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We present a detailed study of the rest-optical (3600-7000 Angstrom) nebularspectra of ~380 star-forming galaxies at z~2-3 obtained with Keck/MOSFIRE aspart of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample isrepresentative of star-forming galaxies at these redshifts, with stellar massesM*=10^9-10^11.5 M_sun and star formation rates SFR=3-1000 M_sun/yr. We focus onrobust measurements of many strong diagnostic emission lines for individualgalaxies: [O II]3727,3729, [Ne III]3869, H-beta, [O III]4960,5008, [NII]6549,6585, H-alpha, and [S II]6718,6732. Comparisons with observations oftypical local galaxies from the Sloan Digital Sky Survey (SDSS) and betweensubsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number ofsignificant differences in addition to the well-known offset in log([OIII]/H-beta) and log([N II]/H-alpha). We argue that the primary differencebetween H II regions in z~2.3 galaxies and those at z~0 is an enhancement inthe degree of nebular excitation, as measured by [O III]/H-beta and R23=log[([OIII]+[O II])/H-beta]. At the same time, KBSS-MOSFIRE galaxies are ~10 timesmore massive than z~0 galaxies with similar ionizing spectra and have higherN/O (likely accompanied by higher O/H) at fixed excitation. These resultsindicate the presence of harder ionizing radiation fields at fixed N/O and O/Hrelative to typical z~0 galaxies, consistent with Fe-poor stellar populationmodels that include massive binaries, and highlight a population of massive,high-specific star formation rate galaxies at high-redshift with systematicallydifferent star formation histories than galaxies of similar stellar mass today.

astrophysical journal/the astrophysical journal

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