Emission-line Diagnostics at $z\gtrsim 2$: A Probe of the Ionizing Spectrum and $\alpha$ Enhancement Beyond Cosmic Noon

TL;DR

This study uses emission-line ratios from galaxies at $1.4<z<7$ to investigate the evolution of ionizing spectra and chemical abundances, revealing a harder spectrum at $z hickapprox 3.5$ and stability at higher redshifts.

Contribution

It provides new insights into the evolution of the ionizing spectrum and chemical abundance patterns in early galaxies using JWST data.

Findings

Harder ionizing spectrum at $z hickapprox 3.5$ compared to $z hickapprox 2$.

ISM ionizing conditions are similar across $z hickapprox 3.5$ to 7.

Evolution towards softer spectra at lower redshifts reflects chemical abundance changes.

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]$\lambda5008$/H$\beta$, [N II]$\lambda 6585$/H$\alpha$, [S II]$\lambda\lambda 6718,6733$/H$\alpha$, [O I]$\lambda 6302$/H$\alpha$, O32, R23, Ne3O2, and RO2Ne3. We find evidence for a harder ionizing spectrum at $z\sim 3.5$ compared to $z\sim 2$ at fixed gas-phase metallicity, resulting in a pronounced shift in the star-forming galaxy locus on the [N II]/H$\alpha$ BPT diagram and the O32 vs. R23 diagram. At $z\gtrsim 3.5$, star-forming galaxies occupy a common locus, indicating that 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., $\alpha$/Fe) in massive stars providing the ionizing radiation field. Therefore, the lack of evolution in ISM ionizing conditions at $z\gtrsim 3.5$, followed by evolution towards a softer ionizing spectrum at fixed nebular metallicity as cosmic time proceeds to $z\sim 2$ 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.