2021ApJ...922...53X

We present a new high-sensitivity H I observation toward nearby spiral galaxy M101 and its adjacent 2° x 2° region using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From the observation, we detect a more extended and asymmetric H I disk around M101. While the H I velocity field within the M101's optical disk region is regular, indicating that the relatively strong disturbance occurs in its outer disk. Moreover, we identify three new H I clouds located on the southern edge of the M101's H I disk. The masses of the three H I clouds are 1.3 x 10⁷ M_☉, 2.4 x 10⁷ M_☉, and 2.0 x 10⁷ M_☉, respectively. The H I clouds similar to dwarf companion NGC 5477 rotate with the H I disk of M101. Unlike NGC 5477, they have no optical counterparts. Furthermore, we detect a new H I tail in the extended H I disk of M101. The H I tail detected gives reliable evidence for M101 interaction with the dwarf companion NGC 5474. We argue that the extraplanar gas (three H I clouds) and the H I tail detected in the M101's disk may originate from a minor interaction with NGC 5474.