Emission-line Diagnostics at z ≳ 2: A Probe of the Ionizing Spectrum and α-enhancement beyond Cosmic Noon

Authors

• Leonardo Clarke, University of California - Los AngelesFollow
• Natalie Lam, University of California - Los Angeles
• Alice E. Shapley, University of California - Los AngelesFollow
• Michael W. Topping, University of Arizona
• Gabriel B. Brammer, Cosmic Dawn Center
• Ryan L. Sanders, University of KentuckyFollow
• Naveen Reddy, University of California - RiversideFollow
• Shreya Karthikeyan, University of California - Los Angeles

Abstract

We analyze several key rest-optical emission-line ratios in a sample of 828 galaxies as well as composite spectra from JADES DR3 in the range 1.4 <  z <  7. These emission-line ratios include: [O iii]λ5008/Hβ, [N ii]λ6585/Hα, [S ii]λλ6718, 6733/Hα, [O i]λ6302/Hα, O32, R23, Ne3O2, and RO2Ne3. We find evidence for a harder ionizing spectrum at z ∼ 3.5 compared to z ∼ 2 at fixed gas-phase metallicity, resulting in a pronounced shift in the star-forming galaxy locus on the [N ii]/Hα Baldwin-Phillips-Terlevich (BPT) diagram and the O32 versus R23 diagram. At z ≳ 3.5, star-forming galaxies occupy a common locus, indicating that interstellar medium (ISM) ionizing conditions at fixed gas-phase metallicity do not evolve strongly at these early cosmic times. There is a connection between ISM ionizing conditions and the chemical abundance patterns (i.e., α/Fe) in massive stars, providing the ionizing radiation field. Therefore, the lack of evolution in ISM ionizing conditions at z ≳ 3.5, followed by evolution towards a softer ionizing spectrum at fixed nebular metallicity as cosmic time proceeds to z ∼ 2 and lower redshift mirrors the chemical abundance patterns in Milky Way stars as a function of iron abundance. Our results highlight the diagnostic power of emission-line diagrams in the era of JWST to further our understanding of the ISM conditions into the Epoch of Reionization.

Document Type

Article

Publication Date

2026

Notes/Citation Information

© 2026. The Author(s). Published by the American Astronomical Society.

Digital Object Identifier (DOI)

https://doi.org/10.3847/2041-8213/ae57a6

Funding Information

We would like to acknowledge the JADES team for their efforts in designing, executing, and making public their JWST/NIRSpec and JWST/NIRCam survey data. We also acknowledge support from NASA grants JWST-GO-01914 and JWST-GO-03833, and NSF AAG grants 2009313, 2009085, 2307622, and 2307623. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope as well as the NASA/ESA Hubble Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-03127 for JWST and NAS 5–26555 for HST. Data were also obtained from the DAWN JWST Archive maintained by the Cosmic Dawn Center. The specific observations analyzed can be accessed via M. Rieke et al. (2023; JADES), P. Oesch & D. Magee (2023; FRESCO), and C. Williams et al. (2023; JEMS). This work used computational and storage services associated with the Hoffman2 Cluster, which is operated by the UCLA Office of Advanced Research Computing’s Research Technology Group.

Repository Citation

Clarke, Leonardo; Lam, Natalie; Shapley, Alice E.; Topping, Michael W.; Brammer, Gabriel B.; Sanders, Ryan L.; Reddy, Naveen; and Karthikeyan, Shreya, "Emission-line Diagnostics at z ≳ 2: A Probe of the Ionizing Spectrum and α-enhancement beyond Cosmic Noon" (2026). Physics and Astronomy Faculty Publications. 765.
https://uknowledge.uky.edu/physastron_facpub/765