Perseus I: A distant satellite dwarf galaxy of Andromeda
  We present the discovery of a new dwarf galaxy, Perseus I/Andromeda XXXIII,
found in the vicinity of Andromeda (M31) in stacked imaging data from the
Pan-STARRS1 3{\pi} survey. Located 27.9{\deg} away from M31, Perseus I has a
heliocentric distance of 785 +/- 65 kpc, compatible with it being a satellite
of M31 at 374 +14/-10 kpc from its host. The properties of Perseus I are
typical for a reasonably bright dwarf galaxy (M_V = -10.3 +/- 0.7), with an
exponential half-light radius of r_h = 1.7 +/- 0.4 arcminutes or r_h = 400
+105/-85 pc at this distance, and a moderate ellipticity (\epsilon = 0.43
+0.15/-0.17). The late discovery of Perseus I is due to its fairly low surface
brightness (\mu_0=25.7 +1.0/-0.9 mag/arcsec^2), and to the previous lack of
deep, high quality photometric data in this region. If confirmed to be a
companion of M31, the location of Perseus I, far east from its host, could
place interesting constraints on the bulk motion of the satellite system of
M31.
