Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters

TL;DR

This study tests Newtonian gravity in globular clusters using AAOmega data, finding no evidence of modified gravity or dark matter effects, and providing detailed mass-to-light profiles and rotation measurements.

Contribution

It provides the first comprehensive analysis of velocity dispersion profiles in four globular clusters, challenging claims of modified gravity effects at large radii.

Findings

No significant flattening of velocity dispersion profiles.

Mass-to-light ratios derived from dynamical models.

Detection of rotation in M22 and possible rotation in M68.

Abstract

Testing Newtonian gravity in the weak-acceleration regime is vital to our understanding of the nature of the gravitational interaction. It has recently been claimed that the velocity dispersion profiles of several globular clusters flatten out at large radii, reminiscent of galaxy rotation curves, even though globular clusters are thought to contain little or no dark matter. We investigate this claim, using AAOmega observations of four globular clusters, namely M22, M30, M53 and M68. M30, one such cluster that has had this claim made for its velocity dispersion, was included for comparison with previous studies. We find no statistically significant flattening of the velocity dispersion at large radii for any of our target clusters and therefore we infer the observed dynamics do not require that globular clusters are dark matter dominated, or a modification of gravity. Furthermore, by applying a simple dynamical model we determine the radial mass-to-light profiles for each cluster. The isothermal rotations of each cluster are also measured, with M22 exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any rotation, within the uncertainties.