Nebular Emission Line Ratios in z ≃ 2–3 Star-forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio

Abstract

We present a detailed study of the rest-optical (3600–7000 Å) nebular spectra of ~380 star-forming galaxies at z ≃ 2–3, obtained with Keck/Multi-object Spectrometer for Infrared Exploration (MOSFIRE) as part of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample is representative of star-forming galaxies at these redshifts, with stellar masses M_* = M^9 – M^(11.5) M_⊙ and star formation rates SFR = 3–1000 M_⊙ yr^(-1). We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O II]λλ3727, 3729, [Ne III]λ3869, Hβ, [O III]λλ 4960, 5008, [N II]λλ 6549, 6585, Hα, and [S II]λλ6718, 6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey and between subsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number of significant differences in addition to the well-known offset in log([O III]λ5008/Hβ) and log([N II]λ6585/Hα). We argue that the primary difference between H II regions in z ~ 2.3 galaxies and those at z ~ 0 is an enhancement in the degree of nebular excitation, as measured by [O III]/Hβ and R23 ≡ log [([O III]λλ4960, 5008+[O II]λλ3727, 3729)/Hβ]. At the same time, KBSS-MOSFIRE galaxies are ~10 times more massive than z ~ 0 galaxies with similar ionizing spectra and have higher N/O (likely accompanied by higher O/H) at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed N/O and O/H relative to typical z ~ 0 galaxies, consistent with Fe-poor stellar population models that include massive binaries, and highlight a population of massive, high-specific star formation rate galaxies at high redshift with systematically different star formation histories than galaxies of similar stellar mass today.

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