We use photoionization models designed to reconcile the joint rest-UV-optical spectra of high- z star-forming galaxies to self-consistently infer the gas chemistry and nebular ionization and excitation conditions for \sim 150 galaxies from the Keck Baryonic Structure Survey ( KBSS ) , using only observations of their rest-optical nebular spectra .
We find that the majority of z \sim 2 - 3 KBSS galaxies are moderately O-rich , with an interquartile range in 12 + \log ( \textrm { O / H } ) = 8.29 - 8.56 , and have significantly sub-solar Fe enrichment , with an interquartile range of [ Fe/H ] = [ -0.79 , -0.53 ] , contributing additional evidence in favor of super-solar O/Fe in high- z galaxies .
Model-inferred ionization parameter and N/O are strongly correlated with common strong-line indices ( such as O32 and N2O2 ) , with the latter exhibiting similar behavior to local extragalactic H ii regions .
In contrast , diagnostics commonly used for measuring gas-phase O/H ( such as N2 and O3N2 ) show relatively large scatter with the overall amount of oxygen present in the gas and behave differently than observed at z \sim 0 .
We provide a new calibration for using R23 to measure O/H in typical high- z galaxies , although it is most useful for relatively O-rich galaxies ; combining O32 and R23 does not yield a more effective calibration .
Finally , we consider implications for the intrinsic correlations between physical conditions across the galaxy sample and find that N/O varies with O/H in high- z galaxies in a manner almost identical to local H ii regions .
However , we do not find a strong anti-correlation between ionization parameter and metallicity ( O/H or Fe/H ) in high- z galaxies , which is one of the principal bases for using strong-line ratios to infer oxygen abundance .