Evidence for Ultra-Diffuse Galaxy `Formation' Through Galaxy
  Interactions
  We report the discovery of two ultra-diffuse galaxies (UDGs) which show clear
evidence for association with tidal material and interaction with a larger
galaxy halo, found during a search of the Wide portion of the
Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The two new UDGs,
NGC2708-Dw1 and NGC5631-Dw1, are faint ($M_g$=$-$13.7 and $-$11.8 mag),
extended ($r_h$=2.60 and 2.15 kpc) and have low central surface brightness
($\mu(g,0)$=24.9 and 27.3 mag arcsec$^{-2}$), while the stellar stream
associated with each has a surface brightness $\mu(g)$$\gtrsim$28.2 mag
arcsec$^{-2}$. These observations provide evidence that the origin of some UDGs
may connect to galaxy interactions, either by transforming normal dwarf
galaxies by expanding them, or because UDGs can collapse out of tidal material
(i.e. they are tidal dwarf galaxies). Further work is needed to understand the
fraction of the UDG population `formed' through galaxy interactions, and wide
field searches for diffuse dwarf galaxies will provide further clues to the
origin of these enigmatic stellar systems.
