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

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Piero Gobetti 93/3 40129 Bologna, Italy
² European Southern Observatory Karl-Schwarzschild-Strasse 2 85748 Garching bei München, Germany
³ INAF – Osservatorio Astronomico di Roma Via Frascati 33 00078 Monteporzio Catone Rome, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 October 2025
Accepted: 21 November 2025

Abstract

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_−.014^+0.17 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_−0.8^+1.7 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.