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 3p survey. Located 27 +/- 9 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 (MV = -10.3 +/- 0.7), with an exponential halflight radius of r(h) = 1.7 +/- 0.4 arcmin or rh = 400+ 105 -85 pc at this distance, and a moderate ellipticity (mu = 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.