A galaxy lacking dark matter

TL;DR

This paper presents evidence that the ultra-diffuse galaxy NGC1052-DF2 has an extremely low dark matter content, challenging the common understanding that dark matter is universally coupled with baryonic matter in galaxies.

Contribution

It provides the first dynamical measurement showing a galaxy with a ratio of dark matter to stellar mass close to zero, contradicting standard galaxy formation models.

Findings

NGC1052-DF2 has a velocity dispersion less than 10.5 km/s.

Total mass within 7.6 kpc is less than 3.4x10^8 solar masses.

The dark matter to stellar mass ratio is near zero, much lower than expected.

Abstract

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 M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. 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 galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers 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 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.