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

We report [O,$\text{textsc}iii$] narrow-band imaging of an extremely large-scale nebula, the Silk Nebula, discovered by the SWONS (Super Wide-Field [O,$\text{textsc}iii$] Narrow-band Survey), together with multi-fiber medium-resolution spectroscopic data covering the H$\alpha$, [N,$\text{textsc}ii$], [S,$\text{textsc}ii$], and [O,$\text{textsc}iii$] 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 2,684 high-quality spectra, we measured its physical parameters, including relative line intensities, radial velocities (RVs), and full widths at half-maximum (FWHMs). The [S,$\text{textsc}ii$]/H$\alpha$ ratio of the nebula significantly exceeds the shock-excitation threshold of 0.4. Its location in BPT diagnostic diagrams and spatial morphology of [O,$\text{textsc}iii$] 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,$\text{textsc}iii$] is the only emission line that clearly traces the nebular morphology, we base our distance estimate exclusively on the [O,$\text{textsc}iii$] kinematics, obtaining a fiducial distance of $\sim$160 pc. At this distance, the nebula has a physical extent of $\sim$28 pc along the north-south direction. We suggest that the Silk Nebula is a large [O,$\text{textsc}iii$] filament possibly associated with a supernova remnant, although a diffuse-ionized-gas (DIG) or ionized-bubble origin cannot be ruled out with the present data, and one of the largest angular-scale [O,$\text{textsc}iii$] emission structures documented to date. Future deep X-ray observations and shock-modeling efforts are required to firmly establish its nature.