Rest-Frame Optical Emission Lines in z~3.5 Lyman Break selected
  Galaxies: The Ubiquity of Unusually High [OIII]/Hbeta 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 1 telescope.
Strong rest-frame optical [O III] and Hbeta emission lines were detected in 18
LBGs. The median flux ratio of [O III]5007 to Hbeta is 5.1+/-0.5, a factor of
5-10x higher than in local galaxies with similar stellar masses. The observed
Hbeta luminosities are in good agreement with expectations from the estimated
star-formation rates, and 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
[OIII]/Hbeta = 4.8+0.8-1.7. This high ratio seems to be an 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, though ~5x lower stellar masses.
High specific star-formation rates either result in a much higher ionization
parameter or other unusual conditions for the interstellar medium, which result
in a much higher [OIII]/Hbeta line ratio. This implies a strong relation
between a global property of a galaxy, the specific star-formation rate, and
the local conditions of ISM in star-forming regions.
