Discovery of a high-velocity cloud of the Milky Way as a potential dark galaxy

High-velocity clouds (HVCs) are composed of neutral hydrogen (H I) moving at velocities that deviate from the general rotational motion of the Milky Way. Now, the origins of the HVCs remain poorly known due to the difficulty in determining their distance and the lack of any other suitable identification. Here we report the detection of a compact gas clump in HVC AC-I, which displays characteristics typical of a disk galaxy, named AC G185.0-11.5, using the H I observations. We estimated the distance of AC G185.0-11.5 to be 277.7−141.6+291.3 kiloparsecs using the baryonic Tully-Fisher relation and constrained its H I gas mass to be between 3.0 × 107 and 4.7 × 108 solar masses. The distance determination indicates that the HVC AC-I hosting AC G185.0-11.5 is an extragalactic object in the Local Group. The absence of molecular gas and an optical counterpart for AC G185.0-11.5 implies that it may be a rare dark galaxy.