Testing for Dark Matter in the Outskirts of Globular Clusters

TL;DR

This study analyzes star motions in globular cluster outskirts to test for dark matter presence, finding most clusters show no evidence of dark matter, while a few suggest possible local dark matter halos.

Contribution

First detailed velocity dispersion profiles of globular clusters using proper motions, providing new constraints on dark matter in these systems.

Findings

Most clusters show decreasing velocity dispersion with radius.

Some clusters have flat profiles, ambiguous for dark matter presence.

A few clusters show rising profiles, hinting at possible dark matter halos.

Abstract

The proper motions of stars in the outskirts of globular clusters are used to estimate cluster velocity dispersion profiles as far as possible within their tidal radii. We use individual color-magnitude diagrams to select high probability cluster stars for 25 metal-poor globular clusters within 20 kpc of the sun, 19 of which have substantial numbers of stars at large radii. Of the 19, 11 clusters have a falling velocity dispersion in the 3-6 half mass radii range, 6 are flat, and 2 plausibly have a rising velocity dispersion. The profiles are all in the range expected from simulated clusters started at high redshift in a zoom-in cosmological simulation. The 11 clusters with falling velocity dispersion profiles are consistent with no dark matter above the Galactic background. The 6 clusters with approximately flat velocity dispersion profiles could have local dark matter, but are ambiguous. The 2 clusters with rising velocity dispersion profiles are consistent with a remnant local dark matter halo, but need membership confirmation and detailed orbital modeling to further test these preliminary results.