The Dragonfly Nearby Galaxies Survey. II. Ultra diffuse galaxies near
  the elliptical galaxy NGC 5485
  We present the unexpected discovery of four ultra diffuse galaxies (UDGs) in
a group environment. We recently identified seven extremely low surface
brightness galaxies in the vicinity of the spiral galaxy M101, using data from
the Dragonfly Telephoto Array. The galaxies have effective radii of $10"-38"$
and central surface brightnesses of $25.6-27.7$ mag arcsec$^{-2}$ in g-band. We
subsequently obtained follow-up observations with $HST$ to constrain the
distances to these galaxies. Four remain persistently unresolved even with the
spatial resolution of $HST$/ACS, which implies distances of $D > 17.5$ Mpc. We
show that the galaxies are most likely associated with a background group at
$\sim 27$ Mpc containing the massive ellipticals NGC 5485 and NGC 5473. At this
distance, the galaxies have sizes of $2.6-4.9$ kpc, and are classified as UDGs,
similar to the populations that have been revealed in clusters such as Coma,
Virgo and Fornax, yet even more diffuse. The discovery of four UDGs in a galaxy
group demonstrates that the UDG phenomenon is not exclusive to cluster
environments. Furthermore, their morphologies seem less regular than those of
the cluster populations, which may suggest a different formation mechanism or
be indicative of a threshold in surface density below which UDGs are unable to
maintain stability.
