Extended star clusters ín quiescent, active, and interacting galaxies as probes for structure formation
Final Report Abstract
In the last decade, very extended old stellar clusters, which cover a large mass range, have been found in various types of galaxies in different environments. Objects with masses comparable to normal globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). Moreover, observations indicate that star clusters tend to form in larger conglomerations called star cluster complexes (CCs) – especially in heavily interacting galaxies. The CCs have typical masses between M CC = 10^5 and a few 10^7 M⊙, radii of tens to a few hundred parsecs, and they typically consist of few to several dozens and possibly hundreds of young massive star clusters. We compiled the first comprehensive catalog of 835 ECs and UCDs with effective radii larger than 10 pc from the literature. This catalogue has been absorbed by the CDS, Strasbourg. At each magnitude objects are found with effective radii between 10 pc and an upper size limit which increases for brighter luminosities. For objects associated with early- and late-type galaxies, the turnover of the luminosity function of the extended objects is about one magnitude fainter than that of the GC luminosity function. The extended objects and GCs form a coherent structure in the reff vs. MV parameter space, while there is a clear gap between extended objects and early-type dwarf galaxies except for the high-mass end, where the most extended objects are close to the parameters of some compact elliptical galaxies. The rapidly increasing number of observed objects allowed for the first time an in-depth investigation of their nature. This DFG project investigated the question whether CCs are the progenitors of ECs and of UCDs by performing extensive numerical simulations. It is the first systematic research on the evolution of CCs. In this formation scenario these extended objects evolve from CCs by merging of their constituent star clusters. The basic and most important parameters of a CC are its mass and size. These two parameters were varied in an extensive parametric study to investigate how the structural parameters of the final merger objects correlate with the underlying CC parameter space. The third important factor is the external tidal field which has a large impact on the evolution of a CC. The influence of the external tidal field was studied by varying the orbital parameters of the CCs. These three parameters were varied systematically and the resulting merger objects were compared with specific extended objects and with the overall properties of the objects of the EO-catalog. A comparison of the observed sample of extended objects of the catalog with the numerical models demonstrates that the merging star cluster scenario reproduces the structural parameters, the distribution, and the overall trends of the observed extended objects very well. Even specific features of some extended objects are well reproduced in the simulations. All extended objects can be very well explained by a star cluster origin, where they are the results of merged star clusters of cluster complexes. The distinction made between ECs and UCDs is no longer existent in this formation scenario. They can all be explained as stemming from the same formation process and can therefore be united under the name “extended stellar dynamical object” (EO).
Publications
- 2011, A&A, 529, 138: A Parametric Study on the Formation of Extended Star Clusters and Ultra-Compact Dwarf Galaxies
Brüns, R. C., Kroupa, P., Fellhauer, M., Metz, M., & Assmann, P.
- 2011, ApJ, 729, 69: A New Formation Scenario for the Milky Way Cluster NGC 2419
Brüns, R. C., Kroupa, P.
- 2012, A&A, 547, 65: A Catalog of Extended Clusters and Ultra-Compact Dwarf Galaxies. An Analysis of their Parameters in Early- and Late-type Galaxies
Brüns, R. C., & Kroupa, P.
(See online at https://doi.org/10.1051/0004-6361/201219693) - 2014, PhD thesis, Mathematisch-Naturwissenschaftlichen Fakultät der Universität Bonn: A Unified Formation Scenario for the Zoo of Extended Star Clusters and Ultra-Compact Dwarf Galaxies
Brüns, R. C.
