2025ApJ...982L..36X

Based on a high-sensitivity H I survey using the Five-hundred-meter Aperture Spherical radio Telescope, we identified an isolated H I cloud with a system velocity of ∼127.0 km s^–1, which is associated with an optical galaxy KK153 in space. The H I gas of KK153 shows a typical disk-galaxy structure. Using the baryonic Tully-Fisher relation, we obtained that the distance to KK153 is 2.0${}_{-0.8}^{+1.7}$ Mpc. Adopting this distance, we derived a stellar mass of 4.1${}_{-2.6}^{+10.0}\times 1{0}^{5}$M_⊙ and a neutral gas fraction of 0.63, implying that KK153 is a gas-rich ultrafaint dwarf galaxy in the Local Group or its outskirts. KK153 shows a cool (∼200 K) and warm (∼7400 K) two-phase neutral medium. The g - r color distribution of KK153 suggests that new stars are mostly forming in its inner disk. The dynamical mass of KK153 is 6.9${}_{-3.0}^{+5.5}\times 1{0}^{7}$M_⊙, which is about 60 times larger than its baryonic matter. Detection of such a low-mass and gas-rich halo poses a challenge to the theory of cosmic reionization.