The Milky Way’s tilted dark matter halo

The gravity of the Milky Way Galaxy is tearing the Sagittarius Dwarf Galaxy apart. Stars ripped out of the tiny galaxy have ended up in a stream, which wraps around our own much heavier galaxy. For over a decade astronomers have been trying to use the distances and speeds of stars in this stream to measure and model the shape of the Milky Way’s dark matter distribution — its dark matter halo.

Early models assumed that the disc of the Milky Way must be perpendicular to one of the three axes of the halo. In 2010 astronomers showed that the best model for matching the data was triaxial. This model favoured the disc being perpendicular to the intermediate axis of the dark matter, rather than the long or short axes.

New work, however, shows that galaxies starting out in this position within a halo quickly tip over. As a result the Milky Way would have long ago tilted away from this orientation, making this model extremely unlikely.

Additionally, our simulations show that the disc can remain inclined relative to all three axes for a long time provided gas is present. From the point of view of the dark matter halo, the Milky Way disc is tilted at a random angle. This makes it much harder to model the precise shape of the halo. Nevertheless, our models where only the stars and gas exist cannot produce the sideways forces which a tilted halo produces. Proof that the motions of stars in the Sagittarius stream require such a tilt would also prove that dark matter exists.