Low Surface Brightness Galaxies

Ultra Compact Dwarfs

Steve Phillipps


We have a long running programme of surveys for dwarf galaxies in clusters. Most dwarfs are of low surface brightness, that is, very diffuse and spread out. However during the Fornax Cluster Spectroscopic Survey of all the moderately bright objects in the direction of the Fornax Cluster, we discovered that 6 of the objects which appeared to be foreground stars in our Galaxy were actually tiny galaxies in the cluster.


The FCSS covered a region of about 9 square degrees in the centre of the
Fornax cluster. The first few UCDs identified are the tiny dots inside the circles marked on the image. Fainter candidates, of similar size to globular clusters, are marked by the small squares (see below). Our HST imaging shows that even the brighter objects have sizes of only about 20pc.

UCD Formation

UCDs may form via the destruction of larger galaxies, such as nucleated dwarf ellipticals. The illustration shows a numerical computation of the removal of the outer layers of stars of a dE,N galaxy by tidal forces as it plunges past the central giant galaxy NGC 1399. The insets show a `before and after' view of a normal dE,N galaxy (top) and a UCD (bottom) as observed with the Hubble Space Telescope.

UCDs in Virgo

More recently we have explored the central regions of the Virgo Cluster, looking for similar objects. In only four hours observing we were able to obtain the spectra for over 1000 targets and establish that 9 of them were UCDs in the cluster. This indicates that UCDs are a standard population in the dense inner regions of clusters.

Even More!

Our most recent observations, of fainter candidate `stars' in Fornax have revealed the presence of 46 new fainter UCDs. There are so many that some may be the remnants of a primordial population of dwarf galaxies out of which the giant central cluster galaxies grew via hierarchical merging.

Revised 5-November-2008 by Steve Phillipps