RGB Tip distance to the faint gas-rich dwarf KK 153

TL;DR

The paper refines the distance to KK 153, a candidate gas-rich ultra-faint dwarf, by resolving its outskirts into individual stars and applying the RGB Tip method with LBT data. The derived distance $D=3.06^{+0.17}_{-0.14}$ Mpc places KK 153 outside the Local Group and yields a stellar mass of $M_{\star}=2.4\pm0.2\times10^6\,M_{\sun}$, substantially higher than previous estimates. The authors use the CMD to characterize the stellar population, confirming an old RGB with a small recent star-formation component, and they estimate a HI content of $M_{HI}=1.2\pm0.4\times10^6\,M_{\sun}$, giving $M_{HI}/M_{\star}\approx0.5$. The work highlights KK 153 as a relatively isolated, low-mass, gas-rich dwarf near the threshold of reionisation-era survivals and demonstrates the RGB Tip as a precise distance indicator for nearby dwarfs. Overall, the study improves distance accuracy, refines the stellar mass budget, and informs the evolutionary context of the faint end of the dwarf galaxy population.

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, 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.

Figures (9)

• Figure 1: Red-Green-Blue cut-out image of KK 153 and its immediate surroundings obtained from sky-subtracted, stacked deep g and r images. The r image has been used for both the Red and the Green channels, while the g image has been used for the Blue channel.
• Figure 2: Upper panel: distribution of the Daophot SHARP shape parameter as a function of $r$ magnitude. Sources lying in the region enclosed by the red contours are considered as likely stars, while those outside the contours (grey points), mostly distant unresolved galaxies, have been excluded from the subsequent analysis. Middle and lower panels: distribution of the photometric errors in $r$ and $g$ as a function of $r$ and $g$ magnitude, respectively.
• Figure 3: Sky map of the position of the sources in our selected catalogue, in an orthographic projection reference system with the origin at the center of KK 153. North is up, East to the left. Sources within $2.0\arcmin$ from the centre of KK 153 are plotted in red (galaxy), sources with $R>3.0\arcmin$ are plotted in cyan (field).
• Figure 4: Left panel: CMD of stars enclosed within a circle of radius=2.0$\arcmin$ centred on the center of KK 153. Middle panel: CMD of all the stars in our field located at an angular distance larger than =3.0$\arcmin$ from the centre of KK 153 (Field). It is worth noting that the area of the $R>3.0\arcmin$ field is more than 8.3 times the area of the $R<2.0\arcmin$ circle around the galaxy. Right panel: the CMD of KK 153 superimposed to the CMD of the Field population. The two dashed parallel lines display our selection of candidate RGB stars of KK 153. In all the CMDs the colour intensity of the points is proportional to the local star count density (Hess diagram).
• Figure 5: Upper panel: the colour distribution of the stars within 1.5$\arcmin$ from the centre of KK 153 (red histogram) is compared to the colour distribution of the Field stars ($R>3.0$; blue histogram). Lower panel: background-subtracted surface density profile of RGB stars (selected as in Fig. \ref{['fig:cmd']}, with the additional condition $r_0>23.0$) from the centre of KK 153.