Globular clusters in the era of precision astrometry

TL;DR

This paper utilizes Gaia DR2 data to analyze globular clusters' kinematics, demonstrating low systematics, detecting internal rotation in about 50 clusters, and highlighting Gaia's potential for future GC studies.

Contribution

It introduces a method to assess Gaia data systematics using proper motion profiles and reports the first detection of internal rotation in a significant sample of GCs.

Findings

Systematics in Gaia DR2 are below measurement errors for the closest 14 GCs.

Internal rotation detected in approximately 50 GCs.

Gaia DR2 enables detailed kinematic studies of GCs, revealing their angular momentum.

Abstract

The study of the kinematics of globular clusters (GCs) offers the possibility of unveiling their long term evolution and uncovering their yet unknown formation mechanism. Gaia DR2 has strongly revitalized this field and enabled the exploration of the 6D phase-space properties of Milky Way GCs, thanks to precision astrometry. However, to fully leverage on the power of precision astrometry, a thorough investigations of the data is required. In this contribution, we show that the study of the mean radial proper motion profiles of GCs offers an ideal benchmark to assess the presence of systematics in crowded fields. Our work demonstrates that systematics in Gaia DR2 for the closest 14 GCs are below the random measurement errors, reaching a precision of ~0.015 mas/yr for mean proper motion measurements. Finally, through the analysis of the tangential component of proper motions, we report the detection of internal rotation in a sample of ~50 GCs, and outline the implications of the presence of angular momentum for the formation mechanism of proto-GC. This result gives the first taste of the unparalleled power of Gaia DR2 for GCs science, in preparation for the subsequent data releases.