Nuclear star clusters (NSCs) are dense and massive clusters observed with high frequency ($\gt 80{{\ \rm per\ cent}}$) at the centre of galaxies with stellar masses 109–1011 M⊙ (e.g. Böker et al. 2002, 2004; Côté et al. 2006; Turner et al. 2012; Baldassare et al. 2014; den Brok et al. 2014; Georgiev & Böker 2014; Sánchez-Janssen et al. 2019; Pechetti et al. 2020). These extreme environments often harbour a central supermassive black hole (as in the case of our Galaxy), and represent the most dense stellar systems in the Universe. We refer to the recent and complete review by Neumayer, Seth & Böker (2020) for more details about NSCs. There are two main formation channels that are thought to compete in NSC formation: in situ formation via fragmentation of gaseous clouds in the galactic centre (e.g. Loose, Kruegel & Tutukov 1982), or via orbital segregation and merger of massive star clusters that migrate towards the galactic centre via dynamical friction (Tremaine, Ostriker & Spitzer 1975; Capuzzo-Dolcetta 1993). The latter formation channel, named dry-merger scenario, has been widely explored theoretically and numerically (Capuzzo-Dolcetta & Miocchi 2008a, b). For instance, high-resolution N-body models suggested that the Milky Way (MW) NSC might have formed through this mechanism (Antonini et al. 2012; Tsatsi et al. 2017; Arca-Sedda et al. 2020), which might explain both the structure and kinematics of the Galactic NSC. The dry-merger scenario also provides a successful explanation for the potential formation of NSCs in young galaxies and for the seemingly absence of nucleated regions in small dwarf and massive ellipticals (Arca-Sedda & Capuzzo-Dolcetta 2014, 2017). In fact, several semi-analytical models have shown that the dry-merger scenario leads to correlations between the NSC and the host galaxy properties pretty similar to the observed one (Antonini 2013; Arca-Sedda & Capuzzo-Dolcetta 2014; Gnedin, Ostriker & Tremaine 2014; Capuzzo-Dolcetta & Tosta e Melo 2017). None the less, several features of NSCs seem hard to explain as the result of star cluster merging events. For instance, NSCs exhibit a complex star formation history that seems to suggest the occurrence of several episodic star formation events over the entire course of their lifetime (Neumayer et al. 2020). Such a feature can also be easily explained with in situ formation (Böker et al. 2004), thus suggesting that the formation and evolution of NSCs is likely the result of both scenarios operating in concert. Here, we present our numerical approach to test this scenario in one of the clearest examples of two massive star clusters caught in the process of merging in the nucleus of the nearby MW-like spiral galaxy NGC 4654 shown in Fig. 1 and a zoom in on its nucleus in Fig. 2. Their projected separations, photometric mass, and assumption for the local velocity field of an MW-like galaxy suggest that they should be on a short, few tens of Myr, collision course before they completely merge (Georgiev & Böker 2014). However, this pure analytical expectation needs to be tested in order to gain a deeper knowledge on the following: (1) what will be the merging time-scales given the cluster current observational properties; (2) how the physical and observational properties of the final product depend on the merger dynamics and how such properties compare to those of current NSCs in galaxies of similar mass and type as NGC 4654; and (3) in the hypothesis that the two clusters contain different stellar populations, what are the expected distributions and fractions of the stars coming from the two progenitors in the merger product.
Citation: Tremaine, Ostriker & Spitzer 1975
Citation function: Background; Background