A TIDALLY DISRUPTING DWARF GALAXY in the HALO of NGC 253

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 colormagnitude 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 23 Gyr population in the center of the dwarf. Our non-detection of HI in a deep Green Bank Telescope spectrum implies gas fraction MHI LV 0.02 M^ø/L^ø. 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 ε = 0.66 ± 0.06), and it has an extremely low surface brightness (μ0.v=27.7 0.6 m0,V = ± mag arcsec^?2). Its elongation and diffuseness make it an outlier in the ellipticity-luminosity and surface brightnessluminosity scaling relations. These properties suggest that this dwarf is being tidally disrupted by NGC 253.