We report the discovery of a large population of Ultra-diffuse Galaxies ( UDGs ) in the massive galaxy cluster Abell 2744 ( z = 0.308 ) as observed by the Hubble Frontier Fields program .
Since this cluster is \sim 5 times more massive than Coma , our observations allow us to extend 0.7 dex beyond the high-mass end of the relationship between UDG abundance and cluster mass reported by ( 41 ) .
Using the same selection criteria as ( 41 ) , A2744 hosts an estimated 2133 \pm 613 UDGs , ten times the number in Coma .
As noted by ( 20 ) , A2744 contains numerous unresolved compact objects , which those authors identified predominantly as globular clusters .
However , these objects have luminosities that are more consistent with ultra-compact dwarf ( UCD ) galaxies .
The abundances of both UCDs and UDGs scale with cluster mass as a power law with a similar exponent , although UDGs and UCDs have very different radial distributions within the cluster .
The radial surface density distribution of UCDs rises sharply toward the cluster centre , while the surface density distribution of the UDG population is essentially flat .
Together , these observations hint at a picture where some UCDs in A2744 may have once been associated with infalling UDGs .
As UDGs fall in and dissolve , they leave behind a residue of unbound ultra-compact dwarfs .