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Title: 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.  more » « less
Award ID(s):
2009313 2009085
PAR ID:
10249682
Author(s) / Creator(s):
; ; ; ; ; ; ;
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
495
Issue:
4
ISSN:
0035-8711
Page Range / eLocation ID:
4430-4444
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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