A catalog of extended clusters and ultra-compact dwarf galaxies

An analysis of their parameters in early- and late-type galaxies^⋆,⋆⋆

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

Received: 28 May 2012
Accepted: 14 September 2012

Abstract

Context. In the past decade, very extended old stellar clusters with masses in the range from a few 10⁴ to 10⁸ M_⊙ and effective radii larger than 10 pc 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).

Aims. The paper analyses the observational parameters total luminosity, M_V, effective radius, r_eff, and projected distance to the host galaxy, R_proj, of all known ECs and UCDs and the dependence of these parameters on the type and the total luminosity of their host galaxy.

Methods. We searched the available literature to compile a catalog of star clusters with effective radii larger than 10 pc. As there is no clear distinction between ECs and UCDs, both types of objects will be called extended stellar objects – abbreviated “EOs” – in this paper.

Results. In total, we found 813 EOs of which 171 are associated with late-type galaxies and 642 EOs associated with early-type galaxies. EOs cover a luminosity range from about M_V =  −4 to  −14 mag. However, the vast majority of EOs brighter than M_V =  −10 mag are associated with giant elliptical galaxies. At each magnitude extended objects are found with effective radii between 10 pc and an upper size limit, which shows a clear trend: the more luminous the object the larger is the upper size limit. For EOs associated with early- and late-type galaxies, the EO luminosity functions peak at  − 6.40 mag and  − 6.47 mag, respectively, which is about one magnitude fainter than the peak of the GC luminosity function. EOs and GCs form a coherent structure in the r_eff vs. M_V parameter space, while there is a clear gap between EOs and early-type dwarf galaxies. However, there is a small potential overlap at the high-mass end, where the most extended EOs are close to the parameters of some compact elliptical galaxies. We compare the EO sample with the numerical models of a previous paper and conclude that the parameters of the EO sample as a whole can be very well explained by a star cluster origin, where EOs are the results of merged star clusters of cluster complexes.