Precisely classify the physical nature of the SDSO nebula

Determine the precise astrophysical classification of the Strottner–Drechsler–Sainty Object (SDSO), the extended, faint [O III]-emitting nebula near M31, by using its observed kinematics, emission-line ratios, and offset morphology (among [O II], Hα, and [O III]) to ascertain which Galactic nebular category (e.g., interstellar medium filament, diffuse ionized gas structure, planetary nebula, or H II region) it belongs to.

Background

The paper presents new narrowband imaging ([O II] and Hα+[N II]) and high-resolution IFU spectroscopy (Hβ to [O III] and Hα to [S II]) of a large, faint [O III]-emitting nebula discovered near M31 (SDSO). The measured barycentric velocities (approximately −6 to −30 km s−1) are inconsistent with M31’s systemic and local rotation velocities, and the velocities are remarkably consistent across multiple pointings, supporting a Galactic origin.

Emission-line widths are narrow (<20 km s−1 for Hβ and [O III]), disfavoring shock-dominated scenarios typical of supernova remnants or extragalactic extended emission-line regions. Line ratios such as [O III]5007/Hβ ≈ 1.1–1.4 and [S II]6716/[S II]6731 ≈ 1.4 suggest low electron densities and are compatible with several Galactic classes (e.g., ISM filaments, H II regions, planetary nebulae, diffuse ionized gas). The [O II] and [O III] morphologies are offset by ~6–7 arcmin, suggestive of a resolved ionization structure at Galactic distances.

Despite favoring a Galactic ISM filament interpretation, the data do not uniquely determine the object’s class or ionizing source, prompting the authors to state that the precise classification remains undetermined and motivating further spectroscopic observations across additional regions and wavelengths.

References

Still, with the data gathered up to now we cannot determine precisely to which class of objects the nebula belongs to.

— Andromeda's tenuous veil: extensive nebular emission near (yet far from) M31  (2412.08327 - Lumbreras-Calle et al., 2024) in Section 4.4 (Physical nature of the nebula), page 11 (Article number, page 11 of 13)

Overall, the H$\alpha$ and [S\,ii], emission intensities are significantly stronger on the western side of the nebula than on the eastern side. Hence, on the basis of the analysis presented above, the SNR interpretation remains plausible but is not yet definitive; the most possible conclusion is that Silk is a large-scale, low-surface-brightness [O\,iii] filament, possibly the shell of a nearby evolved supernova remnant. As can be seen from the orange circle in Fig~\ref{fig:DIGNOSE}, the Silk Nebula shows a minor overlap with the DIG region; therefore, the influence of DIG cannot be excluded, even though it is minor (as discussed in detail in Section~\ref{sec4.2}).

— The Silk Nebula: A Newly Discovered Giant [OIII] Emission Filament  (2610.13329 - Wu et al., 9 Oct 2026) in Section 4.1, “Is the Silk Nebula really a SNR?”; see also Section 4.2 and Section 5

Both scenarios are physically plausible but cannot be distinguished with the current data, and neither excludes a contribution from DIG.

— The Silk Nebula: A Newly Discovered Giant [OIII] Emission Filament  (2610.13329 - Wu et al., 9 Oct 2026) in Section 4.2, “DIG contamination”