The global stability of M33 in MOND

TL;DR

This study models M33 in MOND, demonstrating that external gravitational fields influence its stability and rotation curve, aligning simulation results more closely with observed properties of the galaxy.

Contribution

It provides the first detailed MOND simulation of M33, showing how external fields affect its stability, rotation curve, and velocity dispersion, which was challenging in dark matter models.

Findings

Weak external field stabilizes the galaxy's bar.

External field reduces central velocity dispersion.

Simulation matches several observed properties of M33.

Abstract

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian Dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line of sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of $8.4 \times 10^{-12}$ m/s$^2$ at $30^\circ$ to the disk as a first order estimate for the gravity exerted by M31. This suppresses buildup of material at the centre, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.