How Stellar Stream Torsion may reveal aspherical Dark Matter Haloes

Flat rotation curves follow from elongated Dark Matter distributions, as shown by our earlier competitive fits to the SPARC database. Intending to probe that distortion of the DM halo one needs observables not contained by the galactic plane. Stellar streams are caused by tidal stretching of massive substructures such as satellite dwarf galaxies, and would lie on a plane should the DM-halo gravitational field be spherically symmetric. But if the field does not display such spherical symmetry, stellar trajectories, as well as stellar streams, should torsion out of the plane. This is where the torsion of the stream can be of use: it is a local observable that measures the deviation from planarity of a curve; thus, it quantifies how noncentral the gravitational potential is. We have performed small simulations to confirm that indeed a galactic central force produces negligible torsion, and quantified the torsion for prolate haloes instead. Examining observational data, we select several streams at large distance from the galactic center, as most promising for the study, and by means of helicoidal fits extract their differential torsion. We see that their torsion is much larger than expected for a central spherical bulb alone, pointing to an elongated Milky Way halo.