No need for dark matter: resolved kinematics of the ultra-diffuse galaxy
  AGC 114905
  We present new HI interferometric observations of the gas-rich ultra-diffuse
galaxy AGC 114905, which previous work, based on low-resolution data,
identified as an outlier of the baryonic Tully-Fisher relation. The new
observations, at a spatial resolution $\sim 2.5$ times higher than before,
reveal a regular HI disc rotating at about 23 km/s. Our kinematic parameters,
recovered with a robust 3D kinematic modelling fitting technique, show that the
flat part of the rotation curve is reached. Intriguingly, the rotation curve
can be explained almost entirely by the baryonic mass distribution alone. We
show that a standard cold dark matter halo that follows the concentration-halo
mass relation fails to reproduce the amplitude of the rotation curve by a large
margin. Only a halo with an extremely (and arguably unfeasible) low
concentration reaches agreement with the data. We also find that the rotation
curve of AGC 114905 deviates strongly from the predictions of Modified
Newtonian dynamics. The inclination of the galaxy, which is measured
independently from our modelling, remains the largest uncertainty in our
analysis, but the associated errors are not large enough to reconcile the
galaxy with the expectations of cold dark matter or Modified Newtonian
dynamics.
