RGB 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, lying in the outskirts of the Local Group. We used the Large Binocular Telescope under sub-arcsec seeing conditions to resolve for the first time the outer regions of KK 153 into individual stars, 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, 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 measure of the integrated magnitude, implies a stellar mass of M_=2.4 \pm 0.2 X 10^6 M_{\sun}. 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 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 Gyr later.

💡 Research Summary

The paper presents a precise distance measurement for the gas‑rich dwarf galaxy KK 153, previously identified as a potential ultra‑faint dwarf (UFD) candidate in the outskirts of the Local Group (LG). Using the Large Binocular Telescope (LBT) equipped with the Large Binocular Camera (LBC), the authors obtained deep, sub‑arcsecond seeing images in the Sloan g and r bands on 21 May 2025. The total exposure time per filter was 1800 s, split into six 300‑s exposures. After standard bias, flat‑field, and cosmic‑ray corrections, point‑spread‑function (PSF) photometry was performed with DAOPHOT II and ALLFRAME, yielding a catalog of stellar sources with well‑characterized photometric uncertainties.

To isolate genuine stellar detections, the authors applied a magnitude‑dependent SHARP parameter cut and rejected objects with photometric errors larger than 0.2 mag. They defined a “galaxy” region within a 2′ radius of the nominal centre and a “field” region beyond 3′ to assess background contamination. The colour‑magnitude diagrams (CMDs) of the two regions reveal a clear red‑giant branch (RGB) in the galaxy sample, with an excess of stars in the colour range 0.8 ≤ (g − r)₀ ≤ 1.5 relative to the field. The RGB tip is identified at r₀ ≈ 24.5 mag. The authors employed both a Sobel edge‑detection filter and a maximum‑likelihood approach to locate the tip, adopting the calibrated absolute I‑band magnitude of the tip, M_I^TRGB = ‑4.05 mag (including colour corrections). This yields a distance modulus (m − M)₀ = 27.43 ± 0.12 mag, corresponding to a distance D = 3.06 +0.17/‑0.14 Mpc. This measurement is substantially more precise than the earlier estimate based on the baryonic Tully‑Fisher relation (D ≈ 2.0 +1.7/‑0.8 Mpc) and firmly places KK 153 outside the conventional LG boundary.

Integrated photometry was performed on the stacked images using large elliptical apertures (semi‑major axis 40.5″) matched to the galaxy’s axis ratio and position angle. The resulting extinction‑corrected magnitudes are g = 16.35 ± 0.14 mag and r = 16.86 ± 0.09 mag, about 0.5 mag brighter than previously reported values, likely due to the larger aperture encompassing more of the extended stellar halo. Combining the new distance with these magnitudes and an appropriate mass‑to‑light ratio yields a stellar mass M_* = 2.4 ± 0.2 × 10⁶ M_⊙. The HI mass, taken from existing 21‑cm measurements, is M_HI = 1.2 ± 0.4 × 10⁶ M_⊙, giving a gas‑to‑stellar mass ratio close to unity. The absolute V‑band magnitude is M_V ≈ ‑10.87 ± 0.20 mag, situating KK 153 at the low‑mass end of gas‑rich dwarf galaxies but still more massive than the faintest known gas‑rich dwarfs such as Leo T and Leo P.

The spatial distribution of the RGB candidates shows a centrally concentrated profile that declines smoothly over ~4–5 half‑light radii, confirming that the resolved stars belong to KK 153 rather than being background contaminants. The half‑light radius is measured to be R_h ≈ 17.4″ (≈ 260 pc). The authors discuss the implications of KK 153’s location beyond the LG: despite its low mass, it has retained a substantial gas reservoir and continues to form stars, suggesting that it survived the re‑ionisation epoch without complete quenching. This makes KK 153 a valuable laboratory for studying how low‑mass, gas‑rich systems can resist the suppressive effects of the UV background and maintain star formation over gigayear timescales.

In summary, the paper delivers a robust TRGB‑based distance to KK 153, revises its fundamental properties (luminosity, stellar mass, gas content), and places it firmly outside the Local Group. The findings reinforce the notion that a small but non‑negligible population of ultra‑low‑mass, gas‑rich dwarfs exists beyond the LG, providing critical constraints for models of dwarf galaxy formation, the impact of cosmic re‑ionisation, and the survival of baryons in shallow potential wells. The authors propose follow‑up spectroscopy and deeper imaging to refine the star‑formation history, metallicity distribution, and possible environmental interactions of KK 153.