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A hard ionizing spectrum in z=3-4 Lyα emitters with intense [OIII] emission: Analogs of galaxies in the reionization era?

Ellis, R; Nakajima, K; Iwata, I; Kusakabe, H; Ouchi, M; Robertson, B; (2016) A hard ionizing spectrum in z=3-4 Lyα emitters with intense [OIII] emission: Analogs of galaxies in the reionization era? The Astrophysical Journal , 831 (1) , Article L9. 10.3847/2041-8205/831/1/L9. Green open access

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Abstract

We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O iii]λλ5007,4959, [O ii]λ3727, and Hβ for a sample of 15 redshift z≃3.1\mbox-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 ξion—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 > 7 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.

Type: Article
Title: A hard ionizing spectrum in z=3-4 Lyα emitters with intense [OIII] emission: Analogs of galaxies in the reionization era?
Open access status: An open access version is available from UCL Discovery
DOI: 10.3847/2041-8205/831/1/L9
Publisher version: http://dx.doi.org/10.3847/2041-8205/831/1/L9
Language: English
Additional information: Copyright © 2016. The American Astronomical Society. All rights reserved.
Keywords: Galaxies: evolution; galaxies: high-redshift
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/1517651
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