Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma
  Cluster
  We report the discovery of 47 low surface brightness objects in deep images
of a 3 x 3 degree field centered on the Coma cluster, obtained with the
Dragonfly Telephoto Array. The objects have central surface brightness mu(g,0)
ranging from 24 - 26 mag/arcsec^2 and effective radii r_e = 3"-10", as measured
from archival Canada France Hawaii Telescope images. From their spatial
distribution we infer that most or all of the objects are galaxies in the Coma
cluster. This relatively large distance is surprising as it implies that the
galaxies are very large: with r_e = 1.5 - 4.6 kpc their sizes are similar to
those of L* galaxies even though their median stellar mass is only ~6 x 10^7
Solar masses. The galaxies are relatively red and round, with <g-i> = 0.8 and
<b/a> = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble
Space Telescope ACS observation. The ACS imaging shows a large spheroidal
object with a central surface brightness mu(g,0) = 25.8 mag/arcsec^2, a Sersic
index n=0.6, and an effective radius of 7", corresponding to 3.4 kpc at the
distance of Coma. The galaxy is not resolved into stars, consistent with
expectations for a Coma cluster object. We speculate that UDGs may have lost
their gas supply at early times, possibly resulting in very high dark matter
fractions.
