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* **

摘要

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 . 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 = . 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.