Discovery of an ultra-diffuse galaxy in the Pisces-Perseus supercluster
  We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located
10.4 degrees in projection from the Andromeda galaxy (M31). This low-surface
brightness galaxy (mu_V = 24.8 mag/arcsec), found with a small amateur
telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam
images, and hence has been missed by optical surveys relying on resolved star
counts, in spite of its relatively large effective radius (R_e(V) = 12 arcsec)
and proximity (15 arcmin) to the well-known dwarf spheroidal galaxy And II. Its
red color (V-I = 1.0), shallow Sersic index (n_V=0.68), and the absence of
detectable H-alpha emission are typical properties of dwarf spheroidal galaxies
and suggest that it is mainly composed of old stars.
  Initially interpreted as an interesting case of an isolated dwarf spheroidal
galaxy in the local universe, our radial velocity measurement obtained with the
BTA 6-meter telescope (V_h=5450 +/- 40 km/s) shows that this system is an
M31-background galaxy associated with the filament of the Pisces-Perseus
supercluster. At the distance of this cluster (~78 Mpc), DGSAT I would have an
R_e ~ 4.7 kpc and M_V ~-16.3$. Its properties resemble those of the
ultra-diffuse galaxies recently discovered in the Coma cluster. DGSAT I is the
first case of these rare ultra-diffuse galaxies found in this galaxy cluster.
Unlike the ultra-diffuse galaxies associated with the Coma and Virgo clusters,
DGSAT I is found in a much lower density environment, which provides a fresh
constraint on the formation mechanisms for this intriguing class of galaxy.
