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We present new HI interferometric observations of the gas-rich ultra-diffusegalaxy AGC 114905, which previous work, based on low-resolution data,identified as an outlier of the baryonic Tully-Fisher relation. The newobservations, 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 theflat part of the rotation curve is reached. Intriguingly, the rotation curvecan be explained almost entirely by the baryonic mass distribution alone. Weshow that a standard cold dark matter halo that follows the concentration-halomass relation fails to reproduce the amplitude of the rotation curve by a largemargin. Only a halo with an extremely (and arguably unfeasible) lowconcentration reaches agreement with the data. We also find that the rotationcurve of AGC 114905 deviates strongly from the predictions of ModifiedNewtonian dynamics. The inclination of the galaxy, which is measuredindependently from our modelling, remains the largest uncertainty in ouranalysis, but the associated errors are not large enough to reconcile thegalaxy with the expectations of cold dark matter or Modified Newtoniandynamics.

No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905