Identification of Galactic H I Counterpart of the Large [O iii] Emission Structure 1 . ° 2 Southeast of M31

Strottner–Drechsler–Sainty Object 1 (SDSO1) is a faint [O iii ] emission nebula spanning 1 . ° 5 , discovered southeast of M31. In previous surveys, no counterpart has been observed across radio, infrared, UV, and X-ray wavelengths, thus complicating distance determination. Utilizing the M31 H I survey with the Five-hundred-meter Aperture Spherical radio Telescope, we identified an atomic gas structure associated with SDSO1. This potential neutral gas counterpart includes three strips spatially aligned with the [O iii ] emission arcs. The H I structure has a velocity between −79 and −45 km s −1 with respect to the local standard of rest. It angularly extends to 2 . ° 9 , surpassing SDSO1 in size. Several structures around the H I counterpart share similar velocities, hinting at a link between the H I counterpart and the Galactic disk’s large-scale structure. The combined analysis of the three-dimensional dust map and the reddening–distance relations for these H I strips and their nearby H I structures suggests a distance of 2.0 ± 0.2 kpc. The mass of the H I counterpart is calculated to be 614 M ⊙ . All of these results collectively demonstrate that SDSO1 is most likely a structure in the Milky Way rather than one associated with M31.