Observational Properties of Field UDGs: Colours and Number Densities
  While much of the focus around Ultra-Diffuse Galaxies (UDGs) has been given
to those in galaxy groups and clusters, relatively little is known about them
in less-dense environments. These isolated UDGs provide fundamental insights
into UDG formation because environmentally driven evolution and survivability
play less of a role in determining their physical and observable properties. We
have recently conducted a statistical analysis of UDGs in the field using a new
catalogue of sources detected in the deep Kilo-Degree Survey (KiDS) and Hyper
Suprime-Cam Subaru Strategic Program (HSC-SSP) optical imaging surveys. Using
an empirical model to assess our contamination from interloping sources, we
show that a scenario in which cluster-like quiescent UDGs occupy a large
fraction of the field UDG population is unlikely, with most being significantly
bluer and some showing signs of localised star formation. We estimate an
upper-limit on the total field abundance of UDGs of
8$\pm$3$\times10^{-3}$cMpc$^{-3}$ within our selection range. The mass
formation efficiency of UDGs implied by this upper-limit is similar to what is
measured in groups and clusters, meaning that secular formation channels may
significantly contribute to the overall UDG population.
