Kinematics of Outer Halo Globular Clusters in M31

TL;DR

This study analyzes the kinematics of outer halo globular clusters in M31, revealing rotation, velocity dispersion trends, and infall signatures, and estimates M31's mass within 200 kpc based on these tracers.

Contribution

First kinematic analysis of outer halo GCs in M31, showing rotation, substructure association, and providing mass estimates within 200 kpc.

Findings

Outer halo GCs exhibit net rotation around M31's minor axis.

Velocity dispersion decreases with increasing radius.

Evidence of kinematic coherence and infall along halo substructures.

Abstract

We present the first kinematic analysis of the far outer halo globular cluster (GC) population in the Local Group galaxy M31. Our sample contains 53 objects with projected radii of ~20-130 kpc, of which 44 have no previous spectroscopic information. GCs with projected radii >30 kpc are found to exhibit net rotation around the minor axis of M31, in the same sense as the inner GCs, albeit with a smaller amplitude of 79 +/-19 km/s. The rotation-corrected velocity dispersion of the full halo GC sample is 106 +/-12 km/s, which we observe to decrease with increasing projected radius. We find compelling evidence for kinematic-coherence amongst GCs which project on top of halo substructure, including a clear signature of infall for GCs lying along the North-West stream. Using the tracer mass estimator, we estimate the dynamical mass of M31 within 200 kpc to be M_M31 = (1.2-1.5) +/- 0.2 x 10^12 M_sun. This value is highly dependent on the chosen model and assumptions within.