Globular clusters in the inner Galaxy classified from dynamical orbital criteria

TL;DR

This study uses Gaia DR2 data and orbital analysis to classify globular clusters in the inner Galaxy, revealing many are misclassified by location and are not associated with the bar structure, providing insights into their origins.

Contribution

The paper introduces a dynamical orbital classification method for globular clusters in the inner Galaxy using Gaia data, improving understanding of their true Galactic component memberships.

Findings

Approximately 30% of clusters near the Galactic center are actually halo or thick disk objects.

Most confirmed bulge GCs do not follow the bar structure and are older than the bar formation epoch.

Orbital analysis reveals misclassification of some clusters based solely on location.

Abstract

Globular clusters (GCs) are the most ancient stellar systems in the Milky Way. Therefore, they play a key role in the understanding of the early chemical and dynamical evolution of our Galaxy. Around $40\%$ of them are placed within $\sim4$ kpc from the Galactic center. In that region, all Galactic components overlap, making their disentanglement a challenging task. With Gaia DR2, we have accurate absolute proper motions for the entire sample of known GCs that have been associated with the bulge/bar region. Combining them with distances, from RR Lyrae when available, as well as radial velocities from spectroscopy, we can perform an orbital analysis of the sample, employing a steady Galactic potential with a bar. We applied a clustering algorithm to the orbital parameters apogalactic distance and the maximum vertical excursion from the plane, in order to identify the clusters that have high probability to belong to the bulge/bar, thick disk, inner halo, or outer halo component. We found that $\sim 30\%$ of the clusters classified as bulge GCs based on their location are just passing by the inner Galaxy, they appear to belong to the inner halo or thick disk component, instead. Most of GCs that are confirmed to be bulge GCs are not following the bar structure and are older than the epoch of the bar formation.