2016ApJ...816L...5T

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, -2.3 < [Fe/H] < -1.1, 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 M_H1/L_V < 0.02M_☉/L_☉. The stellar and gaseous properties of Scl-MM-Dw2 suggest that it is a dwarf spheroidal galaxy. Scl-MM-Dw2 has a luminosity of M_V = -12.1 ± 0.5 mag and a half-light radius of r_h = 2.94 ± 0.46 kpc, which makes it moderately larger than dwarf galaxies in the Local Group of the same luminosity. However, Scl-MM-Dw2 is very elongated (e = 0.66 ± 0.06), and it has an extremely low surface brightness (µ_0,V = 27.7 ± 0.6 mag/arcsec2). 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.