Low Surface Brightness Galaxies
Ultra Compact Dwarfs
Steve Phillipps
Background
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.
UCDs
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
