SIMBAD references

We present a joint analysis of rest-ultraviolet (UV) and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at z ∼ 2.3. We divide our sample into two bins based on their location in the [O III]5007/Hβ versus [N II]6584/Hα 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 Binary Population and Spectral Synthesis (BPASS) stellar population synthesis models, including nebular continuum emission, to our rest-UV composite spectra, we find that high-redshift galaxies offset towards higher [O III]λ5007/Hβ and [N II]λ6584/Hα 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+0.0006^_–0.0006). 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 an ionization parameter of log(U)=-3.11^+0.08_–0.08 and nebular metallicity of 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 α-enhanced (7.28^+2.52_–2.82 O/Fe_☉) compared to those consistent with the local sequence (3.04^+0.95_–0.54 O/Fe_☉). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be α-enhanced - in contrast with typical local systems. Such differences must be considered when estimating gas-phase oxygen abundances 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.