SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies

Amrita Singh; Guillermo A. Blanc; Kathryn Kreckel; J. E. Méndez-Delgado; Christophe Morisset; Sebastián F. Sánchez; Oleg Egorov; Niv Drory; Nimisha Kumari; Fu-Heng Liang; Enrico Congiu; Alfredo Mejía-Narváez; Evelyn J. Johnston; José G. Fernández-Trincado; Hemanth Bommireddy; Abhinna Sundar Samantaray; Thomas Hilder; Rodolfo de J. Zermeño; Ivan Katkov; Dhanesh Krishnarao; Juna A. Kollmeier

AI summary · arXiv: 2610.40045v1 · astro-ph.GA

SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies

Amrita Singh, Guillermo A. Blanc, Kathryn Kreckel, J. E. Méndez-Delgado, Christophe Morisset, Sebastián F. Sánchez, Oleg Egorov, Niv Drory,

and 13 more authors

Nimisha Kumari, Fu-Heng Liang, Enrico Congiu, Alfredo Mejía-Narváez, Evelyn J. Johnston, José G. Fernández-Trincado, Hemanth Bommireddy, Abhinna Sundar Samantaray, Thomas Hilder, Rodolfo de J. Zermeño, Ivan Katkov, Dhanesh Krishnarao, Juna A. Kollmeier

Original paper Submitted to arXiv on 29 Oct. 2026 Accepted to ApJ. 12 pages, 6 Figures License: CC BY 4.0 Also in: fr

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The essentials

Spatially resolved observations of the Lagoon Nebula link temperature fluctuations to the oxygen abundance discrepancy and show that accounting for them reconciles CEL- and RL-based abundances.

Question

What causes the long-standing difference between oxygen abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) in the Lagoon Nebula? Can spatially resolved temperature diagnostics identify that cause and constrain the nebula’s average temperature?

Method

The authors analyze SDSS-V Local Volume Mapper integral-field observations of M8 at 0.21 pc per spaxel, using individual spaxels, 1′ (0.36 pc) annuli, and the average spectrum. They compare CEL-based and hybrid RL/CEL temperatures and CEL- and RL-based O++ abundances, then model temperature fluctuations over grids of T₀ = 5000–12000 K and t² = 0–0.1; uncertainties are propagated through 50 Monte Carlo realizations.

Findings

Result The spatially resolved ADF and temperature discrepancy are strongly correlated, with Pearson r = 0.93.
See the passage in the paper
The spatially resolved measurements yield a Pearson correlation coefficient of 0.93 (p-value=$10^{-6}$), confirming a robust point-to-point correspondence throughout the full FOV.
Authors’ interpretation The observed ADF–temperature-discrepancy relation agrees with the temperature-fluctuation model, supporting temperature inhomogeneities as the primary driver of the discrepancy in M8.
See the passage in the paper
The agreement of this observed relation with the theoretical predictions of the temperature-perturbation formalism, shows that $T_{e}$ fluctuations are the primary drivers of the CEL–RL abundance discrepancy in M8.
Result The mean temperature inferred from the average spectrum is 6240 K, substantially below the CEL-based temperature of 8025 K.
See the passage in the paper
The average M8 spectrum yields a best-fit average electron temperature of $T_{0}=6240^{+310}_{-360}$ K.
Result Using the inferred mean temperature for both abundance estimates reduces the average-spectrum ADF from 0.47 dex to −0.04 dex.
See the passage in the paper
Adopting a value of $T_{0}=6240$ K to derive both RL-and CEL-based O++ abundances from the average spectrum of the nebula, reduces the abundance discrepancy in M8 from $0.47\pm 0.02$ dex to $-0.04\pm 0.02$ dex, effectively solving the abundance discrepancy problem.
Authors’ interpretation The authors propose the temperature difference between the CEL and hybrid diagnostics as an observational proxy for the amplitude of temperature fluctuations.
See the passage in the paper
It therefore serves as a direct observational proxy of $t^{2}$ and a means of constraining the thermal structure of the nebula.

Limitations

The evidence comes from one nebula, so the authors say it remains to be established whether the measured temperature fluctuations are common in other interstellar environments.
See the passage in the paper
Determining whether the temperature fluctuations identified in M8 are common across different ISM environments is therefore a critical next step.
The usable CEL and RL diagnostics are limited to the central portion of the observed field where the signal-to-noise requirements are met.
See the passage in the paper
However, the spaxels satisfying the signal-to-noise ($S/N$) requirements for the CEL and RL-based diagnostics are concentrated within the central part of the observed field.

