Ultra-Diffuse Galaxies at Ultraviolet Wavelengths
  We measure NUV aperture magnitudes from {\sl GALEX} images for 258
ultra-diffuse galaxy (UDG) candidates drawn from the initial SMUDGes survey of
$\sim 300$ square degrees surrounding, and including, the Coma galaxy cluster.
For the vast majority, 242 of them, we present flux upper limits due either to
a lack of significant flux in the aperture or confusion with other objects
projected within the aperture. These limits often place interesting constraints
on the UDG candidates, indicating that they are non-star forming or quiescent.
In particular, we identify field, quiescent UDG candidates, which are a
challenge for formation models and are therefore compelling prospects for
spectroscopic follow-up and distance determinations. We present FUV and NUV
magnitudes for 16 detected UDG candidates and compare those galaxies to the
local population of galaxies on color-magnitude and specific star formation
rate diagrams. The NUV detected UDG candidates form mostly an extension toward
lower stellar masses of the star forming galaxy sequence and none of these lie
within regions of high local galaxy density. UDG candidates span a range of
properties, although almost all are consistent with being quiescent, low
surface brightness galaxies, regardless of environment.
