A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: rcbruens@astro.uni-bonn.de; pavel@astro.uni-bonn.de
² Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
e-mail: mfellhauer@astro-udec.cl; passmann@astro-udec.cl
³ Deutsches Zentrum für Luft- und Raumfahrt, Königswinterer Str. 522-524, 53227 Bonn, Germany
e-mail: manuel.metz@dlr.de

Received: 28 November 2010
Accepted: 21 March 2011

Abstract

Context. In the last decade, very extended old stellar clusters with masses in the range from a few 10⁴ to 10⁸ M_⊙ 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). In heavily interacting galaxies star clusters tend to form in larger conglomerations called star cluster complexes (CCs). The individual star clusters in a CC can merge and form a variety of spheroidal stellar objects.

Aims. The parametric study aims to analyze how the structural parameters of the final merger objects correlate with the underlying CC parameter space.

Methods. In this work we systematically scan a suitable parameter space for CCs and perform numerical simulations to study their further fate. The varied sizes and masses of the CCs cover a matrix of 5 × 6 values with CC Plummer radii between 10–160 pc and CC masses between 10^5.5–10⁸ M_⊙, which are consistent with observed CC parameters. The CCs of the parametric study are on orbits with galactocentric distances between 20 kpc and 60 kpc. In addition, we studied also the evolution of CCs on a circular orbit at a galactocentric distance of 60 kpc to verify that also extremely extended ECs and UCDs can be explained by our formation scenario.

Results. All 54 simulations end up with stable merger objects, wherein 26 to 97% of the initial CC mass is bound. The objects show a general trend of increasing effective radii with increasing mass. Despite the large range of input Plummer radii of the CCs (10 to 160 pc) the effective radii of the merger objects are constrained to values between 10 and 20 pc at the low mass end and to values between 15 and 55 pc at the high mass end. The structural parameters of the models are comparable to those of the observed ECs and UCDs. The results of the circular orbits demonstrate that even very extended objects like the M 31 ECs found by Huxor in 2005 and the very extended (r_eff > 80 pc), high-mass UCDs can be explained by merged cluster complexes in regions with low gravitational fields at large galactocentric radii.

Conclusions. We conclude that the observed ECs and UCDs can be well explained as evolved star cluster complexes.