A peculiar faint satellite in the remote outer halo of M31
  We present Hubble Space Telescope imaging of a newly-discovered faint stellar
system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals
this object to be comprised of an ancient and very metal-poor stellar
population with age > 10 Gyr and [Fe/H] < -2.3. Our inferred distance modulus
of 24.57 +/- 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite
with a 3D galactocentric radius of 149 (+19 -8) kpc. We observe an apparent
spread in color on the upper red giant branch that is larger than the
photometric uncertainties should allow, and briefly explore the implications of
this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with
a half-light radius rh = 26 (+4 -3) pc, integrated luminosity Mv = -4.8 +/-
0.5, and ellipticity = 0.30 (+0.08 -0.15). On the size-luminosity plane it
falls between the extended globular clusters seen in several nearby galaxies,
and the recently-discovered faint dwarf satellites of the Milky Way; however,
its characteristics do not allow us to unambiguously class it as either type of
system. If PAndAS-48 is a globular cluster then it is the among the most
elliptical, isolated, and metal-poor of any seen in the Local Group, extended
or otherwise. Conversely, while its properties are generally consistent with
those observed for the faint Milky Way dwarfs, it would be a factor ~2-3
smaller in spatial extent than any known counterpart of comparable luminosity.
