The outer halo globular cluster system of M31 – II. Kinematics

Abstract

We present a detailed kinematic analysis of the outer halo globular cluster\n(GC) system of M31. Our basis for this is a set of new spectroscopic\nobservations for 78 clusters lying at projected distances between Rproj ~20-140\nkpc from the M31 centre. These are largely drawn from the recent PAndAS\nglobular cluster catalogue; 63 of our targets have no previous velocity data.\nVia a Bayesian maximum likelihood analysis we find that GCs with Rproj > 30 kpc\nexhibit coherent rotation around the minor optical axis of M31, in the same\ndirection as more centrally- located GCs, but with a smaller amplitude of\n86+/-17 km s-1. There is also evidence that the velocity dispersion of the\nouter halo GC system decreases as a function of projected distance from the M31\ncentre, and that this relation can be well described by a power law of index ~\n-0.5. The velocity dispersion profile of the outer halo GCs is quite similar to\nthat of the halo stars, at least out to the radius up to which there is\navailable information on the stellar kinematics. We detect and discuss various\nvelocity correlations amongst subgroups of GCs that lie on stellar debris\nstreams in the M31 halo. Many of these subgroups are dynamically cold,\nexhibiting internal velocity dispersions consistent with zero. Simple Monte\nCarlo experiments imply that such configurations are unlikely to form by\nchance, adding weight to the notion that a significant fraction of the outer\nhalo GCs in M31 have been accreted alongside their parent dwarf galaxies. We\nalso estimate the M31 mass within 200 kpc via the Tracer Mass Estimator,\nfinding (1.2 - 1.6) +/- 0.2 10^{12}M_sun. This quantity is subject to\nadditional systematic effects due to various limitations of the data, and\nassumptions built in into the TME. Finally, we discuss our results in the\ncontext of formation scenarios for the M31 halo.\n