The transition between star clusters and dwarf galaxies

On the existence of a mass-radius relation for star clusters of masses > M: are these objects formed in mergers of stellar systems?

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: [mkissler;ajordan]@eso.org
² Astrophysics, Denys Wilkinson Building, University of Oxford, 1 Keble Road, Oxford, OX1 3RH, UK
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK e-mail: bastian@star.ucl.ac.uk

Received: 20 October 2005
Accepted: 24 November 2005

Abstract

Context.At which masses does the regime of globular clusters end and the one of dwarf galaxies begin? And what separates these two classes of hot stellar systems?Aims.We examine to what extend very massive (>) young star clusters are similar to their lower mass counter parts and to which degree they resemble other objects in their mass regime (dwarf-globular transition objects (DGTOs), ultra compact dwarf galaxies (UCDs), galaxy nuclei).Methods.The comparison is performed by placing the recently observed very massive young clusters onto known scaling relation defined by globular clusters (with typical masses ) and/or by hot stellar systems with sizes up to those of giant galaxies.Results.The very massive () young clusters seem to show a mass-radius relation compatible with the one defined by hot stellar systems of galaxy mass. This, in turn, can explain their location on the other scaling relations investigated. It contrasts with the behaviour of the less massive young clusters and of globular clusters, which do not exhibit any mass-radius relation. However, the behaviour of the most massive globular clusters is similar to that of most other objects in that mass regime ().Conclusions.We show that the properties of the most massive young clusters are compatible with other objects in the same mass regime such as DGTOs/UCDs. They present a possible direct avenue of formation for those objects, which does not require the transformation of a previously existing stellar system. Simulations and observations support the possibility of the formation of such very massive young clusters by early mergers of lower mass stellar clusters, which could explain the emergence of a mass-radius relation.