A Statistical Framework to Identify Kinematically Outlying LMC Globular Clusters and Implications for the LMC's Dark Matter Profile

TL;DR

This paper introduces a new statistical method to identify kinematically outlying globular clusters in the LMC using Gaia-DR3 data, revealing their impact on dark matter estimates and suggesting possible accretion origins.

Contribution

A novel robust statistical framework combining proper motions and line-of-sight velocities to identify outlying GCs in the LMC, accounting for measurement uncertainties and intrinsic dispersion.

Findings

Identified 5 GCs with outlying proper motions and 6 with outlying line-of-sight velocities.

Outlying GCs are mostly located 3-4 kpc from the LMC center.

Including outliers biases the LMC's mass estimates by up to 30%.

Abstract

The LMC's Globular Clusters (GCs) bring a novel opportunity to understand the LMC's assembly history and dark matter (DM) properties, provided the kinematically outlying GCs can be reliably identified. However, traditional diagnostics like the Energy-Angular Momentum space fail because of large uncertainties on the GC velocities. In this work, we develop a new, robust statistical framework for identifying kinematically outlying LMC GCs, by using their Gaia-DR3 Proper Motions (PMs) combined with previous Line-of-Sight (LoS) velocity measurements. We use the difference between a GC's velocity vector and the average velocity vector of the surrounding red clump stars as a metric for quantifying a GC's kinematic peculiarity. We account for both the velocity measurement uncertainties and the LMC's intrinsic velocity dispersion. We find 5 LMC GCs to be kinematically outlying based on PM differences alone, and additional 6 GCs if LoS velocity information is also used. Majority of the GCs with outlying PMs are clustered at a distance of 3-4 kpc from the LMC center. The inclusion of outlying LMC GCs introduces a bias of upto 30% in the LMC's enclosed mass estimates derived using GCs as dynamical tracers; caution must be exercised in choosing the GC sample for precisely determining the LMC's DM content. We discuss the possibility that the kinematically outlying LMC GCs may have been accreted from external galaxies, and motivate future spectroscopic follow-up of the GC population to better understand the assembly history of massive satellite galaxies of Milky Way like hosts.