2023ApJ...952..130Z

We report the first high-sensitivity H I observation toward the spiral galaxy M94 with the Five-hundred-meter Aperture Spherical radio Telescope. From these observations, we discovered that M94 has a very extended H I disk, twice larger than that observed by THINGS, which is accompanied by a H I filament and seven high velocity clouds (HVCs) at different distances. The projected distances of these clouds and filaments are less than 50 kpc from the galactic center. We measured a total integrated flux (including all clouds/filament) of 127.3 ( ±1) Jy km s^–1, corresponding to a H I mass of (6.51 ± 0.06)×10⁸M_⊙ , which is 63.0% more than that observed by THINGS. By comparing numerical simulations with the H I maps and the optical morphology of M94, we suggest that M94 is likely a remnant of a major merger of two galaxies, and the HVCs and H I filament could be the tidal features that originated from the first collision of the merger, which happened about 5 Gyr ago. Furthermore, we found a seemingly isolated H I cloud at a projection distance of 109 kpc without any optical counterpart detected. We discuss the possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC (REionization-Limited H I Cloud). Our results demonstrate that high-sensitivity and wide-field H I imaging is important in revealing diffuse cold gas structures and tidal debris, which is crucial to understand the dynamical evolution of galaxies.