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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
Ist Teil von
The Astrophysical journal, 2016-03, Vol.820 (1), p.73
Ort / Verlag
United Kingdom
Erscheinungsjahr
2016
Quelle
EZB Electronic Journals Library
Beschreibungen/Notizen
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.