The origin of the Galactic nucleus
Abstract
Nuclear Star Clusters (NSCs) are dense massive stellar clusters (with typically a few tens of million stars), residing in galactic nuclei and most of them hosting a massive black hole (MBH) at their centre (Neumayer et al. 2020). The fairly strong correlations existing between their masses and the properties of their host galaxies suggest that the formation of NSCs and their host galaxies are linked in important ways. Although their study could help to unveil the build-up process of galaxies, their origin is still unknown. One of their possible formation scenarios is the cluster-inspiral model, where massive and compact globular clusters (GCs) migrate to the centre via dynamical friction and merge to form a dense nucleus. Some basic aspects of this scenario have been explored in detail, especially by means of N-body simulations, but many of the fundamental questions and aspects, implications and observational predictions are yet to be explored.
In Antonini et al. 2012 we ran, for the first time, self-consistent N-body simulations using phiGRAPE a direct N-body code parallelized on GPUs (Harfst et al. 2008). We followed the decay and merger of 12 GCs in the central regions of the Galaxy and studied the properties of the final NSC. The initial conditions of the simulations are based on observations of the Milky Way (MW) centre and include a central Sgr A*-like MBH.
In Tsatsi et al. 2017, we ran more updated simulations producing outputs that can be directly compared to observations. The very good agreement between the MW and simulated NSCs kinematics proves that the inspiral scenario is a viable mechanism for the formation of NSCs. In Mastrobuono-Battisti et al. 2021, we analysed these simulations to study stellar collisions and encounters at the Galactic centre.
All these simulations suffer from several issues, mainly due to the limited computational resources available. For instance, besides using a limited number of particles, we simulated extremely dense, single mass, GCs in order to be sure they would reach the galactic centre without being destroyed by the Galactic tides. Here we aim to test and run new, more realistic simulations using several sets of parameters for the internal properties of the GCs (e.g. different masses, concentrations) and including a recipe for the MBH mass growth. We propose to follow these directions to obtain NSCs arising from more realistic conditions; for example, smaller clusters may disrupt earlier than more massive and dense clusters, injecting their stars at different regions of the NSC, and significantly changing its evolution and growth. These new simulations will finally allow for constraining the contribution of GCs inspirals and for the understanding of how the MW and external galaxies NSCs formed.
References
Harfst et al. 2008, MNRAS, 389, 2
Mastrobuono-Battisti, Church, Davies, 2021, MNRAS, 505, 3314
Neumayer et al. 2020, A&ARv, 28, 4
Schödel et al. 2014, Classical and Quantum Gravity, 31, 244007
Tsatsi, Mastrobuono-Battisti et al. 2017, MNRAS, 464, 3720
