The ultra-diffuse galaxy AGC 114905 needs dark matter

TL;DR

Recent observations of the ultra-diffuse galaxy AGC 114905 suggest it appears to lack dark matter, but simulations and stability considerations raise questions about this interpretation, indicating possible measurement uncertainties.

Contribution

This study challenges the claim of a dark matter-free galaxy by combining observational data with N-body simulations and stability analysis.

Findings

The galaxy's rotation curve can be explained without dark matter based on observations.

Simulations show that a dark matter-free disk is unstable, contradicting the observed regular velocity pattern.

Overestimating the galaxy's inclination could imply the presence of a stabilizing dark matter halo.

Abstract

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.