The global stability of M33 in MOND
  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.
