SEAMLESS Survey: Four Faint Dwarf Galaxies Tracing Low-Mass Galaxy Evolution Across Environments

TL;DR

This study extends the SEAMLESS census to four faint Local Volume dwarfs, including the newly identified Hydrus A, by combining deep optical imaging with UV data to measure distances, stellar populations, SFRs, and environmental context. Distances are derived via the TRGB method, structural parameters and stellar masses are quantified, and HI constraints are placed from HIPASS non-detections, providing a multi-faceted view of each system. The galaxies span a range of environments—from isolated to group-periphery—with Hydrus A quenched and LEDA 486718 still forming stars, while Cetus B and Sculptor 26 show signs of environmental processing; this highlights multiple quenching pathways at $M_* \\lesssim 10^7 M_ $. Collectively, these results demonstrate SEAMLESS' capability to assemble a representative census of ultra-faint dwarfs beyond the Local Group and to constrain low-mass galaxy evolution across environments, guiding future deeper observations of gas content, kinematics, and star-formation histories.

Abstract

We report on four Local Volume dwarf galaxies identified through our ongoing SEmi-Automated Machine LEarning Search for Semi-resolved galaxies (SEAMLESS): Hydrus A, LEDA 486718, Cetus B, and Sculptor 26, with the discovery of Hydrus A reported here for the first time. These four galaxies span a wide range of environments and evolutionary states. Hydrus A (MV = -9.39+/-0.20, D = 3.38-0.30+0.32 Mpc) and LEDA 486718 (MV = -11.62+/-0.08, D = 4.80+/-0.17 Mpc) are among the most isolated dwarfs known within 5 Mpc, while Cetus B (MV = -8.26+/-0.17, D = 3.32-0.23+0.25 Mpc) and Sculptor 26 (MV = -11.25+/-0.10, D =3.21+/-0.13 Mpc) lie < 2 Rvir of NGC 253. Hydrus A shows properties consistent with quenching driven by cosmic reionization, cosmic-web interactions, or internal feedback. LEDA 486718 is an isolated star forming dwarf. Cetus B appears quenched and morphologically disturbed, making it a low-mass satellite or backsplash candidate, while Sculptor 26 is red and seemingly gas-poor but displays signs of recent activity, consistent with a transitional evolutionary state. Together, these systems demonstrate the power of SEAMLESS for building a census of faint galaxies beyond the Local Group.

Figures (6)

• Figure 1: Top row: Magellan/Megacam $g + r$ color images of Hydrus A (upper left) and Cetus B (upper right). Bottom row: Gemini/GMOS $g + r$ color images of Sculptor 26 and LEDA 486718. All images are oriented such that north is up and east is left. These galaxies are dominated by red stellar populations, with LEDA 486718 containing some central, blue star formation. It is also evident that Cetus B and Sculptor 26 have somewhat elongated morphologies.
• Figure 2: Each set of 3 panels depicts the CMD of each galaxy. In each panel the dotted line marks the 90% completeness limit and the dashed line indicates the 50% completeness limit. The error bars on the left indicate the typical photometric uncertainties in bins of 0.5 mag in $r$-band. Left: The CMD of stars selected within a circular aperture of the galaxy, where obvious contaminants are excluded (foreground Milky Way stars and background galaxies). Middle: CMD of background stars in the full field of view. Right: Binned, background-subtracted CMD. This is performed by first binning both the galaxy CMD and full background CMD. Then the binned background CMD is scaled to cover the same area as the CMD of the galaxy before the scaled background CMD is then subtracted from the galaxy CMD. In all instances the galaxy CMD clearly separates from the background CMD. Overlaid in each panel is a 10 Gyr, [Fe/H]=$-$2.5 Dartmouth isochrone dotter2008 in red, set at the TRGB distance (\ref{['tab:props']}) of the respective galaxy.
• Figure 3: Size-luminosity relation of Local Volume dwarfs, with Hydrus A, Cetus B, Sculptor 26, and LEDA 486718 depicted with large red symbols. Diagonal dotted lines show constant surface brightness assuming an exponential profile (24, 26, 28, 30, 32 mag arcsec$^{-2}$ from bottom to top). Grey points are sourced from the Local Volume Database pace2024. Other noteworthy Local Volume dwarfs are also highlighted irwin2007mcconnachie2012giovanelli2013mcquinn2015bsand2015bsand2022sand2024bennet2022jones2023jones2024li2024mutlupakdil2025.
• Figure 4: Supergalactic XY (left) and YZ (right) projections of objects in the nearby galaxy catalog karachentsev2019. In the XY plane we label notable nearby galaxy groups. Hydrus A, Cetus B, Sculptor 26, and LEDA 486718 are marked with red points. Other notable nearby dwarf galaxies are included for comparison. Hydrus A and LEDA 486718 are isolated, while Cetus B and Sculpture 26 are in the periphery of the NGC 253 group.
• Figure 5: Known dwarf galaxies in proximity to NGC 253. The dashed black and gray circles mark the virial radii of NGC 253 and NGC 247 respectively. The sample includes dwarfs from the PISCeS survey sand2014toloba2016mutlupakdil2022, and dwarfs discovered by martinezdelgado2021 (DoIII and DoIV) and carlsten2022 (dw0036m2828) with associations confirmed in mutlupakdil2024. Cetus B and Sculptor 26 are marked with red points. In this projection Cetus B appears to lie within the virial radius of both NGC 253 and NGC 247. Cetus B is within the foreground of both objects, but accounting for uncertainties on the distance may fall within the virial radius of NGC 247. Sculptor 26 is also in the foreground and within neither virial boundary.