Paper: astro-ph/0512360
Date: Wed, 14 Dec 2005 12:11:45 GMT (44kb)

Title: The Transition between Star Clusters and Dwarf Galaxies -- On the
existence of a mass-radius relation for star clusters of masses >10^7

Mo: are
these objects formed in mergers of stellar systems?

Authors: Markus Kissler-Patig (ESO), Andres Jordan (ESO, Oxford), Nate Bastian
(Univ.College London)

Comments: research note, accepted by A&A, 5 pages incl. 3 figures
\\
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?
We examine to what extend very massive (>10^7 Mo) 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).
The comparison is performed by placing the recently observed very massive
young clusters onto known scaling relation defined by globular clusters (with
typical masses <10^6 Mo) and/or by hot stellar systems with sizes up to those
of giant galaxies.
The very massive (>10^{6.5-7} Mo) 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 clusters is similar to
that of most other objects in that mass regime (10^6-10^8 Mo).
We show that the properties of young massive 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.

\\ ( http://arXiv.org/abs/astro-ph/0512360 , 44kb)