Observation of H i around three Satellite Galaxies of M31 with FAST: Andromeda II, NGC 205, and NGC 185

With the exceptional sensitivity of the Five-hundred-meter Aperture Spherical radio Telescope, we conducted observations of the neutral hydrogen (H i) in the circumgalactic medium of Andromeda’s (M31) satellite galaxies, specifically Andromeda II, NGC 205, and NGC 185. Initially, three drift scans were executed for these satellites, with a detection limit of 4 × 1018 cm−2 (approximately 1.88 × 103 M ⊙ of H i mass), followed by a more in-depth scan of a specific region. We discovered a C-shaped H i arc structure sharing a position and line-of-sight velocity similar to a stellar ring structure around Andromeda II, hinting at a potential connection with Andromeda II. In the context of NGC 205, we identified two mass concentrations in the northeast direction, which could be indicative of tidal streams resulting from the interaction between this galaxy and M31. These new lumps discovered could be very helpful in solving the missing interstellar medium problem for NGC 205. Observations regarding NGC 185 are consistent with previous studies, and we did not detect any additional H i material around this galaxy. These observational results enhance our understanding of the evolution of these satellite galaxies and provide insight into their historical interactions with the galaxy M31.