A HARD IONIZING SPECTRUM in z = 3-4 Lyα EMITTERS with INTENSE [O III ]EMISSION: ANALOGS of GALAXIES in the REIONIZATION ERA?

Kimihiko Nakajima, Richard S. Ellis, Ikuru Iwata, Akio K. Inoue, Haruka Kusakabe, Masami Ouchi, Brant E Robertson

Research output: Contribution to journalArticle

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Abstract

We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O iii], [O ii], and Hβ for a sample of 15 redshift 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 - 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 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.

Original languageEnglish (US)
Article numberL9
JournalAstrophysical Journal Letters
Volume831
Issue number1
DOIs
StatePublished - Nov 1 2016
Externally publishedYes

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analogs
galaxies
photons
recombination
set theory
escape
metallicity
photoionization
indication
emitters
ionization
diagram
diagrams
luminosity
continuums
estimates
distribution
effect

Keywords

  • galaxies: evolution
  • galaxies: high-redshift

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

A HARD IONIZING SPECTRUM in z = 3-4 Lyα EMITTERS with INTENSE [O III ]EMISSION : ANALOGS of GALAXIES in the REIONIZATION ERA? / Nakajima, Kimihiko; Ellis, Richard S.; Iwata, Ikuru; Inoue, Akio K.; Kusakabe, Haruka; Ouchi, Masami; Robertson, Brant E.

In: Astrophysical Journal Letters, Vol. 831, No. 1, L9, 01.11.2016.

Research output: Contribution to journalArticle

Nakajima, Kimihiko ; Ellis, Richard S. ; Iwata, Ikuru ; Inoue, Akio K. ; Kusakabe, Haruka ; Ouchi, Masami ; Robertson, Brant E. / A HARD IONIZING SPECTRUM in z = 3-4 Lyα EMITTERS with INTENSE [O III ]EMISSION : ANALOGS of GALAXIES in the REIONIZATION ERA?. In: Astrophysical Journal Letters. 2016 ; Vol. 831, No. 1.
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abstract = "We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O iii], [O ii], and Hβ for a sample of 15 redshift 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 - 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 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.",
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AU - Robertson, Brant E

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AB - We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O iii], [O ii], and Hβ for a sample of 15 redshift 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 - 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 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization.

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