A NEW CLASS OF GALAXY FORMED BY THE DISRUPTION OF DWARF GALAXIES IN GALAXY CLUSTERS [Embargoed by Nature until 10 a.m. U.K. time on Thursday, 29th May.] New observations with the Hubble Space Telescope and two of the world's largest optical telescopes have provided important results on a new class of galaxy. A team of astronomers has used the ultra-sharp imaging provided by the Hubble Space Telescope to measure the sizes of five unusually small galaxies, which they call ultra-compact dwarfs, in a nearby cluster of galaxies, 60 million light years away. These objects appear star-like through ground-based telescopes because of blurring by the Earth's atmosphere. The Space Telescope, however, is able to show them clearly. The 8-metre diameter Very Large Telescope in Chile and the 10-metre diameter Keck II Telescope in Hawaii were used to measure the combined motions of millions of stars inside the objects. The team consists of Dr. Michael Drinkwater of the University of Queensland (Australia), Dr. Michael Gregg of the Lawrence Livermore National Laboratory (USA), Dr. Michael Hilker of the University of Bonn Observatory (Germany), Dr. Kenji Bekki and Prof. Warrick Couch of the University of New South Wales (Australia), Dr. Henry Ferguson of the Space Telescope Science Institute (USA), Dr. Bryn Jones of the University of Nottingham (UK), and Dr. Steve Phillipps of the University of Bristol (UK). The ultra-compact dwarf galaxies were originally discovered during the Fornax Cluster Spectroscopic Survey with the 2dF spectrograph on the 3.9-metre Anglo-Australian Telescope. The survey is measuring the velocities along the line of sight of 12000 objects in the direction of the Fornax Cluster of galaxies, to determine which of these are dwarf galaxies in the cluster. Unlike almost all previous galaxy surveys, the project is targeting both objects that look like galaxies and ones that look like stars in images from ground-based telescopes. The survey team was surprised to find seven objects in the cluster that appear star-like from the ground. The new results, published in the 29th May edition of Nature, show that the five ultra-compact objects in the new study have sizes of only 100 to 200 light years, more than ten times smaller than typical dwarf galaxies. They are larger and more luminous than globular clusters, a type of star cluster containing hundreds of thousands of stars crowded in a compact region of space. The motions of the stars in the ultra-compact galaxies suggest that they are not merely unusually luminous globular clusters associated with a giant elliptical galaxy at the centre of the Fornax Cluster. The new results also show that the objects are not the cores of otherwise undetected ordinary dwarf galaxies. The team interprets the results as showing that the ultra-compact galaxies are likely to be the left-over nuclei of a common type of dwarf galaxy which has been disrupted by the strong gravitational tides of much larger galaxies. This type, called nucleated dwarf elliptical galaxies, are plentiful in galaxy clusters and computer simulations show that they can be completely disrupted if they come too close to a massive galaxy, to leave behind a bare nucleus. The new compact galaxies have the same brightness as nuclei of nucleated dwarf ellipticals in the Fornax Cluster and the speeds of the stars in their orbits inside these two objects are similar. These results should be important in understanding how galaxies evolve in galaxy clusters. The new observations imply that dwarf galaxies can be disrupted inside clusters by close encounters with massive galaxies. When this happens their stars will be swallowed by the massive galaxies or distributed loosely between the cluster galaxies. The team members are now turning their attention to a search for similar objects in the Virgo Cluster, another cluster of galaxies at a similar distance to the Fornax Cluster. This could demonstrate whether these ultra-compact dwarfs are common in galaxy clusters. Dr. J. B. Jones, Astronomy Group, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom. E-mail : Bryn.Jones@astro.nottingham.ac.uk