A Hard Ionizing Spectrum in z = 3-4 Lyα Emitters with Intense [O III] Emission: Analogs of Galaxies in the Reionization Era?
Abstract
We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O III]λ λ 5007,4959, [O II]λ 3727, and Hβ for a sample of 15 redshift z≃ 3.1{--}3.7 Lyα emitters (LAEs) and Lyman break galaxies (LBGs). In conjunction with spectra from other surveys, we confirm earlier indications that LAEs have a much higher [O III]/[O II] line ratio than is seen in similar redshift LBGs. By comparing their distributions on a [O III]/[O II] versus R23 diagram, we demonstrate that this difference cannot arise solely because of their lower metallicities but most likely is due to a harder ionizing spectrum. Using measures of Hβ and recombination theory, we demonstrate, for a subset of our LAEs, that {ξ }{ion}—the number of Lyman continuum photons per UV luminosity—is indeed 0.2-0.5 dex larger than for typical LBGs at similar redshifts. Using photoionization models, we estimate the effect this would have on both [O III]/[O II] and R23 and conclude such a hard spectrum can only partially explain such intense line emission. The additional possibility is that such a large [O III]/[O II] ratio is in part due to density rather than ionization bound nebular regions, which would imply a high escape fraction of ionizing photons. We discuss how further observations could confirm this possibility. Clearly LAEs with intense [O III] emission represent a promising analog of those z\gt 7 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.
- Publication:
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The Astrophysical Journal
- Pub Date:
- November 2016
- DOI:
- 10.3847/2041-8205/831/1/L9
- arXiv:
- arXiv:1608.08222
- Bibcode:
- 2016ApJ...831L...9N
- Keywords:
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- galaxies: evolution;
- galaxies: high-redshift;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 7 pages, 4 figures. Accepted for publication in ApJ Letters