We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey ( KBSS ) using MOSFIRE on the Keck 1 telescope , focusing on a sample of 251 galaxies with redshifts 2.0 < z < 2.6 , star-formation rates 2 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } { SFR } \lower 2.15 pt \hbox { $% \buildrel < \over { \sim } $ } 200 M _ { \odot } yr ^ { -1 } , and stellar masses { 8.6 < log ( M _ { \ast } / M _ { \odot } ) < 11.4 } , with high-quality spectra in both H- and K-band atmospheric windows .
We show unambiguously that the locus of z \sim 2.3 galaxies in the ‘ ‘ BPT ’ ’ nebular diagnostic diagram exhibits a disjoint , yet similarly tight , relationship between the ratios [ NII ] \lambda 6585 / H \alpha and [ OIII ] / H \beta as compared to local galaxies .
Using photoionization models , we argue that the offset of the z \sim 2.3 locus relative to z \sim 0 is explained by a combination of harder ionizing radiation field , higher ionization parameter , and higher N/O at a given O/H than applies to most local galaxies , and that the position of a galaxy along the z \sim 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 ionizing radiation field resembles a blackbody with effective temperature T _ { eff } = 50000 - 60000 K and N/O close to the solar value at all O/H .
We critically assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities , and consider the implications of the small intrinsic scatter in the empirical relationship between excitation-sensitive line indices and M _ { \ast } ( i.e. , the ‘ ‘ mass-metallicity ’ ’ relation ) , at z \simeq 2.3 .