Temperature based radial metallicity gradients in nearby galaxies

TL;DR

This study uses PHANGS-MUSE data to map gas-phase metallicities in 19 nearby spiral galaxies through $T_{ m e}$[N II]-based abundances, analyzing 534 individual H II regions and 215 stacked samples. By stacking in bins of $L([N II]6583)$ and radius, the authors obtain robust [N II] 5755 detections and demonstrate that the resulting gradients agree with strong-line scalings, confirming a well-mixed ISM with low scatter ($\sigma(O/H)\sim0.05$–$0.1$ dex). They find that $T_{ m e}$[N II] can serve as a reliable single-line direct-method abundance diagnostic in the high-metallicity regime, while the absolute offset to strong-line metallicities (~0.3 dex) reflects the longstanding abundance discrepancy factor. The results show no strong arm–interarm metallicity offsets and highlight the utility of auroral-line stacking for precise abundance studies, supporting the consistency between direct-method and strong-line approaches for typical star-forming spirals.

Abstract

Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals build up and redistribute within galaxy disks over cosmic time. We use a catalog of 22,958 HII regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced using only the [NII]5755 electron temperature as a proxy for `direct method' metallicities. Using 534 direct detections of the temperature sensitive [NII]5755 auroral line, we measure gradients in 15 of the galaxies. Leveraging our large catalog of individual HII regions, we stack in bins of HII region [NII]6583 luminosity and radius to recover stacked radial gradients. We find good agreement between the metallicity gradients from the stacked spectra, those gradients from individual regions and those from strong line methods. In addition, particularly in the stacked Te([NII]) measurements, some galaxies show very low (<0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examine individual high confidence (S/N > 5) outliers and identify 13 regions across 9 galaxies with anomalously low metallicity, although this is not strongly reflected in the strong line method metallicities. By stacking arm and interarm regions, we find no systematic evidence for offsets in metallicity between these environments, suggesting enrichment within spiral arms is due to very localized processes. This work demonstrates the potential to systematically exploit the single [NII]5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.

Figures (12)

• Figure 1: A comparison of $T_{\rm e}$[N ii] measurements used in this work Brazzini2024 with three different sets of measurements available in the literature. While auroral line fits in Kreckel2022 (left) and RRV2024 (center) are both based on the same underlying MUSE data set, different assumptions have been made in the SSP fitting and region boundaries. Berg2015 measurements (right) are based on independent long-slit spectroscopy. As there are only five regions in common with Berg2015, no offset or scatter is calculated in comparison with this sample.
• Figure 2: $T_{\rm e}$[N ii] as a function of radius for all 19 galaxies. We compare individual H ii regions (S/N$>$3 open circles, S/N$>$5 filled circles) with measurements from H ii region stacks (squares and lines). Points are color coded by their [N ii]$\lambda$6583 Luminosity (L[[N ii]]). Note that H ii regions in NGC 4254 and NGC 4535 do not cover the full disk due to the AO notch filter. Galaxies are ordered from low (top-left) to high (bottom-right) stellar mass, and all galaxies are shown with matched scales, so it is possible to directly compare the absolute values and slopes across the sample.
• Figure 3: A comparison of metallicity gradients. The Scal values (grey) are compared with individual $T_{\rm e}$[N ii] metallicities (red) and $T_{\rm e}$[N ii] stacked metallicities (black). For context, the subsample of individual regions with $T_{\rm e}$[N ii] detections are highlighted within the Scal measurements in light blue. Linear radial gradients are fit when there are at least 5 measurements that cover at least 0.5 r$_{\rm eff}$. Note that H ii regions in NGC 4254 and NGC 4535 do not cover the full disk due to the AO notch filter. Galaxies are ordered from low (top-left) to high (bottom-right) stellar mass, and axis scalings are not matched between galaxies.
• Figure 4: A comparison of the metallicity gradient (left), intercept (center) and scatter (right) between Scal metallicities and the $T_{\rm e}$[N ii] metallicities in individual H ii regions (red) and stacks (black). For reference, the 1-to-1 line is shown, as well as a fixed offset of 0.3 dex for the intercept. In the right panel, the scatter is measured using individual regions with S/N$>$3 (open) or S/N$>$5 (filled).
• Figure 5: The distribution of $\Delta$(O/H) based on the Scal metallicities for the full H ii region sample (filled blue). This is compared with the offsets measured from the strong-line Scal prescription for $T_{\rm e}$[N ii] outliers to low (blue lines) and high (red lines) metallicities. The full H ii region sample distribution has been normalized, while the outlier distribution reflects direct counts.