SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies
Abstract
Knowing the chemical composition of interstellar gas is essential for understanding the chemical evolution of galaxies. Ionized nebulae provide key laboratories for this purpose, where chemical abundances are commonly derived from emission lines of heavy ions such as doubly ionized oxygen, O++. Abundances can be inferred either from collisionally excited lines (CELs) or from recombination lines (RLs). However, for more than 80 years, the former have yielded systematically lower abundances than the latter. We map this ''abundance discrepancy'' using the first spatially resolved integral field unit (IFU) observations of O++ RLs and CELs across a Milky Way H II region: the Lagoon Nebula (a.k.a. M8). Modeling the data using a plasma temperature perturbation model, we present unequivocal evidence that electron temperature (Te) fluctuations drive the abundance discrepancy in M8. We find a tight correlation between the abundance discrepancy and the difference between Te derived from the classical CEL method and a hybrid RL/CEL approach. We identify this Te discrepancy as an observational proxy for the amplitude of the thermal fluctuations, providing a new, direct way to quantify their effect on nebular abundances. The correlation constrains the mean electron temperature of the gas. Adopting this mean temperature reconciles CEL and RL abundances, effectively eliminating the abundance discrepancy.
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BibTeX
@misc{singh2026sdssv,
title = {{SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies}},
author = {Amrita Singh and Guillermo A. Blanc and Kathryn Kreckel and J. E. Méndez-Delgado and Christophe Morisset and Sebastián F. Sánchez and Oleg Egorov and Niv Drory and Nimisha Kumari and Fu-Heng Liang and Enrico Congiu and Alfredo Mejía-Narváez and Evelyn J. Johnston and José G. Fernández-Trincado and Hemanth Bommireddy and Abhinna Sundar Samantaray and Thomas Hilder and Rodolfo de J. Zermeño and Ivan Katkov and Dhanesh Krishnarao and Juna A. Kollmeier},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.40045},
eprint = {2610.40045},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.40045},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - SDSS-V LVM: Mapping of plasma temperatures in the Lagoon Nebula reveals the origin of long-standing chemical abundance discrepancies
AU - Amrita Singh
AU - Guillermo A. Blanc
AU - Kathryn Kreckel
AU - J. E. Méndez-Delgado
AU - Christophe Morisset
AU - Sebastián F. Sánchez
AU - Oleg Egorov
AU - Niv Drory
AU - Nimisha Kumari
AU - Fu-Heng Liang
AU - Enrico Congiu
AU - Alfredo Mejía-Narváez
AU - Evelyn J. Johnston
AU - José G. Fernández-Trincado
AU - Hemanth Bommireddy
AU - Abhinna Sundar Samantaray
AU - Thomas Hilder
AU - Rodolfo de J. Zermeño
AU - Ivan Katkov
AU - Dhanesh Krishnarao
AU - Juna A. Kollmeier
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.40045
UR - https://arxiv.org/abs/2610.40045
AB - Knowing the chemical composition of interstellar gas is essential for understanding the chemical evolution of galaxies. Ionized nebulae provide key laboratories for this purpose, where chemical abundances are commonly derived from emission lines of heavy ions such as doubly ionized oxygen, O^++. Abundances can be inferred either from collisionally excited lines (CELs) or from recombination lines (RLs). However, for more than 80 years, the former have yielded systematically lower abundances than the latter. We map this ''abundance discrepancy'' using the first spatially resolved integral field unit (IFU) observations of O^++ RLs and CELs across a Milky Way H II region: the Lagoon Nebula (a.k.a. M8). Modeling the data using a plasma temperature perturbation model, we present unequivocal evidence that electron temperature (T_e) fluctuations drive the abundance discrepancy in M8. We find a tight correlation between the abundance discrepancy and the difference between T_e derived from the classical CEL method and a hybrid RL/CEL approach. We identify this Te discrepancy as an observational proxy for the amplitude of the thermal fluctuations, providing a new, direct way to quantify their effect on nebular abundances. The correlation constrains the mean electron temperature of the gas. Adopting this mean temperature reconciles CEL and RL abundances, effectively eliminating the abundance discrepancy.
ER -