Does the Sagittarius Stream constrain the Milky Way halo to be triaxial?

TL;DR

This study reevaluates the shape of the Milky Way's dark matter halo using the Sagittarius stream, finding that a spherical halo can fit the data if a rising rotation curve is assumed, challenging previous claims of a triaxial halo.

Contribution

It introduces a new stream-fitting algorithm that allows for more flexible halo density profiles, showing that triaxiality is not necessary to explain the Sagittarius stream.

Findings

Spherical halos can fit the Sagittarius stream data with a rising rotation curve.

Triaxiality is not required to match the stream, contrary to previous claims.

Degeneracy exists between halo shape and density profile in modeling the stream.

Abstract

Recent analyses of the stellar stream of the Sagittarius dwarf galaxy have claimed that the kinematics and three-dimensional location of the M-giant stars in this structure constrain the dark matter halo of our Galaxy to possess a triaxial shape that is extremely flattened, being essentially an oblate ellipsoid oriented perpendicular to the Galactic disk. Using a new stream-fitting algorithm, based on a Markov Chain Monte Carlo procedure, we investigate whether this claim remains valid if we allow the density profile of the Milky Way halo greater freedom. We find stream solutions that fit the leading and trailing arms of this structure even in a spherical halo, although this would need a rising Galactic rotation curve at large Galactocentric radius. However, the required rotation curve is not ruled out by current constraints. It appears therefore that for the Milky Way, halo triaxiality, despite its strong theoretical motivation, is not required to explain the Sagittarius stream. This degeneracy between triaxiality and the halo density profile suggests that in future endeavors to model this structure, it will be advantageous to relax the strict analytic density profiles that have been used to date.