Globular clusters of the Gaia Enceladus/Sausage -- I. Orbital and dynamical evolution on cosmological timescales

TL;DR

This work analyzes Gaia-Enceladus/Sausage (GE/S) globular clusters by reconstructing their orbits back 9 Gyr in a time-variable Milky Way potential and identifying a robust set of GE/S members. It uses φ-GPU for backward integration of 36 candidate GCs, classifying them into 'most probable' and 'tentative' GE/S associations, and then performs N-body simulations of 24 clusters to study the distribution of stars escaped from GCs over cosmological times. The resulting present-day phase-space distribution of escaped stars yields concrete energy and angular-momentum bounds that align with GE/S debris and overlap with very metal-poor GE/S stars from Ye2024, Ernandes2024, and Molaro2020, supporting a GE/S origin for many GC debris. The study demonstrates the value of time-dependent Galactic potentials in constraining merger-origin GC populations and outlines paths for more detailed, higher-resolution modeling of the GE/S–MW merger history.

Abstract

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. To reconstruct the orbital evolution of the known globular clusters of the dwarf galaxy Gaia-Enceladus/Sausage, we used the parallel $N$-body code $\varphi$-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. 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 $\times10^4$ km$^2$ s$^{-2}$, L$_{\rm z}$ from -0.98 to 0.72 $\times10^3$ kpc km s$^{-1}$, and L$_{\rm perp}$ from 0 to 1.8 $\times10^3$ 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.

Figures (13)

• Figure 1: Evolution of halo and disc masses, and their characteristic scales for 411321 external potential that are shown as thick black lines. Lines with dots represent the original TNG-100 data. The dashed line represents the 9 Gyr.
• Figure 2: Orbital evolution in the 411321 TVP external potential, presented in $X-Y$ an $R-Z$, where $R$ is the planar Galactocentric radius. The total time of integration is a 9 Gyr lookback time which is represented by the coloured bar. The filled black circle shows the current position, non-filled -- at -9 Gyr ago.
• Figure 3: Evolution of the GCs phase-space parameters during the 9 Gyr lookback time integration in 411321 TVP external potential for the four different pre-sets of GE/S GC's (see Table \ref{['tab:gc-pre-sets']}). The upper row shows the evolution of the L$_{z}$ and total energy; the bottom row of L$_{perp}$ and total energy. All values are in specific units. Transparent coloured areas represent the limits of the phase-space for GE/S GCs based on publications from Table \ref{['tab:ges-GC-today']}.
• Figure 4: Evolution of the GCs orbital parameters during the 9 Gyr lookback time integration in 411321 TVP external potential for four different pre-sets of GE/S GC's. The upper row shows the evolution of r$_{apo}$ and r$_{peri}$; the bottom row $a$ and ecc. Transparent coloured areas represent the limit of orbital parameters for GE/S GCs based on the publications from Table \ref{['tab:ges-GC-today']}. The GC colour coding corresponds to the legend from Fig. \ref{['fig:E-lz-sets']}. Black dots show the GC position today. GCs Palomar 2, Palomar 15, and NGC 6101 are not shown in the bottom row for pre-set 4 due to their outside position on a plot boundaries (they have very large r$_{apo}$ values).
• Figure 5: Evolution of the GC's energy angular momentum and orbital parameters, during the 9 Gyr lookback time integration in the 411321 TVP external potential. The green colour represents the most probable GCs of the GE/S category, the blue are tentative, and the grey excluded GCs.