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 ($T_e$) 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.