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 ~380 star-forming galaxies at z~2-3 obtained with Keck/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_sun and star formation rates SFR=3-1000 M_sun/yr. We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O II]3727,3729, [Ne III]3869, H-beta, [O III]4960,5008, [N II]6549,6585, H-alpha, and [S II]6718,6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey (SDSS) 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-beta) and log([N II]/H-alpha). 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]/H-beta and R23=log[([O III]+[O II])/H-beta]. 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. 
Allison L. Strom, Charles C. Steidel, Gwen C. Rudie, Ryan F. Trainor,   Max Pettini, and Naveen A. Reddy
astro-ph.GA astro-ph.CO