LACERTA I AND CASSIOPEIA III. TWO LUMINOUS AND DISTANT ANDROMEDA SATELLITE DWARF GALAXIES FOUND IN THE 3π PAN-STARRS1 SURVEY

We report the discovery of two new dwarf galaxies, Lacerta I/Andromeda XXXI (Lac I/And XXXI) and Cassiopeia III/Andromeda XXXII (Cas III/And XXXII), in stacked Pan-STARRS1 r(P1)- and i(P1)-band imaging data. Both are luminous systems (M-V similar to -12) located at projected distances of 20 degrees.3 and 10 degrees.5 from M31. Lac I and Cas III are likely satellites of the Andromeda galaxy with heliocentric distances of 756(-28)(+44) kpc and 772(-56)(+61) kpc, respectively, and corresponding M31-centric distances of 275 +/- 7 kpc and 144(-4)(+6) kpc. The brightest of recent Local Group member discoveries, these two new dwarf galaxies owe their late discovery to their large sizes (r(h) = 4.2(-0.5)(+0.4) arcminor 912(-93)(+124) pc for Lac I; r(h) = 6.5(-1.0)(+1.2) arcmin or 1456 +/- 267 pc for Cas III) and consequently low surface brightness (mu(0) similar to 26.0 mag arcsec(-2)), as well as to the lack of a systematic survey of regions at large radii from M31, close to the Galactic plane. This latter limitation is now alleviated by the 3 pi Pan-STARRS1 survey, which could lead to the discovery of other distant Andromeda satellite dwarf galaxies.