Does the galaxy NGC1052-DF2 falsify Milgromian dynamics?

Abstract

Galaxies normally have far more dark matter than normal matter, but the dynamics of objects within the ultra-diffuse galaxy NGC1052–DF2 suggest that it has a very little dark matter component. Most galaxies studied need a component of dark matter associated with them in order to explain their observed properties. Normally there is far more dark matter than normal matter, with an average of 30 times more for galaxies about the mass of the Milky Way. Dwarf galaxies are thought to have even higher ratios of dark to normal matter (more than 400 times as much). Pieter van Dokkum and colleagues report that the ultra-diffuse galaxy NGC1052–DF2 has a dynamical mass, determined by the motions of globular-cluster-like objects, that is essentially the same as the mass in stars, meaning that it does not have a dark matter component. Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses1,2. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies3,4. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy5 NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected2. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.