1. Department of Physics, University of Queensland, Queensland 4072, Australia
2. Department of Physics, University of California, Davis, California 95616, USA
3. Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-413, Livermore, California 94550, USA
4. Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
5. School of Physics, University of New South Wales, Sydney 2052, Australia
6. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
7. School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
8. Astrophysics Group, Department of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

Correspondence to: M. J. Drinkwater¹ Correspondence and requests for materials should be addressed to M.J.D. (Email: mjd@physics.uq.edu.au).

Abstract

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters^{1, 2, 3}. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails⁴ and intra-cluster stars^{5, 6} being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies⁷. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster⁸. This revealed a new population of compact members^{9, 10}, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.