Why it matters

The authors argue that temperature structure must be accounted for to obtain reliable CEL-based abundances, and that combining the ADF with the temperature discrepancy offers a way to constrain that structure and the mean gas temperature. They present M8 as a resolved benchmark for testing how this effect shapes abundance measurements in other nebulae and, ultimately, galaxy-scale metallicity estimates.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: abundance discrepancy temperature fluctuations ionized nebulae oxygen abundances Lagoon Nebula

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper investigates the long-standing abundance discrepancy in ionized nebulae: oxygen abundances inferred from collisionally excited lines (CELs) are often lower than those derived from recombination lines (RLs). It examines whether spatial variations in electron temperature can explain this difference in the Lagoon Nebula (M8), and whether observations can quantify the nebula’s thermal structure. The authors study doubly ionized oxygen (O++) using spatially resolved integral-field observations from the SDSS-V Local Volume Mapper (LVM). They compare two temperature diagnostics. The CEL temperature is derived from the ratio of the [O III] line at 4363 Å to the stronger [O III] lines at 4959 and 5007 Å. The hybrid diagnostic combines O II recombination-line emission with [O III] emission. The temperature difference, ΔTe, is defined as the CEL temperature minus the hybrid temperature. The authors also calculate the abundance discrepancy factor (ADF), defined as the logarithmic difference between RL- and CEL-derived O++ abundances. Measurements are analyzed for individual spatial elements, in one-arcminute-wide annuli centered on the main ionizing source, Herschel 36, and in a spectrum averaged across the observed nebula. To interpret the observations, the authors use a temperature-fluctuation model based on *t*², which describes the relative variance of electron temperatures around a mean temperature, *T*₀. Across M8, RL-based O++ abundances exceed CEL-based abundances. Both the ADF and ΔTe are greatest near Herschel 36 and decline with distance. The CEL temperature rises toward the central region, while the hybrid temperature is lower and comparatively uniform. The authors interpret this pattern as evidence of stronger temperature inhomogeneity near the ionizing source. These inhomogeneities raise temperatures inferred from CELs and, in turn, lower CEL-based oxygen abundance estimates. The central result is a strong relationship between the ADF and ΔTe. Their spatially resolved measurements have a Pearson correlation coefficient of 0.93 (p = 10⁻⁶); the annular measurements show the same correlation (p = 4.4 × 10⁻⁸). The observed relationship agrees with the temperature-fluctuation model. In that model, ΔTe increases approximately linearly with *t*² and depends only weakly on *T*₀, making it a practical observational indicator of fluctuation amplitude. For the average M8 spectrum, the authors estimate a mean temperature of 6,240 K, with uncertainties of +310 and −360 K. This is about 1,800 K below the CEL temperature of 8,025 ± 67 K and is consistent, within uncertainties, with the hybrid temperature of 6,364 ± 49 K. The inferred temperature-fluctuation amplitude is 22 ± 1%, corresponding to *t*² = 0.050. Annular estimates range from 24 ± 0.4% near the center to 20 ± 3% in the outermost bin. Using the inferred mean temperature to calculate both abundance estimates reduces the average ADF from 0.47 ± 0.02 dex to −0.04 ± 0.02 dex, effectively bringing the RL and CEL results into agreement. The authors conclude that temperature inhomogeneities are the primary driver of the abundance discrepancy in M8, and that jointly using the ADF and ΔTe can help constrain nebular thermal structure and improve abundance estimates. The findings apply to one nebula and do not establish whether the measured fluctuations are typical of all H II regions. Conditions in other environments, particularly unresolved distant galaxies, may differ.

Mapping the Lagoon Nebula’s Hidden Temperature Structure

In astronomy, a basic question can have a stubbornly complicated answer: how much oxygen does a nebula contain? Different methods often produce different results. Measurements based on collisionally excited lines tend to give lower oxygen abundances than those based on recombination lines. This long-standing mismatch is known as the abundance discrepancy.

In “SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies,” Amrita Singh, Guillermo A. Blanc, Kathryn Kreckel, J. E. Méndez-Delgado, Christophe Morisset, Sebastián F. Sánchez et al. investigate whether uneven temperatures inside a nebula can explain the disagreement. Their target is the Lagoon Nebula, also known as M8, where they map the gas around the bright ionizing source Herschel 36.

