The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies1

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 ∼ 2.3. We divide our sample into two bins based on their location in the [OIII]5007/Hβ vs. [NII]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 BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV composite spectra, we find that high-redshift galaxies offset towards higher [OIII]λ5007/Hβ and [NII]λ6584/Hα have younger ages ($\log (\textrm {~Age/yr})=7.20^{+0.57}_{-0.20}$) 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 ($\log (\textrm {Age/yr})=8.57^{+0.88}_{-0.84}$, $Z_*=0.0019^{+0.0006}_{-0.0006}$). Additionally, we find that the offset galaxies have an ionization parameter of $\log (U)=-3.04^{+0.06}_{-0.11}$ and nebular metallicity of ($12+\log (\textrm {~O/H})=8.40^{+0.06}_{-0.07}$), and the non-offset galaxies have an ionization parameter of $\log (U)=-3.11^{+0.08}_{-0.08}$ and nebular metallicity of $12+\log (\textrm {~O/H})=8.30^{+0.05}_{-0.06}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more α-enhanced ($7.28^{+2.52}_{-2.82}\textrm {~O/Fe}_{\odot }$) compared to those consistent with the local sequence ($3.04^{+0.95}_{-0.54}\textrm {~O/Fe}_{\odot }$). 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.

The MOSDEF Survey: The First Direct Measurements of the Nebular Dust Attenuation Curve at High Redshift*

“…Here we reevaluate the picture described above in the context of recent studies that demonstrate the necessity of binary stellar populations (or rotating massive stars) to jointly reproduce the rest-frame UV and optical spectra of typical galaxies at z∼2. Specifically, several studies argue that low stellar metallicity massive binary stellar populations are required to reproduce the observed strong line ratios (e.g., [O III]/Hβ versus [N II]/Hα) of z∼2 galaxies while simultaneously matching the rest-frame far-UV spectra (e.g., Steidel et al 2014Steidel et al , 2016Topping et al 2020). Three consequences of these massive binary star models are that they increase massive star main-sequence lifetimes, result in a broader range of stellar masses over which ionizing photon production occurs, and boost the ionizing flux per unit SFR (Eldridge et al 2017).…”

Binary evolution pathways of blue large-amplitude pulsators

“…BPASS is extensively used for interpretation of massive star populations (e.g. Pahl et al 2021;Cullen et al 2021;Vijayan et al 2021;Neugent 2021;Marshall et al 2020;Stevance et al 2020b;Neugent et al 2020;Shivaei et al 2020;Topping et al 2020;Dorn-Wallenstein & Levesque 2020;Wilkins et al 2020;Chrimes et al 2020;Sanders et al 2020). Its accuracy and capabilities in the low mass regime have been explored through the white dwarf progenitor initial to final mass ratio (Eldridge et al 2017), the impact of low mass binary stars on understanding ages of globular clusters and elliptical galaxies (Stanway & Eldridge 2018) and consideration of the type Ia supernova rate (Tang et al 2020), producing good matches to the data in each case.…”

The NIRVANDELS Survey: a robust detection of α-enhancement in star-forming galaxies at z ≃ 3.4

“…Secondly, a deficit of Fe relative to O will mean that the stellar ionizing spectrum of typical high-redshift galaxies is harder at fixed oxygen abundance compared to galaxies in the local Universe. This relative hardening of the ionizing continuum at fixed O/H offers a natural explanation for the observed offset of > 2 star-forming galaxies relative to 0 galaxies in the common line ratio diagrams (e.g., the BPT diagram; Topping et al 2020a;Runco et al 2021). Non-solar /Fe ratios will also force us to re-think current stellar population techniques when applied to high-redshift galaxies.…”