SDSS-V LVM: Resolving Physical Conditions in the Trifid Nebula

TL;DR

This study uses high-resolution spatial mapping of the Trifid Nebula to analyze its physical conditions, revealing internal density variations but overall homogeneity in temperature and oxygen abundance, and comparing resolved and integrated measurements.

Contribution

First spatially resolved mapping of physical conditions in the Trifid Nebula at 0.24 pc resolution, testing effects of inhomogeneities on abundance estimates.

Findings

Electron density varies with ionization front and shows a negative radial gradient.

Electron temperature and oxygen abundance are relatively uniform across the nebula.

No significant difference between integrated and spatially resolved measurements.

Abstract

The chemical abundance of the interstellar medium sets the initial conditions for star formation and provides a probe of chemical galaxy evolution models. However, unresolved inhomogeneities in the electron temperature can lead to a systematic underestimation of the abundances. We aim to directly test this effect. We use the SDSS-V Local Volume Mapper to spatially map the physical conditions of the Trifid Nebula (M 20), a Galactic H II region ionized by a single mid-type O star, at 0.24 pc resolution. We exploit various emission lines (e.g., Hydrogen recombination lines and collisionally excited lines, including also faint auroral lines) and compute spatially resolved maps of [O II] and [S II] electron densities; [N II], [O II], [S II], [S III] electron temperatures; and the ionic oxygen abundances. We find internal variations of electron density that result from the ionization front, along with a negative radial gradient. However, we do not find strong gradients or structures in the electron temperature and the total oxygen abundance, making the Trifid Nebula a relatively homogeneous H II region at the observed spatial scale. We compare these spatially resolved properties with equivalent integrated measurements of the Trifid Nebula and find no significant variations between integrated and spatially resolved conditions. This isolated H II region, ionized by a single O-star, represents a test case of an ideal Str\"omgren sphere. The physical conditions in the Trifid Nebula behave as expected, with no significant differences between integrated and resolved measurements.