A Tidally Disrupting Dwarf Galaxy in the Halo of NGC 253

ABSTRACT We report the discovery of Scl-MM-Dw2, a new dwarf galaxy at a projected separation of ∼50 kpc from NGC 253, as part of the Panoramic Imaging Survey of Centaurus and Sculptor project (PISCeS). We measure a tip of the red giant branch (RGB) distance of 3.12 ± 0.30 Mpc, suggesting that Scl-MM-Dw2 is likely a satellite of NGC 253. We qualitatively compare the distribution of RGB stars in the color–magnitude diagram with theoretical isochrones and find that it is consistent with an old, ∼12 Gyr, and metal-poor, Fe/H , stellar population. We also detect a small number of asymptotic giant branch stars consistent with a metal-poor 2–3 Gyr population in the center of the dwarf. Our non-detection of HI in a deep Green Bank Telescope spectrum implies a gas fraction . The stellar and gaseous properties of Scl-MM-Dw2 suggest that it is a dwarf spheroidal galaxy. Scl-MM-Dw2 has a luminosity of mag and a half-light radius of r h , which makes it moderately larger than dwarf galaxies in the Local Group of the same luminosity. However, Scl-MM-Dw2 is very elongated ( ), and it has an extremely low surface brightness ( mag arcsec −2 ). Its elongation and diffuseness make it an outlier in the ellipticity–luminosity and surface brightness–luminosity scaling relations. These properties suggest that this dwarf is being tidally disrupted by NGC 253.