The globular cluster migratory origin of nuclear star clusters

Abstract

Nuclear Star Clusters (NSCs) are often present in spiral galaxies as well as resolved Stellar Nuclei (SNi) in elliptical galaxies centres. Ever growing observational data indicate the existence of correlations between the properties of these very dense central star aggregates and those of host galaxies, which constitute a significant constraint for the validity of theoretical models of their origin and formation. In the framework of the well known 'migratory and merger' model for NSC and SN formation, in this paper we obtain, first, by a simple argument the expected scaling of the NSC/SN mass with both time and parent galaxy velocity dispersion in the case of dynamical friction as dominant effect on the globular cluster system evolution. This generalizes previous results by Tremaine et al. (1975) and is in good agreement with available observational data showing a shallow correlation between NSC/SN mass and galactic bulge velocity dispersion. Moreover, we give statistical relevance to predictions of this formation model, obtaining a set of parameters to correlate with the galactic host parameters. We find that the correlations between the masses of NSCs in the migratory model and the global properties of the hosts reproduce quite well the observed correlations, supporting the validity of the migratory-merger model. In particular, one important result is the flattening or even decrease of the value of the NSC/SN mass obtained by the merger model as function of the galaxy mass for high values of the galactic mass, i.e. 3 × 10 11 M ⊙ , in agreement with some growing observational evidence.

Masses and scaling relations for nuclear star clusters, and their co-existence with central black holes

“…An obvious question to ask therefore is which, if any, evolutionary effects could explain this difference? An increase in NSC size and mass over time has been demonstrated by a number of numerical simulations of merging clusters (Fellhauer & Kroupa 2002;Baumgardt et al 2003;Bekki et al 2004;Brüns et al 2011;Antonini 2013;Gnedin et al 2014;Arca-Sedda & Capuzzo-Dolcetta 2014) or/and mass build up from gas accretion (Hartmann et al 2011). In particular, the slope of the r eff,NSC -MNSC relation found in this paper (α 0.34 Table 1) is consistent with the slope of 0.4 found from cluster merger simulations (e.g.…”

On the formation of compact, massive subsystems in stellar clusters and its relation with intermediate-mass black holes

“…This mechanism, called dynamical friction (df), arises directly from two-body encounters and plays a significant role in shaping the evolution of astrophysical systems on very different scales (Bekenstein 1973;Tremaine 1976;Capuzzo-Dolcetta 1993;Milosavljević & Merritt 2001;Gualandris & Merritt 2008;Antonini 2013;Arca-Sedda & Capuzzo-Dolcetta 2014b;Arca-Sedda et al 2015).…”

Star formation at the Galactic Centre: coevolution of multiple young stellar discs

“…The presence of the young stellar disc as well as the existence of H ii regions and young stars within the central 100 pc of the Milky Way (Figer et al 2004) suggest that the GC went through recent star formation processes in this region (Genzel et al 2010). However, while the majority of current studies focused on the understanding of the build-up of NSCs through the infall of stellar clusters (Tremaine et al 1975;Capuzzo-Dolcetta 1993;Antonini et al 2012;Arca-Sedda & Capuzzo-Dolcetta 2014;Mastrobuono-Battisti et al 2014;Gnedin et al 2014;Arca-Sedda et al 2015;Antonini et al 2015;Arca-Sedda & Capuzzo-Dolcetta 2017a,b;Tsatsi et al 2017;Abbate et al 2018), very few studies so far explore the long-term effects and evolution of NSCs with insitu star formation (Aharon & Perets 2015;Guillard et al 2016;Baumgardt et al 2018). In particular the potential insitu star formation producing stellar discs such as the one observed in the GC had not been investigated.…”