Satellite accretion in action: a tidally disrupting dwarf spheroidal
  around the nearby spiral galaxy NGC 253
  We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy
candidate undergoing tidal disruption around a nearby spiral galaxy, NGC 253 in
the Sculptor group: the first such event identified beyond the Local Group. The
dwarf was found using small-aperture amateur telescopes, and followed up with
Suprime-Cam on the 8 m Subaru Telescope in order to resolve its brightest
stars. Using g- and R_c-band photometry, we detect a red giant branch
consistent with an old, metal-poor stellar population at a distance of ~ 3.5
Mpc. From the distribution of likely member stars, we infer a highly elongated
shape with a semi-major axis half-light radius of (2 +/- 0.4) kpc. Star counts
also yield a luminosity estimate of ~ 2x10^6 L_Sun,V (M_V ~ -10.7). The
morphological properties of NGC 253-dw2 mark it as distinct from normal dSphs
and imply ongoing disruption at a projected distance of ~ 50 kpc from the main
galaxy. Our observations support the hierarchical paradigm wherein massive
galaxies continously accrete less massive ones, and provide a new case study
for dSph infall and dissolution dynamics. We also note the continued efficacy
of small telescopes for making big discoveries.
