The massive elliptical galaxy NGC 4649 from the perspective of extended gravity

TL;DR

This study models the elliptical galaxy NGC 4649 using a MONDian gravity law, demonstrating that observed dynamics can be explained without dark matter by applying modified gravity principles.

Contribution

The paper provides a self-consistent MOND-based model of NGC 4649, aligning observed velocity dispersion and brightness profiles without requiring dark matter.

Findings

MONDian gravity reproduces observed velocity dispersion.

Galaxy dynamics are consistent with baryonic matter only.

No dark matter needed to explain NGC 4649's structure.

Abstract

Elliptical galaxies are systems where dark matter is usually less necessary to explain observed dynamics than in the case of spiral galaxies, however there are some instances where Newtonian gravity and the observable mass are insufficient to explain their observed structure and kinematics. Such is the case of NGC 4649, a massive elliptical galaxy in the Virgo cluster for which recent studies report a high fraction of dark matter, 0.78 at $4R_e$. However this galaxy has been studied within the MOND hypothesis, where a good agreement with the observed values of velocity dispersion is found. Using a MONDian gravity force law, here we model this galaxy as a self-consistent gravitational equilibrium dynamical system. This force law reproduces the MOND phenomenology in the $a<a_{0}$ regime, and reduces to the Newtonian case when $a>a_{0}$. Within the MONDian $a<a_{0}$ scales, centrifugal equilibrium or dispersion velocities become independent of radius, and show a direct proportionality to the fourth root of the total baryonic mass, $V^{4}\propto(M G a_{0})$. We find that the recent detailed observations of the surface brightness profile and the velocity dispersion profile for this galaxy are consistent with the phenomenology expected in MONDian theories of modified gravity, without the need of invoking the presence of any hypothetical dark matter.