Barred spiral galaxies in modified gravity theories
  When bars form within galaxy formation simulations in the standard
cosmological context, dynamical friction with dark matter (DM) causes them to
rotate rather slowly. However, almost all observed galactic bars are fast in
terms of the ratio between corotation radius and bar length. Here, we
explicitly display an $8\sigma$ tension between the observed distribution of
this ratio and that in the EAGLE simulation at redshift 0. We also compare the
evolution of Newtonian galactic discs embedded in DM haloes to their evolution
in three extended gravity theories: Milgromian Dynamics (MOND), a model of
non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although
our models start with the same initial baryonic distribution and rotation
curve, the long-term evolution is different. The bar instability happens more
violently in MOND compared to the other models. There are some common features
between the extended gravity models, in particular the negligible role played
by dynamical friction $-$ which plays a key role in the DM model. Partly for
this reason, all extended gravity models predict weaker bars and faster bar
pattern speeds compared to the DM case. Although the absence of strong bars in
our idealized, isolated extended gravity simulations is in tension with
observations, they reproduce the strong observational preference for `fast' bar
pattern speeds, which we could not do with DM. We confirm previous findings
that apparently `ultrafast' bars can be due to bar-spiral arm alignment leading
to an overestimated bar length, especially in extended gravity scenarios where
the bar is already fast.
