SDSS-V LVM: A spatially resolved study of the physical conditions and the chemical abundance discrepancy in the Lagoon Nebula (M 8)

Abstract

The abundance discrepancy problem refers to the systematic differences observed between chemical abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) of heavy ions. It remains a major unsolved problem in the study of ionized nebulae and is quantified by the abundance discrepancy factor (ADF). In this work, we present a deep integral field spectroscopic dataset covering the entire Lagoon Nebula (M 8), obtained by the SDSS-V Local Volume Mapper project, at a spatial resolution of 0.21 pc per spaxel. This unique dataset allows us, for the first time, to investigate spatially resolved maps of oxygen RL intensities (O II V1), together with maps of H I RLs, heavy-ion CELs, and dust attenuation across a whole H II region. We map the electron temperature using CELs and RLs of $O^{2+}$, CELs of $N^{+}$, and the electron density using CELs of $S^{+}$. We derive CEL-based ionic and elemental oxygen abundances and, for the first time, a spatially resolved map of the RL-based $O^{2+}$ abundance in an H II region. These measurements enable the construction of the first spatially resolved ADF($O^{2+}$) map of an H II region and yield a global mean ADF of ~0.47 +/- 0.02 dex. Focusing on the central region of M 8, where ionization is dominated by the O-type star Her 36, we find radial variations in the ADF ranging between ~0.35-0.50 dex. Our findings provide novel constraints on the spatial behavior and origin of the abundance discrepancy in H II regions.

Figures (24)

• Figure 1: Left panel: The LVM footprint for M 8 overlaid on the recently released https://rubinobservatory.org/news/rubin-first-look/trifid-lagoon. Tile numbers are annotated in yellow, on the upper right of the panel is the Trifid nebula (M 20, see one tile coverage of LVM in sattler25). Right panel: Full LVM mosaic of the M 8 nebula, consisting of 10 tiles and 17,492 fibers. The RGB composite maps emission from [O iii] $\lambda\lambda$4959,5007 (blue), H$\beta$ (green), and [S ii] $\lambda\lambda$6717,6731 (red), highlighting the filamentary structures. Black dots denote the dead fibers and the masked spaxels excluded due to contamination from bright stars (see Section \ref{['subsec:stellar_cont_removal']}). The blue horizontal solid and dashed lines indicate the approximate projected width of the annotated H ii regions (NGC 6526 and NGC 6523), the Great Rift and M 8 East IR; these widths are schematics and not drawn to scale relative to the physical dimensions of the nebula.
• Figure 2: Average spectrum of M 8 in blue (b, upper panel), red (r, middle panel) and near-infrared (z, bottom panel) channels. Grey vertical bands highlight regions where channels overlap. Prominent emission lines are marked with vertical red lines. In the z channel, only a few lines are labeled, since most features correspond to sky emission lines that are not used in this work.
• Figure 3: Color composite map of M 8 (extended from Figure\ref{['fig:figure1']}), highlighting the central HG region powered by Her 36 (both in the central bin). The red circles mark three 1 arcmin wide annular bins at a distance of $2\arcmin$, $15\arcmin$, and $28\arcmin$ from Her 36. The green ellipse marks the NGC 6523 H ii region ionized by Her 36, while the area within the $15\arcmin$ annulus encompasses the NGC 6533 H ii region.
• Figure 4: Spectral fits to the O ii$\lambda\lambda$4600–4700 region are presented in six panels: (i) first -- for the M 8 average spectrum, second -- the average spectrum of NGC 6523, third -- for the central bin covering the HG region, last three are -- representative spectra at $2\arcmin$ (top), $15\arcmin$ (middle), and $28\arcmin$ bins, respectively. In the spectra, observed data are in blue, and the best-fit model is in red. Black lines show error bars associated with the flux. Vertical red dashed lines indicate key O ii transitions of the V1 multiplet; blue dashed lines mark multiplet 4 N ii permitted lines and the [Fe iii] $\lambda$4658 CEL.
• Figure 5: Upper panel: Best-fit $R_V$ and $E(B-V)$ obtained using Balmer and Paschen lines to H$\beta$ ratios for the average spectrum. Lower panel: Observed and modeled line ratios of HI lines w.r.t. the H$\beta$ relative to theoretical line ratios without dust. Solid circles represent the modeled ratios, and hollow circles represent the observed ratios. The best fit is calculated using the H$\gamma$ , H$\delta$ , H$\beta$ , P 11 and P 9; these line labels are annotated in blue, while the rest are in red.