A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

Abstract

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.

Globular cluster clustering and tidal features around ultra-compact dwarf galaxies in the halo of NGC 1399

“…Recent mergers of star clusters in star cluster complexes can host a variety of substructures in the form of faint envelopes, tidal tails, and nonmerged companion GCs (e.g., Brüns et al 2011Brüns et al , 2009Fellhauer & Kroupa 2005). Whereas in some simulations of UCD-like objects the surrounding GC companions have merged into the central object within the first Gyr (Brüns et al 2011), and thus the merging process is finished.…”

“…Whereas in some simulations of UCD-like objects the surrounding GC companions have merged into the central object within the first Gyr (Brüns et al 2011), and thus the merging process is finished. Other simulations of lower mass star cluster complexes suggest that some member star clusters still can be identified as substructures up to 5 Gyr after the onset of merging.…”

“…In Brüns et al (2011), it has been shown with a numerical model that the products of such mergers at large galactocentric distances can have effective radii up to 80 pc. In Fellhauer & Kroupa (2005), the evolution of the surface brightness profiles of merging stellar superclusters are shown.…”

“…An alternative explanation for the clustering of GCs around UCDs could be that those UCDs initially formed in a star cluster complex that then subsequently merged to create a UCD (see, e.g., Fellhauer & Kroupa 2002;Brüns et al 2009Brüns et al , 2011. Massive star cluster complexes are common in gas-rich mergers (e.g., The Antennae, Whitmore et al 1999).…”

“…In Brüns et al (2011) it was shown that this kind of merging super star cluster complex has several surrounding close GC companions at 70 Myr after the start of the merging process. After 280 Myr of merging, it is left with one GC companion.…”