by finding the nearest neighbour within 0.15, or 5 ACS pixels. The distribution of photo-$z$’s of sources that pass the UDG selection cuts is shown in Figure \[fig:photo-z\]. Photo-$z$’s were not used in the computation of physical sizes and absolute magnitudes since not every UDG has a match in the <span style="font-variant:small-caps;">ASTRODEEP</span> catalog.

![ The <span style="font-variant:small-caps;">ASTRODEEP</span> photo-$z$ distributions of sources that pass our UDG selection cuts are shown in blue (sources with $z_\mathrm{phot} \geq
    1$ were not visually inspected). Objects in the XDF that pass the cuts for each cluster are shown in black. The vertical line is the redshift of the cluster. \[fig:photo-z\] ](figure2.png){width="50.00000%"}

![image](figure3a.pdf){width="50.00000%"} ![image](figure3b.pdf){width="50.00000%"}

![image](figure3c.pdf){width="50.00000%"} ![image](figure3d.pdf){width="50.00000%"}

![image](figure3e.pdf){width="50.00000%"} ![image](figure3f.pdf){width="50.00000%"}

All UDG candidates were visually inspected. Of the 1190 candidates, 636 UDGs were kept. The most common contaminants are de-blended objects (i.e.spiral arms, lensing arcs and tidal features split into multiple objects by <span style="font-variant:small-caps;">SExtractor</span>) and compact ($n \sim 3.5$) galaxies with high central surface brightnesses that do not visually resemble UDGs. Finally, 18 duplicate UDGs were removed where a UDG was de-blended into multiple sources, in which case the brighter object was kept. In a few of these cases, <span style="font-variant:small-caps;">SExtractor</span> picked out an offset overdensity, similar 