2026A&A...706A..81S

Aims. 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.
Methods. We used 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 a 0.24 pc resolution. We exploited various emission lines (e.g., Hydrogen recombination lines and collisionally excited lines, including also faint auroral lines) and computed the spatially resolved maps of [O II] and [S II] electron densities, the [N II], [O II], [S II], and [S III] electron temperatures, and the ionic oxygen abundances.
Results. We found internal variations of electron density that result from the ionization front, along with a negative radial gradient. However, we did not find any 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 compared these spatially resolved properties with equivalent integrated measurements of the Trifid Nebula and found no significant variations between integrated and spatially resolved conditions.
Conclusions. This isolated H II region, ionized by a single O-star, represents a test case of an ideal Strömgren sphere. The physical conditions in the Trifid Nebula behave as expected, with no significant differences between integrated and resolved measurements.