MOND vs. Newtonian dynamics in early-type galaxies. The case of NGC 4649 (M60)

TL;DR

This study compares MOND and Newtonian models using globular cluster data for NGC 4649, finding that both fit the observed velocity dispersion well and suggesting little need for dark matter in the galaxy's outer regions.

Contribution

It provides a detailed comparison of MOND and Newtonian dynamics in an early-type galaxy using globular cluster kinematics, challenging the necessity of dark matter in this context.

Findings

Both models fit the velocity dispersion data well.

Mass-to-light ratio in B-band is about 7.

No significant dark matter needed beyond 3 effective radii.

Abstract

Context: Regarding the significant interest in both dark matter and the application of MOND to early-type galaxies, we investigate the MOND theory by comparing its predictions, for models of constant mass-to-light ratio, with observational data of the early-type galaxy NGC 4649. Aims: We study whether measurements for NGC 3379 and NGC 1399 are typical of early-type systems and we test the assumption of a Newtonian constant M/L ratio underlying most of the published models. Methods: We employ the globular clusters of NGC 4649 as a mass tracer. The Jeans equation is calculated for both MOND and constant mass-to-light ratio assumptions. Spherical symmetry is assumed and the calculations are performed for both isotropic and anisotropic cases. Results: We found that both Jeans models with the assumption of a constant mass-to-light ratio and different MOND models provide good agreement with the observed values of the velocity dispersion. The most accurate fits of the velocity dispersion were obtained for the mass-to-light ratio in the B-band, which was equal to 7, implying that there is no need for significant amounts of dark matter in the outer parts (beyond 3 effective radii) of this galaxy. We also found that tangential anisotropies are most likely present in NGC 4649.