We report the discovery of Scl-MM-Dw2 , a new dwarf galaxy at a projected separation of \sim 50 kpc from NGC 253 , as part of the PISCeS ( Panoramic Imaging Survey of Centaurus and Sculptor ) project .
We measure a tip of the red giant branch distance of 3.12 \pm 0.30 Mpc , suggesting that Scl-MM-Dw2 is likely a satellite of NGC 253 .
We qualitatively compare the distribution of red giant branch ( RGB ) stars in the color-magnitude diagram with theoretical isochrones and find that it is consistent with an old , \sim 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 _ { HI } / L _ { V } < 0.02 M _ { \odot } / L _ { \odot } .
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 \pm 0.5 mag and a half-light radius of r _ { h } = 2.94 \pm 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 ( \epsilon = 0.66 \pm 0.06 ) and it has an extremely low surface brightness ( \mu _ { 0 ,V } = 26.5 \pm 0.7 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 .