A Unified Formation Scenario for the Zoo of Extended Star Clusters and Ultra-Compact Dwarf Galaxies

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,<br /> observations indicate that star clusters tend to form in larger conglomerations called star cluster complexes (CCs) in heavily interacting galaxies. The CCs have typical masses between M^CC = 10⁵ and a few 10⁷ M_⊙, radii of tens to a few hundred parsecs, and they typically consist of few to several dozens of young massive star clusters.<br /> I have compiled a catalog of 835 ECs and UCDs with effective radii larger than 10 pc from the literature. 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<br /> 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 allows for the first time an in-depth investigation of their nature.<br /> In this thesis I investigate the question whether CCs are the progenitors of ECs associated with galactic disks (also known as faint fuzzies, FFs), of ECs located in galactic halos, and of UCDs by performing extensive numerical simulations (in total 154). It is the first systematic research on the evolution of CCs. In this formation scenario these extended objects evolve from<br /> CCs by merging of their constituent star clusters. The basic and most important parameters of a CC are itsmass and size. These two parameters were varied in all parametric studies 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<br /> 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 are varied systematically and the resulting merger objects are compared with specific extended objects like the faint fuzzies in the galaxy NGC1023 and the Milky Way cluster NGC2419 and with the overall properties of the extended objects in the catalog.<br /> 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<br /> star clusters of cluster complexes. The distinction made between FFs, ECs, and UCDs is no longer existent in this formation scenario. They all stem from the same formation process and are therefore united under the name “extended stellar dynamical object” (EO).