REST-FRAME OPTICAL EMISSION LINES IN z ∼ 3.5 LYMAN-BREAK-SELECTED GALAXIES: THE UBIQUITY OF UNUSUALLY HIGH [O III]/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 H β emission lines were detected in 18 Lyman break galaxies (LBGs). The median flux ratio of [O iii ] λ 5007 to H β is . 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 ]/H β = . 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 ]/H β 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.