Dynamical evolution of Milky Way globular clusters on the cosmological timescale

II. Terzan 2, 4, and 5 mass loss and collision tracking

¹ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
² Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³ Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
⁴ Farabi University, al-Farabi ave. 71, 050040 Almaty, Kazakhstan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 January 2026
Accepted: 30 April 2026

Abstract

Context. The dynamical evolution of Galactic globular clusters is one of the key topics in modern astrophysics. It is also essential to understand their mutual gravitational interactions, especially in dense central regions, for reconstructing the assembly history of the Milky Way.

Aims. We investigate the long-term dynamical evolution of Ter2, Ter4, and Ter5, focusing on their mutual interactions, mass-loss behaviour, and survivability in the dense Galactic centre environment. We expect that low-mass clusters are particularly more sensitive to gravitational perturbations from massive neighbours, and our study seeks to quantify these effects under realistic orbital and dynamical conditions.

Methods. We performed a suite of high-resolution direct N-body simulations over 8 Gyr, modelling three individual clusters that we also modelled as combined systems. Orbital reconstructions and dynamical analyses were carried out in a realistic time-evolving Galactic potential. We compared reference runs of isolated clusters with simulations of the full three-cluster system to quantify possible differences in mass loss, potential energy, and orbital behaviour. Pairwise collision statistics were extracted to assess the frequency and impact of possible close encounters.

Results. Our simulations reveal multiple close encounters between the Terzan clusters. The most significant encounters occur between Ter2–Ter4 and Ter4–Ter5, with their tidal radii exceeding the minimum separation. A notable case is the pair Ter2–Ter4, which approaches within 10 pc at a relative velocity of ~320 km/s. Overall, Ter4 is the most dynamically active cluster in close interactions. We found that the mass-loss rate is higher for the low-mass Ter2 and Ter4 systems in the combined three-cluster simulations than in our similar isolated runs, highlighting the importance of mutual cluster interactions. Differences in potential energy evolution further confirm that collective modelling alters the dynamical pathways of individual clusters. The common run clearly demonstrates that mutual gravitational interactions between clusters drive significant triaxial deformations, especially for Ter2 and Ter5, which evolve from nearly spherical to distinctly prolate shapes. In contrast, the isolated runs show clusters that remained almost perfectly spherical, confirming that the observed shape changes are correlated with the mutual interactions.

Conclusions. The survivability and dynamical evolution of Galactic centre globular clusters cannot be fully understood without accounting for collective interactions among all systems within a few kiloparsecs. Low-mass clusters are particularly sensitive to gravitational perturbations from massive neighbours, which accelerate their mass loss and alter their orbital histories. Our results emphasise the necessity of complex multi-cluster modelling in realistic Galactic potentials to capture the long-term fate of surviving and dissolved clusters.