Title:
A parametric study on the formation of ECs and UCDs by merged star cluster complexes

Abstract:
In the last decade, very extended old stellar clusters have been found in various types of galaxies in different environments, 
covering a huge mass range. 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.
The proposed formation scenario for ECs and UCDs starts with newly born complexes of star clusters in the galactic halo on 
an eccentric orbit with galactocentric distances between 20 kpc and 60 kpc. Our parametric study aims to analyze how the
structural parameters of the final merger objects after 5 Gyr of dynamical evolution correlate with the underlying CC parameter 
space. We systematically scan a suitable parameter space for CCs covering CC Plummer radii between 10 - 160 pc and CC 
masses between 105.5 - 108 M_sun, which are consistent with observed CC parameters.
All simulations end up with stable merger objects, having structural parameters comparable to those of observed ECs and 
UCDs. The merger 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 10 and 50 pc at the high mass end.
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. The results demonstrate that even very extended 
objects like the M31 ECs found by Huxor(2004) 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.
We conclude that the observed ECs and UCDs can be well explained by the merged cluster complex scenario.