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 , 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 and star formation rates SFR = 3–1000  yr −1 . We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O ii ] λλ 3727, 3729, [Ne iii ] λ 3869, H β , [O iii ] 4960, 5008, [N ii ] λλ 6549, 6585, H α , and [S ii ] λλ 6718, 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 ] /H β ) and log([N ii ] /H α ). We argue that the primary difference between H ii regions in galaxies and those at is an enhancement in the degree of nebular excitation, as measured by [O iii ]/H β and [([O iii ] +[O ii ] )/H β ]. At the same time, KBSS-MOSFIRE galaxies are ∼10 times more massive than 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 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.