Hedgehog: An Isolated Quiescent Dwarf Galaxy at 2.4 Mpc

Abstract: It is well-known that almost all isolated dwarf galaxies are actively forming stars. We report the discovery of dw1322m2053 (nicknamed Hedgehog), an isolated quiescent dwarf galaxy at a distance of $2.40\pm0.15$ Mpc with a stellar mass of $M_\star \approx 10^{5.8}\, M_\odot$. The distance is measured using surface brightness fluctuations with both Legacy Surveys and deep Magellan/IMACS imaging data. Hedgehog is 1.7~Mpc from the nearest galaxy group, Centaurus A, and has no neighboring galaxies within 1~Mpc, making it one of the most isolated quiescent dwarf galaxies at this stellar mass. It has a red optical color and early-type morphology and shows no UV emission. This indicates that Hedgehog has an old stellar population and no ongoing star formation. Compared with other quiescent dwarfs in the Local Group and Local Volume, Hedgehog appears smaller in size for its luminosity but is consistent with the mass--size relations. Hedgehog might be a backsplash galaxy from the Centaurus A group, but it could also have been quenched in the field by ram pressure stripping in the cosmic web, reionization, or internal processes such as supernova and stellar feedback. Future observations are needed to fully unveil its formation, history, and quenching mechanisms.