundation through the Faculty Early Career Development (CAREER) Program, grant AST-1255591. Additional support comes from the NSF through grants AST-0908805 (C.C.S.) and AST-1313472 (C.C.S., A.L.S.), and an NSF Graduate Student Research Fellowship (A.L.S.). We are grateful to Alaina Henry for providing flux measurements for the Green Pea galaxies, and to the organizers and participants of the First Carnegie Symposium in Honor of Leonard Searle, "Understanding Nebular Emission in High-Redshift Galaxies," held at the Carnegie Observatories in 2015 July, for many useful discussions. Finally, we wish to extend thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests.
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
Subject Keywords:galaxies: evolution; galaxies: formation; galaxies: high-redshift
Issue or Number:1
Record Number:CaltechAUTHORS:20161012-142337724
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Official Citation:Dawn K. Erb et al 2016 ApJ 830 52
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71031
Deposited By: Ruth Sustaita
Deposited On:12 Oct 2016 22:09
Last Modified:11 Nov 2021 04:38

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