Testing QUMOND theory with Galactic globular clusters in a weak external field

TL;DR

This study develops self-consistent dynamical models in QUMOND gravity to analyze globular clusters, comparing their mass-to-light ratios with stellar evolution estimates, and finds no significant incompatibility with observations.

Contribution

The paper introduces a new method to test QUMOND gravity using detailed dynamical models fitted to globular cluster data, incorporating external field effects.

Findings

QUMOND models fit observed cluster profiles well.

No significant discrepancy between dynamical and stellar evolution M/L ratios.

Potential to use globular clusters to test alternative gravity theories.

Abstract

We developed self-consistent dynamical models of stellar systems in the framework of quasi-linear modified Newtonian dynamics (QUMOND). The models are constructed from the anisotropic distribution function of Gunn & Griffin (1979), combined with the modified Poisson equation defining this gravitation theory and take into account the external field effect. We have used these models, and their Newtonian analogues, to fit the projected density and the velocity dispersion profiles of a sample of 18 Galactic globular clusters, using the most updated datasets of radial velocities and Gaia proper motions. We have thus obtained, for each cluster, estimates of the dynamical mass-to-light ratio ($M/L$) for each theory of gravity. The selected clusters have accurate proper motions and a well sampled mass function down to the very low mass regime. This allows us to constrain the degree of anisotropy and to provide, from comparison with stellar evolution isochrones, a dynamics-independent estimate of the minimum mass-to-light ratio $(M/L)_{min}$. Comparing the best-fitting dynamical $M/L$ with $(M/L)_{min}$, we find that for none of the analyzed clusters the two gravity theories are significantly incompatible with the observational data, although for one of them (NGC 5024) the dynamical $M/L$ predicted by QUMOND lies at $2.8\sigma$ below $(M/L)_{min}$. Though the proposed approach suffers from some limitations (in particular the lack of a treatment of mass segregation), the obtained results suggest that the kinematics of globular clusters in a relatively weak external field can be a powerful tool to prove alternative theories of gravitation.