The torsion of stellar streams due to a nonspherical dark matter halo*

We have recently pointed out that flattening rotation curves v(r) are naturally explained by elongated (prolate) Dark Matter (DM) distributions, and provided competitive fits to the SPARC database. To further probe the geometry of the halo one needs out-of-plane observables.

Stellar streams in the Milky Way, poetically analogous to airplane contrails, but caused by tidal dispersion of massive substructures such as satellite dwarf galaxies, would lie on a plane (consistently with angular momentum conservation) should the gravitational field of the DM halo be spherically symmetric. 

Entire orbits are seldom available because their periods are commensurable with Hubble time, with streams often presenting themselves as short segments.

Therefore, the systematic study of the stellar stream torsion, a local observable that measures the deviation from planarity in differential curve geometry, provides sensitivity to aspherical DM distributions and ensures the use of even short streams.