Evidence for Ultra-diffuse Galaxy “Formation” through Galaxy Interactions

We report the discovery of two ultra-diffuse galaxies (UDGs) that show clear evidence of an association with tidal material and an interaction with a larger galaxy halo, found during a search of the Wide portion of the Canada-France-Hawaii Telescope Legacy Survey. The two new UDGs, NGC 2708-Dw1 and NGC 5631-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 (μ(g, 0) = 24.9 and 27.3 mag arcsec^-2), while the stellar stream associated with each has a surface brightness μ(g) ≳ 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.