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

We present a detailed study of the rest-optical (3600-7000 angstrom) nebular spectra of similar to 380 star-forming galaxies at z similar or equal to 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-* = 10(9)-10(11.5) M-circle dot and star formation rates SFR - 3-1000 M-circle dot yr(-1). We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O II]lambda lambda 3727, 3729, [Ne III]lambda 3869, H beta, [O III] lambda lambda 4960, 5008, [N II]lambda lambda 6549, 6585, H alpha, and [S II]lambda lambda 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]lambda 5008/H beta) and log([N II]lambda 6585/H alpha). We argue that the primary difference between H II regions in z similar to 2.3 galaxies and those at z similar to 0 is an enhancement in the degree of nebular excitation, as measured by [O III]/H beta and R23 equivalent to log[([O III] lambda lambda 4960, 5008+[O II] lambda lambda 3727, 3729)/H beta]. At the same time, KBSS-MOSFIRE galaxies are similar to 10 times more massive than z similar to 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 similar to 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.