Context. The history of our Galaxy is shaped by significant merger events, which contribute to its mass and to the distribution of stars, but which also bring globular clusters that act as the main tracers of the accretion history of the Milky Way. Aims. We investigated Gaia-Enceladus/Sausage globular cluster samples and studied their orbital and dynamical evolution over cosmological timescales in external time-variable potential. We estimated the limits of distribution of the escaped stars from the globular clusters’ orbital evolution in energy angular momentum space. Methods. To reconstruct the orbital evolution of the known globular clusters of the dwarf galaxy Gaia-Enceladus/Sausage, we used the parallel N-body code φ-GPU. We investigated the relationship between globular clusters and their progenitor by analysing their orbital parameters and phase-space distribution during 9 Gyr of evolution in the past. We created a N-body model of Gaia-Enceladus/Sausage globular clusters and analysed their dynamical evolution and distribution of the escaped stars today. Results. We summarised the samples of the Gaia-Enceladus/Sausage globular clusters and created two main categories: ‘most probable’ and ‘tentative’, with 15 and 9 clusters, respectively. We analysed the evolution of their kinematic, orbital, and phase-space parameters in the external time-variable potential. We defined phase-space distribution limits of stars that escape from globular clusters during 9 Gyr of evolution: a specific energy from −18 to −12.2 ×10⁴ km² s⁻², Lz from −0.98 to 0.72 ×10³ kpc km s⁻¹, and Lperp from 0 to 1.8 ×10³ kpc km s⁻¹. The limits of the GE/S debris in Galactic area based on orbital parameters of the GC’s escaped stars are: for apocentre and pericetre distances of 10–28 and 1–4 kpc, <18 kpc in Galactocentric radius and <|15| kpc in the Z direction. Generally we compared the phase-space distribution of escaped stars from the GCs GE/S debris energy-angular momentum limits with the observed very metal-poor stars, which belong to the GE/S itself and produce consistent results.

Context. Comets, asteroids, planetesimals, free-floating planets, and brown dwarfs are continuously injected into the intracluster environment after expulsion from their host-planetary systems or binary system. The dynamics of large populations of such free-floating comets (ffcs) in a star cluster environment is not yet fully understood. Aims. We investigated the dynamical evolution of comet populations in star clusters and characterized the kinematics and ejection rates of ffc in a star cluster. Moreover, we determined whether a different initial energy distribution affects the mass segregation of the less massive population. Methods. We carried out simulations using the N-body code NBODY6++GPU-MASSLESS, which allows the fast integration of star clusters that contain large numbers of massless particles, to characterize the dynamics of populations of low-mass particles with sub-virial and super-virial distributions. Results. Comets do not participate in the mass-segregation process, similarly to planet-sized objects, regardless of their initial energy distribution. The latter slightly changed the whole dynamical evolution at the start of the simulation. We only observe an initial relaxation or collapse of the objects for super-virial and sub-virial ratios, respectively. The external regions of the ffcs population tend to be pulled back in the cluster core at the end of the simulation, suggesting the gravitational pull of the stars is pulling them back in the core. This phenomenon occurs at later times if the system is in virial equilibrium. Compared to less massive bodies, brown dwarfs experience more mass segregation. The inner regions tend to be more mixed with the stellar population.

Aims. In this study, we investigate the dynamics of intermediate-mass black hole (IMBH) binaries within nuclear star clusters (NSCs) that contain a population of stellar-mass black holes (BHs). We examine how these stellar and BH populations influence the dynamics of the IMBH binary and, in turn, how the evolving IMBH binary affects the surrounding stellar and BH populations. Methods. We conducted high-resolution N-body simulations of NSCs constructed based on observational parameters from two local dwarf galaxies: NGC205 and NGC404. For the first time, we achieved a star particle mass resolution of 1 M⊙ and a BH mass resolution of 10 M⊙. This level of resolution is crucial for accurately modeling the collisional dynamics of these dense systems. Results. Including stellar-mass BHs within the stellar population significantly influences the IMBH binary dynamics, nearly doubling the sinking rate and halving the merger time. During the initial phase of the inspiral, the IMBH binary disrupts both the stellar and BH cusps. However, the BH cusp quickly regains its steep slope due to its shorter relaxation time and continues to dominate the evolution of the IMBH binary, despite being much less massive than the stellar component. We uncover an interesting mechanism in which BHs first efficiently extract energy from the IMBH binary and then transfer this energy to the surrounding stars, allowing the BHs to spiral back toward the center of the system and restart the process. Our results imply that although stellar-mass BHs are a minor component of a stellar population they can significantly facilitate IMBH growth within NSCs via mergers. We also notice that these dense systems can potentially boost intermediate-mass ratio inspirals (IMRIs) predominantly on radial orbits.

Nuclear star clusters (NSCs) are dense stellar environments located in the center of most galaxies. NSCs are thought to form through two primary methods: through the inspiral of globular clusters (GCs) to the galactic center due to dynamical friction, and through in situ star formation. Recent observations of dwarf elliptical galaxy UGC 7346 suggest that it might be undergoing NSC formation due to the presence of multiple GCs near its photometric center. We perform direct N-body simulations of nine GCs belonging to UGC 7346’s GC system to investigate whether their eventual infall to the galactic center would result in the formation of an NSC. Our simulations indicate that GCs' inspiral leads to the formation of a central stellar overdensity relative to the background profile of the host galaxy within ∼1.5 Gyr, corresponding to an NSC with a typical mass of (4.1–4.5) × 10⁵ M⊙. Several key structural parameters of the newly formed NSC, including the Sérsic index, effective radius, and central stellar density, lie well within the range observed for NSCs. We also test a hypothetical scenario in which some of the infalling GCs have larger masses (M ∼ 10⁶M⊙), resulting in the formation of a more massive NSC whose mass and size are more consistent with observations. Our results suggest that the inspiral of GCs is a viable channel for assembling a significant mass in the shape of NSCs in the centers of dwarf galaxies and that UGC 7346 will host an NSC at its center in about 2–3 Gyr.

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 (rhm), 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 rhm=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.