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 similar to 3.2-3.7 using MOSFIRE on the Keck I telescope. Strong rest-frame optical [O III] and H beta emission lines were detected in 18 Lyman break galaxies (LBGs). The median flux ratio of [O III]lambda 5007 to H beta 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]/H beta = 4.8(-1.7)(+0.8). This high ratio seems to be a. ubiquitous feature of z similar to 3-4 LBGs, very different from typical local star-forming galaxies at similar stellar masses. The only comparable systems at z similar to 0 are those with similarly high specific star formation rates (SSFRs), though similar to 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 beta 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.