Strong Nebular Line Ratios in the Spectra of z ~ 2-3 Star Forming Galaxies: First Results from KBSS-MOSFIRE

We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey using the recently commissioned MOSFIRE spectrometer on the Keck 1 telescope. We focus on a sample of 251 galaxies with redshifts 2.0 < z < 2.6, star formation rates (SFRs) 2 ≾ SFR ≾ 200 M_☉ yr^(–1), and stellar masses 8.6 < log (M_*/M_☉) < 11.4, with high-quality spectra in both H- and K-band atmospheric windows. We show unambiguously that the locus of z ~ 2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits an almost entirely disjointed, yet similarly tight, relationship between the line ratios [N II] λ6585/Hα and [O III]/Hβ as compared to local galaxies. Using photoionization models, we argue that the offset of the z ~ 2.3 BPT locus relative to that at z ~ 0 is caused by a combination of harder stellar ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H compared to most local galaxies, and that the position of a galaxy along the z ~ 2.3 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The observed nebular emission line ratios are most easily reproduced by models in which the net stellar ionizing radiation field resembles a blackbody with effective temperature T_(eff) = 50, 000-60, 000 K, the gas-phase oxygen abundances lie in the range 0.2 < Z/Z_☉ < 1.0, and the ratio of gas-phase N/O is close to the solar value. We critically assess the applicability at high redshift of commonly used strong line indices for estimating gas-phase metallicity, and consider the implications of the small intrinsic scatter of the empirical relationship between excitation-sensitive line indices and M_* (i.e., the "mass-metallicity" relation) at z ≃ 2.3.