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

TL;DR

Using a large JWST/JADES DR3 sample of $1.4<z<7$ galaxies, the study shows that at fixed nebular metallicity the ionizing spectrum is harder around $z\sim 3.5$ than at $z\sim 2$, shifting star-forming galaxies on the BPT and O32–R23 diagrams. Beginning at $z\gtrsim 3.5$, the star-forming loci overlap across redshift, suggesting little evolution in ISM ionization conditions, while later cosmic time shows a shift toward a softer spectrum as iron accumulates in stellar atmospheres, linked to evolving $\alpha$/Fe. The authors interpret these trends as evidence for $\alpha$-enhanced abundance patterns at high redshift and consistent regulation of the ionizing radiation field, with implications for metallicity calibrations into the Epoch of Reionization. Overall, the work demonstrates the continued diagnostic power of rest-optical emission lines with JWST for constraining ISM physics deep into the early universe.

Abstract

We analyze several key rest-optical emission-line ratios in a sample of 763 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$β$, [N II]$λ6585$/H$α$, [S II]$λλ6718,6733$/H$α$, [O I]$λ6302$/H$α$, 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$α$ 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., $α$/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.

Figures (5)

• Figure 1: Stacked composite spectra in each of the four redshift bins examined in this study. As labeled, each stack is composed of galaxies with median stellar masses of $\sim$$10^9\ \rm M_\odot$. The insets showcase several faint emission lines analyzed in this study.
• Figure 2: BPT diagrams for the JADES DR3 sample. The boundaries separating SF galaxies from AGN from 2001ApJ...556..121K and 2003MNRAS.346.1055K are shown as solid and dashed black lines, respectively. The gray 2D histogram shows the distribution of $z \sim 0$ galaxies from SDSS 2009ApJS..182..543A. The pink dashed curve shows the binned median sequence of local H ii regions from the CHAOS survey 2015ApJ...806...16B and a compilation by 2016MNRAS.457.3678P. The [N ii] and [O i] diagrams bin by [O iii]/H$\beta$, while the [S ii] diagram bins by electron temperature. Cloudy curves from 2020ApJ...902L..16J are overlaid and color-coded by stellar metallicity. The sizes of the triangles on the model curves correspond to the ionization parameter. Emission-line sequences from 2013ApJ...774..100K ($z\sim 0$; lower [N ii] panel), 2015ApJ...801...88S ($z\sim 2$; upper [N ii] panel), and 2017ApJ...836..164S ($z\sim 2$; upper [N ii] and [S ii] panels) are shown for reference.
• Figure 3: Emission-line ratios for stacked measurements shown in Figure \ref{['fig:bpt']}
• Figure 4: Top: O32 vs. R23 diagram. Bottom: Ne3O2 vs. RO2Ne3 diagram. The gray 2D histogram shows $z\sim 0$ galaxies, where AGN are removed based on the 2003MNRAS.346.1055K criterion. The color coding follows the legend in Figure \ref{['fig:bpt']}.
• Figure 5: Emission-line ratios for stacked measurements shown in Figure \ref{['fig:o32_r23']}