Globular clusters in \textsc{OrbIT}: complete dynamical characterisation of the globular cluster population of the Milky Way through updated orbital reconstruction

TL;DR

This study addresses the Milky Way's assembly history by dynamically classifying its globular cluster population into in situ and accreted components. It introduces OrbIT, a dedicated orbit integrator that evolves GC orbits in a time-varying Galactic potential including a rotating bar, yielding a full set of orbital parameters, integrals of motion, and adiabatic invariants across six dynamical spaces. The authors update GC–progenitor associations, including new links to Aleph, Cetus, Typhon, Elqui and Antaeus, and demonstrate that a purely dynamical analysis can reveal subtle connections previously missed. The work highlights the importance of a time-varying potential in accurately tracing the Galaxy’s accretion history and suggests combining dynamical results with chronochemical data to fully unravel the MW’s formation.

Abstract

In hierarchical structure formation, the content of a galaxy is determined both by its in-situ processes and by material added via accretions. Globular clusters in particular represent a window for the study of the different merger events that a galaxy underwent. Establishing the correct classification of in-situ and accreted tracers, and distinguishing the various different progenitors that contributed to the accreted population are important tools to deepen our understanding of galactic formation and evolution. Our aim is to refine our knowledge of the assembly history of the Milky Way by studying the dynamics of its globular cluster population and establishing an updated classification among in-situ objects and the different merger events identified. We used a custom built orbit integrator to derive precise orbital parameters, integrals of motions and adiabatic invariants for the globular cluster sample studied. By properly accounting for the rotating bar, which transforms the underlying model in a time-varying potential, we proceeded to a complete dynamical characterisation of the globular clusters. We present a new catalogue of clear associations between globular clusters and structures (both in-situ and accreted) in the Milky Way, and a full table of derived parameters. By using all dynamical information available, we were able to attribute previously unassociated or misclassified globular clusters to the different progenitors, including those responsible for the Aleph, Antaeus, Cetus, Elqui, and Typhon merger events. By using a custom built orbit integrator and properly accounting for the time-varying nature of the Milky Way potential, we have shown the depth of information that can be extracted from a purely dynamical analysis of the globular clusters of our Galaxy.

Figures (10)

• Figure 1: Rotational curve of the MW. In solid black the velocity of the full potential model, the yellow dashed line is the contribution from the halo component, the blue dotted line is the total contribution from the discs and the red dash-dotted is the contribution from the bulge (bar and spheroid), the red symbol with errorbar represents the Sun.
• Figure 2: The GC sample in the angular momentum versus total energy plane. As per the legend, the GCs are marked based on their affiliation to a progenitor: red triangles for Aleph, cyan circles for the bulge, purple triangles for Cetus, cyan triangles for the disc, yellow triangles for Elqui, red circles for Gaia-Enceladus-Sausage (GES), yellow circles for the Helmi Streams (HS), purple circles for the low energy group (low-E), green circles for the retrogrades (retro), blue circles for Sagittarius (Sag), green triangles for Typhon, black circles for the unassociated. The dashed line at $L_z = 0$ guides the eye in dividing the prograde and retrograde sides.
• Figure 3: The GC sample in the apocentre versus pericentre plane, in logarithmic scale. The uncertainties are computed as detailed in Sec. \ref{['out']}. The GCs are marked depending on their identified progenitor as in Fig. \ref{['fig:etotlz']}.
• Figure 4: The GC sample in the parallel versus perpendicular action space. GCs with $J_{\parallel} \approx -1$ are on nearly circular prograde orbits, conversely $J_{\parallel} \approx 1$ identifies nearly circular retrograde orbits. GCs on radial orbits have $J_{\perp} \approx -1$ while those on polar orbits have $J_{\perp} \approx 1$. The GCs are marked depending on their identified progenitor as in Fig. \ref{['fig:etotlz']}.
• Figure 5: The GC sample in the pericentre versus eccentricity plane, in semilogarithmic scale. The uncertainties are computed as detailed in Sec. \ref{['out']}. The GCs are marked depending on their identified progenitor as in Fig. \ref{['fig:etotlz']}.