Context. In the last decade, very extended old stellar clusters with masses in the range from a few 10 4 to 10 8 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 -10 8 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.

In the lenticular galaxy NGC 1023 a third population of globular clusters (GCs), called faint fuzzies (FFs), was discovered next to the blue and red GC populations by Larsen & Brodie. While these FFs have colors comparable to the red population, the new population is fainter, larger (R eff > 7 pc) and, most importantly, shows clear signs of co-rotation with the galactic disk of NGC 1023. We present N-body simulations verifying the hypothesis that these disk-associated FFs are related to the young massive cluster complexes (CCs) observed by Bastian et. al in M51, who discovered a massradius relation for these CCs. Our models have an initial configuration based on the observations from M51 and are placed on various orbits in a galactic potential derived for NGC 1023. All computations end up with a stable object containing 10 to 60% of the initial CC mass after an integration time of 5 Gyr. A conversion to visual magnitudes demonstrates that the resulting objects cover exactly the observed range for FFs. Moreover, the simulated objects show projected half-mass radii between 3.6 and 13.4 pc, in good agreement with the observed FF sizes. We conclude that objects like the young massive CCs in M51 are likely progenitors of the FFs observed in NGC 1023.

We present a new formation scenario for NGC 2419, which is one of the most luminous, one of the most distant, and as well one of the most extended globular clusters of the Milky Way. We propose that NGC 2419 is the remnant of a merged star cluster complex, which was possibly formed during an interaction between a gas-rich galaxy and the Milky Way. To test this hypothesis, we performed numerical simulations of 27 different models of star cluster complexes (CCs) moving on a highly eccentric orbit in the Galactic halo. We vary the CC mass, the CC size, and the initial distribution of star clusters in the CC to analyze the influence of these parameters on the resulting objects.In all cases, the vast majority of star clusters merged into a stable object. The derived parameters mass, absolute V-band magnitude, effective radius, velocity dispersion and the surface brightness profile are, for a number of models, in good agreement with those observed for NGC 2419. Despite the large range of CC sizes, the effective radii of the merger objects are constrained to a relatively small interval. A turnover in the r eff vs. M encl space leads to degenerate states, i.e. relatively compact CCs can produce an object with the same structural parameters as a more massive and larger CC. In consequence, a range of initial conditions can form a merger object comparable to NGC 2419 preventing us to pinpoint the exact parameters of the original CC, which formed NGC 2419.We conclude that NGC 2419 can be well explained by the merged cluster complex scenario. Some of the recently discovered stellar streams in the Galactic halo might be related to the parent galaxy, which produced the cluster complex in our scenario. Measurements of the proper motion of NGC 2419 are necessary to prove an association with one of the stellar streams.

It is widely believed that star clusters form with low star formation efficiencies. With the onset of stellar winds by massive stars or finally when the first supernova blows off, the residual gas is driven out of the embedded star cluster. Due to this fact, a large number, if not all, of the stars become unbound and disperse in the gravitational potential of the galaxy. In this context, Kroupa suggested a new mechanism for the emergence of thickened galactic discs. Massive star clusters add kinematically hot components to the galactic field populations, building up, in this way, the Galactic thick disc as well. In this work, we perform, for the first time, numerical simulations to investigate this scenario for the formation of the Galactic discs of the Milky Way (MW). We find that a significant kinematically hot population of stars may be injected into the disc of a galaxy such that a thick disc emerges. For the MW, the star clusters that formed the thick disc must have had masses of about 10 6 M .