# Formation imprints in the kinematics of the Milky Way globular cluster system

## TL;DR

This study analyzes the kinematics of Milky Way globular clusters using updated velocities, revealing distinct formation histories and orbital characteristics for in-situ versus accreted clusters, enhancing understanding of galaxy formation.

## Contribution

It introduces a new 3D kinematic framework to distinguish in-situ and accreted globular clusters based on their orbital properties and velocity anisotropy.

## Key findings

- In-situ GCs show a correlation between eccentricity and inclination.
- Accreted GCs display a wide range of inclinations at high eccentricities.
- Velocity anisotropy varies with radius and formation origin.

## Abstract

We report results on the kinematics of Milky Way (MW) globular clusters (GCs) based on updated space velocities for nearly the entire GC population. We found that a 3D space with the semi-major axis, the eccentricity and the inclination of the orbit with respect to the MW plane as its axes is helpful in order to dig into the formation of the GC system. We find that GCs formed in-situ show a clear correlation between their eccentricities and their orbital inclination in the sense that clusters with large eccentricities also have large inclinations. These GCs also show a correlation between their distance to the MW center and their eccentricity. Accreted GCs do not exhibit a relationship between eccentricity and inclination, but span a wide variety of inclinations at eccentricities larger than ~ 0.5. Finally, we computed the velocity anisotropy "beta" of the GC system and found for GCs formed in-situ that "beta" decreases from ~ 0.8 down to 0.3 from the outermost regions towards the MW center, but remains fairly constant (0.7-0.9) for accreted ones. These findings can be explained if GCs formed from gas that collapsed radially in the outskirts, with preference for relative high infall angles. As the material reached the rotating forming disk, it became more circular and moved with lower inclination relative to the disk. A half of the GC population was accreted and deposited in orbits covering the entire range of energies from the outer halo to the bulge.