How our proto-nuclear star cluster formed and grew due to early globular cluster disruption

I. Case of low masses

¹ Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
² Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
³ Faculty of Physics and Technology, Al-Farabi Kazakh National University, al-Farabi ave. 71, 050040 Almaty, Kazakhstan
⁴ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
⁵ Rudolf Peierls Centre for Theoretical Physics, Parks Road, Oxford OX1 3PU, UK
⁶ Szechenyi Istvan University, Space Technology and Space Law Research Center, 9026 Gyor, Egyetem ter 1, Hungary

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

Received: 30 June 2025
Accepted: 19 December 2025

Abstract

Context. To date, two main mechanisms have been proposed for the formation and growth of nuclear star clusters (NSCs) in galaxies. The first suggests in situ star formation from gas that has migrated to the central regions from the galaxy’s outskirts, while the second involves the accretion of stars from disrupted globular clusters (GCs) onto the galactic centre. However, the relative importance of these mechanisms in the evolution of NSCs across different galaxy morphologies remains an open question.

Aims. We investigate the accretion of GC stars on early cosmological timescales through detailed N-body simulations of theoretical GC models to assess the role of this mechanism in Milky Way-like (MW-like) galaxies.

Methods. For the dynamical modelling, we used the updated parallel N-body code ϕ-GPU, including stellar evolution. We prepared three sets of GC models with different half-mass radii (r_hm), each consisting of 50 full N-body GC models, and integrated these models in an external, time-variable MW-like potential taken from the cosmological database IllustrisTNG-100. The simulations cover the time interval from −10 Gyr to −5 Gyr, enabling us to assess the rate of early stellar accretion onto the proto-NSC.

Results. We find that GC models with average orbital eccentricities of 0.4−0.5 and orbits oriented perpendicular to the galactic disc contribute most significantly to the mass of the proto-NSC formation. Accretion is especially efficient in the first billion years (Gyr) and in compact GC models with r_hm=1 pc. In all sets, the dominant accreted stellar population consists of low-mass stars (≈ 0.33 M_⊙). However, the accreted mass alone is insufficient to fully account for the current NSC mass.

Conclusions. Based on our extended set of numerical simulations, we obtained an average lower limit of mass contribution (≈ 6%) to the NSC from investigated GCs. The fraction of mass contribution from individual disrupted GCs can significantly vary from 0.1% up to 90%. Generally, we conclude that the GC stellar accretion channel alone might not be sufficient to ensure the present-day MW galaxy’s NSC mass budget.