While we have learned much about Ultra-Diffuse Galaxies ( UDGs ) in groups and clusters , relatively little is known about them in less-dense environments .
More isolated UDGs are important for our understanding of UDG formation scenarios because they form via secular mechanisms , allowing us to determine the relative importance of environmentally-driven formation in groups and clusters .
We have used the public Kilo-Degree Survey ( KiDS ) together with the Hyper Suprime-Cam Subaru Strategic Program ( HSC-SSP ) to constrain the abundance and properties of UDGs in the field , targeting sources with low surface brightness ( 24.0 \leq \bar { \mu } _ { e,r } \leq 26.5 ) and large apparent sizes ( 3.0 \arcsec \leq \bar { r } _ { e,r } \leq 8.0 \arcsec ) .
Accounting for several sources of interlopers in our selection based on canonical scaling relations , and using an empirical UDG model based on measurements from the literature , we show that a scenario in which cluster-like red sequence UDGs occupy a significant number of field galaxies is unlikely , with most field UDGs being significantly bluer and showing signs of localised star formation .
An immediate conclusion is that UDGs are much more efficiently quenched in high-density environments .
We estimate an upper-limit on the total field abundance of UDGs of 8 \pm 3 \times 10 ^ { -3 } cMpc ^ { -3 } within our selection range .
We also compare the total field abundance of UDGs to a measurement of the abundance of Hi-rich UDGs from the literature , suggesting that they occupy at least one-fifth of the overall UDG population .
The mass formation efficiency of UDGs implied by this upper-limit is similar to what is measured in groups and clusters .