Two Ways to Measure Temperature

The team used spatially resolved observations from the SDSS-V Local Volume Mapper to study doubly ionized oxygen, or O++. They compared two temperature diagnostics. One relies on collisionally excited lines (CELs), using the ratio of a faint [O III] line at 4363 angstroms to stronger [O III] lines at 4959 and 5007 angstroms. The other is a hybrid diagnostic that combines O II recombination-line emission with [O III] emission.

The difference between these temperatures, called ΔTe, is the CEL temperature minus the hybrid temperature. The researchers also calculated the abundance discrepancy factor, or ADF: the logarithmic difference between O++ abundances derived from recombination lines (RLs) and collisionally excited lines.

Rather than treating M8 as a single uniform cloud, the team examined individual spatial elements, one-arcminute-wide rings centered on Herschel 36, and a spectrum averaged over the observed nebula. This made it possible to see how the temperature and abundance measurements change with distance from the central source.

A Stronger Discrepancy Near Herschel 36

Across M8, the RL-based oxygen abundances were higher than the CEL-based abundances. But the size of the gap was not the same everywhere: both the ADF and ΔTe were greatest near Herschel 36 and declined farther out.

The temperature measurements showed a matching pattern. The CEL temperature rose toward the central region, while the hybrid temperature was lower and comparatively uniform. The researchers interpret this contrast as evidence that the gas near Herschel 36 has a more uneven temperature structure.

Why would that matter for oxygen? CELs respond strongly to temperature. If hot pockets raise the temperature inferred from CELs, the resulting abundance calculation can underestimate how much oxygen is present. RL-based estimates are less affected by the same temperature variations. In this picture, the abundance discrepancy is not necessarily a disagreement about the oxygen itself; it is partly a consequence of averaging over gas with different temperatures.

Connecting the Observations to Temperature Fluctuations

To test this explanation, the authors used a model based on t2, a measure of how much electron temperatures vary around the mean temperature, T0. The model predicts that ΔTe increases approximately linearly with t2 and depends only weakly on T0. That makes ΔTe a potentially practical observational indicator of the strength of temperature fluctuations.

The observed connection was striking. Across individual spatial elements, the ADF and ΔTe had a Pearson correlation coefficient of 0.93, with a p-value of 10−6. The annular measurements showed the same relationship, with a p-value of 4.4 × 10−8. The trend also agreed with the temperature-fluctuation model.

For the spectrum averaged across M8, the team estimated a mean temperature of 6,240 K, with uncertainties of +310 and −360 K. That is about 1,800 K below the CEL temperature of 8,025 ± 67 K, and consistent within uncertainties with the hybrid temperature of 6,364 ± 49 K. The inferred temperature-fluctuation amplitude was 22 ± 1%, corresponding to t2 = 0.050. Across the annuli, the estimated amplitude ranged from 24 ± 0.4% near the center to 20 ± 3% in the outermost bin.

Bringing the Abundance Estimates Together

The researchers then used the inferred mean temperature to calculate both RL- and CEL-based abundances. The average ADF fell from 0.47 ± 0.02 dex to −0.04 ± 0.02 dex—close enough to zero that the two methods effectively agree.

This result supports the idea that temperature inhomogeneities are the primary driver of the abundance discrepancy in M8. It also suggests a useful approach for studying nebulae: measure both the ADF and ΔTe, then use their relationship to constrain the gas’s thermal structure and improve abundance estimates.

What the Study Does—and Does Not—Show

The findings make a strong case for temperature variations in the Lagoon Nebula, but they do not establish that other H II regions share the same conditions. Nebulae differ, and distant galaxies may contain structures that observations cannot resolve. The next step is to test whether the relationship between abundance discrepancies and temperature differences holds across a broader range of environments.

For M8, however, the message is clear: a nebula’s temperature is not always captured by a single number. Looking at how temperature changes across space helps explain why two widely used methods can report different oxygen abundances—and how those differences can be brought back into alignment.

Created on 03 Nov. 2026

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Weekly letter Presented in "Astronomy and space this week" of 2 November 2026. Astronomy and space this week: Hidden worlds, stellar futures and the temperature of uncertainty

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SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Mapping the Lagoon Nebula’s Hidden Temperature Structure

Two Ways to Measure Temperature

A Stronger Discrepancy Near Herschel 36

Connecting the Observations to Temperature Fluctuations

Bringing the Abundance Estimates Together

What the Study Does—and Does Not—Show

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