Declining rotation curves of galaxies as a test of gravitational theory

TL;DR

This study tests Milgromian dynamics by analyzing galaxy rotation curves, demonstrating that including the External Field Effect improves fit accuracy and aligns inferred gravitational fields with environmental sources.

Contribution

It provides empirical evidence that the External Field Effect is essential in Milgromian dynamics, improving rotation curve fits and environmental gravitational field estimates.

Findings

EFE improves rotation curve fits in 80% of galaxies

External gravitational fields are consistent with nearby sources

Inclusion of EFE yields realistic stellar mass-to-light ratios

Abstract

Unlike Newtonian dynamics which is linear and obeys the strong equivalence principle, in any nonlinear gravitation such as Milgromian dynamics (MOND), the strong version of the equivalence principle is violated and the gravitational dynamics of a system is influenced by the external gravitational field in which it is embedded. This so called External Field Effect (EFE) is one of the important implications of MOND and provides a special context to test Milgromian dynamics. Here, we study the rotation curves (RCs) of 18 spiral galaxies and find that their shapes constrain the EFE. We show that the EFE can successfully remedy the overestimation of rotation velocities in 80\% of the sample galaxies in Milgromian dynamics fits by decreasing the velocity in the outer part of the RCs. We compare the implied external field with the gravitational field for non-negligible nearby sources of each individual galaxy and find that in many cases it is compatible with the EFE within the uncertainties. We therefore argue that in the framework of Milgromian dynamics, one can constrain the gravitational field induced from the environment of galaxies using their RCs. We finally show that taking into account the EFE yields more realistic values for the stellar mass-to-light ratio in terms of stellar population synthesis than the ones implied without the EFE.