Abstract
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 ~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 van der
Burg et al. 2016. Using the same selection criteria as van der Burg et al.
2016, A2744 hosts an estimated 2133 +/- 613 UDGs, ten times the number in Coma.
As noted by Lee & Jang 2016, 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.
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