The ultradiffuse galaxy AGC 114905 needs dark matter

Recent 21 cm line observations of the ultradiffuse 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 H i rotation curve are 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 dark matter (DM)-free galaxy for a number of reasons, one being that a bare Newtonian disc having low velocity dispersion would be expected to be unstable to both axisymmetric and non-axisymmetric perturbations that would change the structure of the disc on a dynamical time-scale, 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, DM. 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.