The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies

Abstract

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 $z\sim 2.3$. 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)=$7.20_{-0.20}^{+0.57}$) and lower stellar metallicities ($Z_{\ast }=0.0010_{-0.0003}^{+0.0011}$) 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)=$8.57_{-0.84}^{+0.88}$, $Z_{\ast }=0.0019\pm 0.0006$). Additionally, we find that the offset galaxies have an ionization parameter of log(U)=$-3.04_{-0.11}^{+0.06}$ and nebular metallicity of 12+log(O/H)=$8.40_{-0.07}^{+0.06}$, and the non-offset galaxies have log(U)=$-3.11\pm 0.08$ and 12+log(O/H)=$8.30_{-0.06}^{+0.05}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more alpha-enhanced ($7.28_{-2.82}^{+2.52}$ O/Fe${}_{\odot }$) compared to those consistent with the local sequence ($3.04_{-0.54}^{+0.95}$ 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.

16 pages, 12 figures. Accepted to MNRAS

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