Globular clusters of the Gaia Enceladus/Sausage

I. Orbital and dynamical evolution on cosmological timescales

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 Szechenyi Istvan University, Space Technology and Space Law Research Center, H-9026 Gyor, Egyetem ter 1, Hungary

^★ Corresponding author: bilodid_m@mao.kiev.ua

Received: 11 September 2025
Accepted: 21 October 2025

Abstract

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, L_z from −0.98 to 0.72 ×10³ kpc km s⁻¹, and L_perp 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.