Magnetic fields and the outer rotation curve of M31

TL;DR

This paper proposes that magnetic fields within M31's disc can explain the unexpected rise in its outer rotation curve, improving the fit beyond standard dark matter models.

Contribution

It introduces magnetic fields as a key factor in modeling galaxy rotation curves, specifically addressing M31's outer rotation behavior.

Findings

Magnetic fields significantly improve the fit of the rotation curve.

A magnetic field amplitude of ~4 microG explains the outer rotation rise.

Magnetic effects are crucial in understanding galactic dynamics beyond dark matter models.

Abstract

Recent observations of the rotation curve of M31 show a rise of the outer part that can not be understood in terms of standard dark matter models or perturbations of the galactic disc by M31's satellites. Here, we propose an explanation of this dynamical feature based on the influence of the magnetic field within the thin disc. We have considered standard mass models for the luminous mass distribution, a NFW model to describe the dark halo, and we have added up the contribution to the rotation curve of a magnetic field in the disc, which is described by an axisymmetric pattern. Our conclusion is that a significant improvement of the fit in the outer part is obtained when magnetic effects are considered. The best-fit solution requires an amplitude of ~4 microG with a weak radial dependence between 10 and 38 kpc.