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

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^+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 [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.