Red giant branch tip distance to the faint gas-rich dwarf KK 153

KK 153 is a star-forming dwarf galaxy that has been recently proposed as a new member of the sparsely populated class of gas-rich ultra-faint dwarfs that lie in the outskirts of the Local Group. We used the Large Binocular Telescope under sub-arcsec seeing conditions to resolve the outer regions of KK 153 into individual stars for the first time, reaching the red giant branch. The magnitude of the red giant branch tip was used to measure a distance of $ D = 3.06^{+0.17}_{-0.14} $ Mpc, which is much more accurate and precise than the estimate previously available in the literature, based on the baryonic Tully-Fisher relation ($ D = 2.0^{+1.7}_{-0.8} $ Mpc). The new distance places KK 153 clearly beyond the boundaries of the Local Group, and, together with a new measurement of the integrated magnitude, implies a stellar mass of M★ = 2.4 ± 0.2 × 106 M⊙. The dwarf populates the extreme low-mass tail of the M★ distribution of gas-rich galaxies but it is significantly more massive than the faintest local gas-rich dwarfs, Leo T and Leo P. In an analogy with similar systems, the star formation history of KK 153 may have been impacted by the re-ionisation of the Universe while keeping a sufficient gas reservoir to form new stars several Gigayear later.