Polar ring galaxies as tests of gravity

TL;DR

Polar ring galaxies serve as effective natural laboratories for testing gravity theories, with Milgromian dynamics successfully explaining observed rotation velocity patterns without dark matter.

Contribution

This study demonstrates that Milgromian dynamics can naturally account for the rotation curve properties of polar ring galaxies, providing an alternative to dark matter explanations.

Findings

Milgromian dynamics predicts higher rotation velocities in polar rings.

The model matches observed velocity shapes and amplitudes.

Dark matter halos are not necessary to explain these features.

Abstract

Polar ring galaxies are ideal objects with which to study the three-dimensional shapes of galactic gravitational potentials since two rotation curves can be measured in two perpendicular planes. Observational studies have uncovered systematically larger rotation velocities in the extended polar rings than in the associated host galaxies. In the dark matter context, this can only be explained through dark halos that are systematically flattened along the polar rings. Here, we point out that these objects can also be used as very effective tests of gravity theories, such as those based on Milgromian dynamics (MOND). We run a set of polar ring models using both Milgromian and Newtonian dynamics to predict the expected shapes of the rotation curves in both planes, varying the total mass of the system, the mass of the ring with respect to the host, as well as the size of the hole at the center of the ring. We find that Milgromian dynamics not only naturally leads to rotation velocities being typically higher in the extended polar rings than in the hosts, as would be the case in Newtonian dynamics without dark matter, but that it also gets the shape and amplitude of velocities correct. Milgromian dynamics thus adequately explains this particular property of polar ring galaxies.