A parametric study on the formation of extended star clusters and
  ultra-compact dwarf galaxies
  In the last decade, extended stellar clusters with masses in the range from a
few 10^4 to 10^8 M_sun 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). 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 5x6 values with CC Plummer radii between 10 - 160 pc and CC
masses between 10^5.5 - 10^8 M_sun, which are consistent with observed CC
parameters. The CCs of the parametric study are on orbits with distances
between 20 kpc and 60 kpc. In addition, we studied also the evolution of CCs on
a circular orbit at a distance of 60 kpc to verify that also extremely extended
ECs and UCDs can be explained by our formation scenario. 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 M31 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
distances.
