Classifying globular clusters and applying them to estimate the mass of the Milky Way

TL;DR

This study combines Gaia EDR3 data with other observations to classify globular clusters by origin and estimate the Milky Way's mass, revealing new sub-structures and refining mass estimates.

Contribution

It introduces a comprehensive classification of GCs based on multiple criteria and provides updated mass estimates of the Milky Way using a refined GC sample.

Findings

45.3% of GCs formed in situ

Mass inside 37.3 kpc is approximately 0.42 trillion solar masses

Identified three new sub-structures related to Gaia-Sausage-Enceladus

Abstract

We combine the kinematics of 159 globular clusters (GCs) provided by the Gaia Early Data Release 3 (EDR3) with other observational data to classify the GCs, and to estimate the mass of the Milky Way (MW). We use the age-metallicity relation, integrals of motion, action space and the GC orbits to identify the GCs as either formed in-situ (Bulge and Disk) or ex situ (via accretion). We find that $45.3\%$ have formed in situ, $38.4\%$ may be related to known merger events: Gaia-Sausage-Enceladus, the Sagittarius dwarf galaxy, the Helmi streams, the Sequoia galaxy, and the Kraken galaxy. We also further identify three new sub-structures associated with the Gaia-Sausage-Enceladus. The remaining $16.3\%$ of GCs are unrelated to the known mergers and thought to be from small accretion events. We select 46 GCs which have radii $8.0<r<37.3$ kpc and obtain the anisotropy parameter $\beta=0.315_{-0.049}^{+0.055}$, which is lower than the recent result using the sample of GCs in Gaia Data Release 2, but still in agreement with it by considering the error bar. By using the same sample, we obtain the MW mass inside the outermost GC as $M(<37.3 kpc)=0.423_{-0.02}^{+0.02}\times10^{12}M_{\odot}$, and the corresponding $M_{200}=1.11_{-0.18}^{+0.25}\times10^{12}M_{\odot}$. The estimated mass is consistent with the results in many recent studies. We also find that the estimated $\beta$ and mass depend on the selected sample of GCs. However, it is difficult to determine whether a GC fully traces the potential of the MW.