Abstract

I will present new results from the MOSFIRE component of the Keck Baryonic Structure Survey (KBSS-MOSFIRE), focusing on the rest-optical spectra of around 350 star-forming galaxies at z=2-3, for which we have robust measurements of many of the strong diagnostic emission lines (including H-alpha, H-beta, [OIII], [OII], [NII], [SII], and [NeIII]). The observations strongly suggest that the primary difference between HII regions in z=2 galaxies and those in local galaxies is a strong enhancement in the degree of excitation, driven by the hard ionizing radiation from populations of metal-poor stars. At the same time, z=2 KBSS -MOSFIRE galaxies have significantly higher gas-phase oxygen abundances than local star-forming galaxies with similar ionization and excitation properties, along with values of N/O at fixed O/H that are consistent with galaxies in the local Universe. I will show that the combination of these two effects naturally explains the combined nebular properties of high-z galaxies—-including the controversial ‘BPT offset’—-and highlights a population of young, massive galaxies at high-redshift that must have accumulated their stellar mass significantly more rapidly than the majority of galaxies today.