Rest-frame Optical Emission Lines in z ∼ 3.5 Lyman-break-selected Galaxies: The Ubiquity of Unusually High [OIII]/Hβ Ratios at 2 Gyr
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 [O III] and Hβ emission lines were detected in 18 Lyman break galaxies (LBGs). The median flux ratio of [O III]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 [O III]/Hβ = {4.8}-1.7+0.8. 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 [O III]/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.
Partially based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555. Partially based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.- Publication:
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The Astrophysical Journal
- Pub Date:
- March 2016
- DOI:
- 10.3847/0004-637X/820/1/73
- arXiv:
- arXiv:1401.5490
- Bibcode:
- 2016ApJ...820...73H
- Keywords:
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- galaxies: evolution;
- galaxies: high-redshift;
- galaxies: star formation;
- techniques: spectroscopic;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 14 pages, 8 figures, 5 color, published in ApJ, updated to reflect published version