Rest-frame optical emission lines in z ∼ 3.5 Lyman-break-selected galaxies

the ubiquity of unusually high [OIII]/Hβ ratios at 2 Gyr

B. P. Holden, P. A. Oesch, V. G. González, G. D. Illingworth, I. Labbé, R. Bouwens, M. Franx, P. Van Dokkum, L. Spitler

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28 Citations (Scopus)

Abstract

We present K-band spectra of rest-frame optical emission lines for 24 star-forming galaxies at z ∼ 3.2-3.7 using MOSFIRE on the Keck I telescope. Strong rest-frame optical [OIII] and Hβ emission lines were detected in 18 Lyman break galaxies (LBGs). The median flux ratio of [OIII]λ5007 to Hβ is 5.1+0.5-0.5. This is a factor of 5-10 times higher than in local galaxies with similar stellar masses. None of our sources are detected in deep X-ray stacks, ruling out significant contamination by active galactic nuclei. Combining our sample with a variety of LBGs from the literature, including 49 galaxies selected in a very similar manner, we find a high median ratio of [OIII]/Hβ = 4.8+0.8-1.7. This high ratio seems to be a ubiquitous feature of z ∼ 3-4 LBGs, very different from typical local star-forming galaxies at similar stellar masses. The only comparable systems at z ∼ 0 are those with similarly high specific star formation rates (SSFRs), though ∼5 times lower stellar masses. High SSFRs may result in a higher ionization parameter, higher electron density, or harder ionizing radiation, which, combined different elemental abundances, result in a much higher [OIII]/Hβ line ratio. This implies a strong relation between a global property of a galaxy, the SSFR, and the local conditions of ISM in star-forming regions.

Original languageEnglish
Article number73
Pages (from-to)1-13
Number of pages13
JournalAstrophysical Journal
Volume820
Issue number1
DOIs
Publication statusPublished - 20 Mar 2016

Keywords

  • galaxies: evolution
  • galaxies: high-redshift
  • galaxies: star formation
  • techniques: spectroscopic

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