Mergers, tidal interactions, and mass exchange in a population of disc globular clusters

Abstract

Globular clusters (GCs), the oldest stellar systems observed in the Milky Way, have for long been considered single stellar populations. As such, they provided an ideal laboratory to understand stellar dynamics and primordial star formation processes. However, during the last two decades, observations unveiled their real, complex nature. Beside their pristine stars, GCs host one or more helium enriched and possibly younger stellar populations whose formation mechanism is still unknown. Even more puzzling is the existence of GCs showing star by star iron spreads. Using detailed N-body simulations we explore the hypothesis that these anomalies in metallicity could be the result of mutual stripping and mergers between a primordial population of disc GCs. In the first paper of this series we proved, both with analytical arguments and short-term N-body simulations, that disc GCs have larger fly-by and close encounter rates with respect to halo clusters. These interactions lead to mass exchange and even mergers that form new GCs, possibly showing metallicity spreads. Here, by means of long-term direct N-body simulations, we provide predictions on the dynamical properties of GCs that underwent these processes. The comparison of our predictions with available and future observational data could provide insights on the origin of GCs and on the Milky Way build-up history as a whole.

A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. II. Kinematic Characterization of the Stellar Populations

“…A large spread in metallicity alone can be explained by self-enrichment during formation (Bailin 2018), but it does not explain the large spread in age. Thus, we suggested that this could be the result of a merger event between two or more clusters that fell into the central region of the host, as suggested through simulations (e.g., Amaro-Seoane et al 2013;Gavagnin et al 2016;Bekki & Tsujimoto 2016;Khoperskov et al 2018;Mastrobuono-Battisti et al 2019a). In addition, a contribution to this high spread in metallicity might be due to the old, metal-poor population of Sgr dSph that mixed with the merged GCs to the resulting OMP population of this NSC.…”

“…The final rotation of two merged clusters strongly depends on the conditions, as the orbital configuration and relaxation states of the merging clusters do not always result in a highly rotating structure (Mastrobuono-Battisti et al 2019a). Based on the kinematics, we cannot be certain that two merging clusters were actually involved in the formation of the OMP population but cannot discard this possibility either.…”

“…Where detailed measurements of kinematics are possible, rotation seems to be a common dynamical ingredient of NSCs (Feldmeier et al 2014;Nguyen et al 2018) and is also commonly found in a high fraction of GCs (van de Ven et al 2006;Bellazzini et al 2012;Bianchini et al 2013;Kacharov et al 2014;Fabricius et al 2014;Kimmig et al 2015;Bellini et al 2017;Kamann et al 2018;Bianchini et al 2018;Sollima et al 2019). The presence of internal rotation can be an indicator of their formation mechanism (Mastrobuono-Battisti & Perets 2013, 2016Hénault-Brunet et al 2015;Gavagnin et al 2016;Khoperskov et al 2018;Mastrobuono-Battisti et al 2019a) and can give important clues on their long-term dynamical evolution (e.g., Einsel & Spurzem 1999;Tiongco et al 2018). Fabricius et al (2014) and Kamann et al (2018) found that GCs with high internal central rotation are more flattened, showing that internal rotation plays an important role in forming the shape of GCs.…”

Mergers, tidal interactions, and mass exchange in a population of disc globular clusters

“…In this figure, GCs appear as density peaks of the stellar distribution, embedded in a low-density field of stars escaped from the clusters over time. This loss of stellar mass from the clusters is the consequence of the action of the galactic gravitational field and of the mutual gravitational interactions among GCs, as we will discuss in more detail in Mastrobuono-Battisti et al (2018). The effect of the galactic tidal field is also responsible for generating extended tidal tails around clusters, which are probes -on large scales -of the GC orbits (Montuori et al 2007).…”

“…Therefore, for the accreted populations, distance from the GC centre increases with their decreasing mass. For the longterm evolution of these accreted populations in the host GC we refer to Mastrobuono-Battisti et al (2018). In the following, we describe in more detail the mergers and mass exchanges taking place in our simulated GC system.…”