The torsion of stellar streams and the overall shape of galactic gravity's source

Abstract

Flat rotation curves v(r) are naturally explained by elongated (prolate) Dark Matter (DM) distributions, and we have provided competitive fits to the SPARC database. To further probe the geometry of the halo one needs out-of-plane observables. Stellar streams, poetically analogous to airplane contrails, but caused by tidal dispersion of massive substructures such as satellite dwarf galaxies, would lie on a plane should the DM-halo gravitational field be spherically symmetric. We aim at establishing stellar stream torsion, a local observable that measures the deviation from planarity in differential curve geometry. We perform small-scale simulations of tidally distorted star clusters to check that indeed a central force center produces negligible torsion. Turning to observational data, we identify among the known streams those that are at largest distance from the galactic center and likely not affected by the Magellanic clouds, as most promising for the study, and by means of polynomial fits we extract their differential torsion. We find that the torsion of the few known streams that should be sensitive to most of the Milky Way's DM Halo is much larger than expected for a central spherical bulb alone. This is consistent with non-sphericity of the halo. Future studies of stellar stream torsion with larger samples and further out of the galactic plane should be able to extract the ellipticity of the halo to see whether it is just a slight distortion of a spherical shape or rather ressembles a more elongated cigar.