The ultra-diffuse galaxy AGC 114905 needs dark matter
  Recent 21 cm line observations of the ultra-diffuse galaxy AGC~114905
indicate a rotating disc largely supported against gravity by orbital motion,
as usual. Remarkably, this study has revealed that the form and amplitude of
the HI rotation curve is completely accounted for by the observed distribution
of baryonic matter, stars and neutral gas, implying that no dark halo is
required. It is surprising to find a DM-free galaxy for a number of reasons,
one being that a bare Newtonian disk having low velocity dispersion would be
expected to be unstable to both axi- and non-axisymmetric perturbations that
would change the structure of the disc on a dynamical timescale, as has been
known for decades. We present $N$-body simulations of the DM-free model, and
one having a low-density DM halo, that confirm this expectation: the disc is
chronically unstable to just such instabilities. Since it is unlikely that a
galaxy that is observed to have a near-regular velocity pattern would be
unstable, our finding calls into question the suggestion that the galaxy may
lack, or have little, dark matter. We also show that if the inclination of this
near face-on system has been substantially overestimated, the consequent
increased amplitude of the rotation curve would accommodate a halo massive
enough for the galaxy to be stable.
