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

TL;DR

This study uses the SDSS-V Local Volume Mapper to map the Trifid Nebula (M20) at 0.24 pc resolution, directly testing how electron-temperature inhomogeneities affect abundance determinations in H II regions. It derives spatially resolved electron densities, temperatures, and oxygen abundances with multiple diagnostics, validating a largely homogeneous temperature structure and a moderate density gradient with localized enhancements. The integrated and resolved metallicities agree within uncertainties, while a Cloudy toy model reconciles the observed nebular size with a higher ionizing photon rate than a single O-star would suggest, implicating dust and density structure. Overall, M20 serves as a benchmark Strömgren sphere where integrated spectra reliably trace average conditions, yet spatially resolved data reveal small-scale inhomogeneities that inform abundance diagnostics in more complex systems.

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 Strömgren sphere. The physical conditions in the Trifid Nebula behave as expected, with no significant differences between integrated and resolved measurements.

Figures (14)

• Figure 1: NSF–DOE Vera C. Rubin Observatory image of the Trifid Nebula, using LSST's (Legacy Survey of Space and Time, Ivezic2019) six filters: u, g, r, i, z and y. It shows the glowing pink emission nebula and the cool blue reflection nebula. The position of the central ionizing source HD 164492A is marked with a grey star. The center of Trifid's reflection nebula Lynds1986 is marked by the A 7 super-giant HD 164514 (white star). The extent of the hexagonal LVM pointing is indicated by a grey dashed line.
• Figure 2: Top: Integrated spectrum of the full LVM pointing covering the Trifid Nebula. Bottom: A zoom-in collection of strong lines (left in magenta) and weak auroral lines (right in blue) taken from the above spectrum, each in a window of 30 Å. The H$\alpha$ emission line is not shown here, as its height would dominate over all other lines.
• Figure 3: Spatial map of the dust extinction in M 20. The position of HD 164492A is marked with a grey star. The extent of the measured Strömgren sphere (Sect. \ref{['sec:Q0']}) is shown by a grey dashed circle. The white regions arise from the limited sensitivity to the faint hydrogen emission lines in these spaxels and were masked (see Sect. \ref{['sec:observations']}). A uncertainty map is shown in Fig. \ref{['fig:reddening_error']}.
• Figure 4: Left: Map of the reddening corrected H$\alpha$ line flux as described in Sect. \ref{['subsec:reddening']}. The position of HD 164492A is marked with a grey star, while the position of HD 164514 is marked with a white star. The extent of the measured Strömgren sphere (Sect. \ref{['sec:Q0']}) is shown by a grey dashed circle. Right: Radial variation of the reddening corrected H$\alpha$ line flux as a function of the distance to the ionizing star HD 164492A. Faint points represent individual spaxels, while opaque points show the uncertainty-weighted average of all spaxels within 0.1 pc wide distance bins. Standard deviations are shown as error bars. The extent of the measured Strömgren sphere (Sect. \ref{['sec:Q0']}) is shown by a grey dashed line together with the error as a grey shaded region.
• Figure 5: Maps of the electron densities. In all maps, the position of HD 164492A is marked with a grey star, while the position of HD 164514 is marked with a white star. The extent of the measured Strömgren sphere (Sect. \ref{['sec:Q0']}) is shown by a grey dashed circle. Uncertainty maps corresponding to each density tracer are shown in Fig. \ref{['fig:density_error']}.