The orbital anisotropy profiles of nearby globular clusters from Gaia Data Release 2

Abstract

Gaia Data Release 2 provides a wealth of data to study the internal structure of nearby globular clusters. We use this data to investigate the internal kinematics of 11 nearby globular clusters, with a particular focus on their poorly-studied outer regions. We apply a strict set of selection criteria to remove contaminating sources and create pure cluster-member samples over a significant fraction of the radial range of each cluster. We confirm previous measurements of rotation (or a lack thereof) in the inner regions of several clusters, while extending the detection of rotation well beyond where it was previously measured and finding a steady decrease in rotation with radius. We also determine the orbital anisotropy profile and determine that clusters have isotropic cores, are radially anisotropic out to ≈ 4 half-light radii or 35% of their limiting radii, and are then isotropic out to the limits of our datasets. We detect for the first time the presence of radial anisotropy in M 22, while confirming previous detections of radial anisotropy in 47 Tuc, M 3, M 13, M 15, and ω Cen's innermost regions. The implications of these measurements are that clusters can be separated into two categories: 1) clusters with observed radial anisotropy that likely formed tidally underfilling or are dynamically young, and 2) clusters that are primarily isotropic that likely formed tidally filling or are dynamically old.

Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?

“…This confirms the projected isotropy previously determined by Heyl et al (2012) from 3. 5 to 7 using HST (with 10% individual errors), as well as Watkins et al (2015a): σ POSt = 0.98 σ POSr , up to R e /10 using HST, and by Jindal et al (2019) at larger radii up to 12 3 R e using Gaia, with signs of radial motions at R > 8 .…”

“…NGC 6397 has been broadly investigated in previous studies. The PMs lead to plane-of-sky (POS) velocities with equal radial and tangential dispersions, leading to the appearance of isotropic orbits (Heyl et al 2012;Watkins et al 2015a with HST for the inner orbits, and Jindal et al 2019 with Gaia for the outer orbits). But this projected velocity isotropy can hide three-dimensional (3D) variations of the anisotropy of the 3D velocity ellipsoid.…”

Structure and Rotation of Young Massive Star Clusters in a Simulated Dwarf Starburst

“…The errorbars have been obtained by bootstrapping the particles 1000 times. We compare the resulting anisotropy profiles to six observed globular clusters with clear rotational signal (NGC 104, NGC 5139, NGC 5904, NGC 6656 and NGC 6752) identified in Bianchini et al (2018) and obtained from Watkins et al (2015) (data at inner radii) and Jindal et al (2019) (data at outer radii).…”

Early dynamics and violent relaxation of multimass rotating star clusters

“…Fabricius et al 2014, Ferraro et al 2018, Baumgardt & Hilker 2018, Kamann et al 2018) and/or high-precision proper motion studies based on Gaia and HST data (see e.g. Watkins et al 2015, Bellini et al 2017, Libralato et al 2018a, Bianchini et al 2018b, Jindal et al 2019, Vasiliev 2019, Cordoni et al 2020, Cohen et al 2021) have allowed to investigate the velocity dispersion's variation with the cluster-centric distance, have shown that clusters are often characterized by anisotropy in the velocity distribution and/or internal rotation, and provided the first quantitative measures of the degree of evolution towards energy equipartition.…”