We present a joint analysis of rest-UV and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at $z\sim2.3$. We divide our sample into 2 bins based on their location in the [OIII]/Hb vs. [NII]/Ha BPT diagram, and perform the first differential study of the rest-UV properties of massive ionizing stars as a function of rest-optical emission-line ratios. Fitting BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV spectra, we find that high-redshift galaxies offset towards higher [OIII]/Hb and [NII]/Ha have younger ages (log(Age/yr)=$7.20^{ +0.57}_{ -0.20}$) and lower stellar metallicities ($Z_*=0.0010^{ +0.0011}_{ -0.0003}$) resulting in a harder ionizing spectrum, compared to the galaxies in our sample that lie on the local BPT star-forming sequence (log(Age/yr)=$8.57^{ +0.88}_{ -0.84}$, $Z_*=0.0019\pm0.0006$). Additionally, we find that the offset galaxies have an ionization parameter of log(U)=$-3.04^{ +0.06}_{ -0.11}$ and nebular metallicity of 12+log(O/H)=$8.40^{ +0.06}_{ -0.07}$, and the non-offset galaxies have log(U)=$-3.11\pm0.08$ and 12+log(O/H)=$8.30^{ +0.05}_{ -0.06}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more alpha-enhanced ($7.28^{ +2.52}_{ -2.82}$ O/Fe$_\odot$) compared to those consistent with the local sequence ($3.04^{ +0.95}_{ -0.54}$ O/Fe$_\odot$). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be alpha-enhanced -- in contrast with typical local systems. Such differences must be considered when estimating gas-phase O/H at high redshift based on strong emission-line ratios. Specifically, a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties.
Categories: astro-ph.GA