Tucana B: A Potentially Isolated and Quenched Ultra-faint Dwarf Galaxy at D ≍ 1.4 Mpc

August 2022 • 2022ApJ...935L..17S

Authors • Sand, David J. • Mutlu-Pakdil, Burçin • Jones, Michael G. • Karunakaran, Ananthan • Wang, Feige • Yang, Jinyi • Chiti, Anirudh • Bennet, Paul • Crnojević, Denija • Spekkens, Kristine

Abstract • We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy at a distance of D = 1.4 Mpc. Tucana B was found during a search for ultra-faint satellite companions to the known dwarfs in the outskirts of the Local Group, although its sky position and distance indicate the nearest galaxy to be ~500 kpc distant. Deep ground-based imaging resolves Tucana B into stars, and it displays a sparse red giant branch consistent with an old, metal-poor stellar population analogous to that seen in the ultra-faint dwarf galaxies of the Milky Way, albeit at fainter apparent magnitudes. Tucana B has a half-light radius of 80 ± 40 pc and an absolute magnitude of {M}_{V}=-{6.9}_{-0.6}^{+0.5}$ mag ( {L}_{V}=({5}_{-2}^{+4})\times {10}^{4}$ L _☉), which is again comparable to the Milky Way's ultra-faint satellites. There is no evidence for a population of young stars, either in the optical color-magnitude diagram or in GALEX archival ultraviolet imaging, with the GALEX data indicating \mathrm{log}({\mathrm{SFR}}_{\mathrm{NUV}}/{M}_{\odot }\,{\mathrm{yr}}^{-1})\lt -5.4$ for star formation on ≲100 Myr timescales. Given its isolation and physical properties, Tucana B may be a definitive example of an ultra-faint dwarf that has been quenched by reionization, providing strong confirmation of a key driver of galaxy formation and evolution at the lowest mass scales. It also signals a new era of ultra-faint dwarf galaxy discovery at the extreme edges of the Local Group. *This paper includes data gathered with the 6.5 m Magellan Telescope at Las Campanas Observatory, Chile.

Links

• PREPRINT http://arxiv.org/abs/2205.09129
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2022ApJ...935L..17S
• ELECTR https://doi.org/10.3847/2041-8213/ac85ee
• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2022ApJ...935L..17S
• PDF https://iopscience.iop.org/article/10.3847/2041-8213/ac85ee/pdf