Understand
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.