The Silk Nebula: A Newly Discovered Giant [O iii ] Emission Filament

We report [O iii ] narrowband imaging of an extremely large-scale nebula, the Silk Nebula, discovered by the Super Wide-Field [O iii] Narrow-band Survey (SWONS), together with multifiber medium-resolution spectroscopic data covering the H α , [N ii ], [S ii ], and [O iii ] emission lines, acquired with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) as part of the Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N). Based on 2684 high-quality spectra, we measured its physical parameters, including relative line intensities, radial velocities, and full widths at half-maximum. The [S ii ]/H α ratio of the nebula significantly exceeds the shock-excitation threshold of 0.4. Its location in BPT diagnostic diagrams and spatial morphology of [O iii ] both support the optical emission is dominated by shock-heated gas. The oxygen abundance is near solar and spatially uniform, consistent with a shock propagating into the ambient interstellar medium. Because [O iii ] is the only emission line that clearly traces the nebular morphology, we base our distance estimate exclusively on the [O iii ] kinematics, obtaining a fiducial distance of ∼160 pc. At this distance, the nebula has a physical extent of ∼28 pc along the north–south direction. We suggest that the Silk Nebula is a large [O iii ] filament possibly associated with a supernova remnant, although a diffuse-ionized-gas (DIG) or ionized-bubble origin cannot be ruled out with the present data, and one of the largest angular-scale [O iii ] emission structures documented to date. Future deep X-ray observations and shock-modeling efforts are required to firmly establish its nature.