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 \gtrsim 10 Gyr and [ Fe / H ] \lesssim - 2.3 .
Our inferred distance modulus ( m - M ) _ { 0 } = 24.57 \pm 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 r _ { h } = 26 ^ { +4 } _ { -3 } pc , integrated luminosity M _ { V } = -4.8 \pm 0.5 , and ellipticity \epsilon = 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 \sim 2 - 3 smaller in spatial extent than any known counterpart of comparable luminosity .