DISCOVERY OF A NEW FAINT DWARF GALAXY ASSOCIATED WITH NGC 253

We report the discovery of a new faint dwarf galaxy, which we dub Scl-MM-Dw1, at a projected distance of similar to 65 kpc from the spiral galaxy NGC 253. The discovery results from the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a program with the Magellan/Megacam imager to study faint substructure in resolved stellar light around massive galaxies outside of the Local Group. We measure a tip of the red giant branch distance to Scl-MM-Dw1 of D = 3.9 +/- 0.5 Mpc, consistent with that of NGC 253, making their association likely. The new dwarf's stellar population is complex, with an old, metal-poor red giant branch (greater than or similar to 10 Gyr, [Fe/H] similar to -2), and an asymptotic giant branch with an age of similar to 500 Myr. Scl-MM-Dw1 has a half-light radius of r(h) = 340 +/- 50 pc and an absolute magnitude of M-V = -10.3 +/- 0.6 mag, comparable to the Milky Way's satellites at the same luminosity. Once complete, our imaging survey of NGC 253 and other nearby massive galaxies will provide a census of faint substructure in halos beyond the Local Group, both to put our own environment into context and to confront models of hierarchical structure formation.