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 .