The Kinematics of the Sagittarius Stream.

Abstract

Tidal streams can provide us with a great deal of information about their dwarf progenitors, and by implication, the progenitors of the building blocks of stellar halos, such as: their stellar populations, their mass and mass loss rate, and their orbital history and future. But, perhaps more importantly, streams can help us probe properties of their host galaxy. Since the locations, and motions of the stars that comprise a stream reflect the underlying gravitational potential in which they orbit, their positions and kinematics can be used to determine, or at least place constraints upon, the distribution of matter in the halo. In particular, the size and shape of the dark matter halo can be constrained.

There have been numerous efforts to use observations of the Sgr streams to constrain the shape of the Milky Way's dark matter halo. Sgr is the only known Milky Way satellite with substantial streams encircling more than 360 degrees around its host, making it the perfect candidate for such a study. However, various models, relying upon the contemporaneously available observations of Sgr, have produced strikingly contradictory results, initiating a seemingly intractable debate over the shape of the Milky Way's dark matter halo. The data has been interpreted in favor of halos of all possible shapes: spherical, oblate, prolate, triaxial, and transitional.

We present the results of an extensive observational undertaking to acquire the single largest spectroscopic data set of Sgr stream stars. Using a combination of telescopes and instruments in both hemispheres we have completed a kinematic survey including velocity measurements for 2368 unique main-sequence stars in 39 fields spanning the full 360 degrees along the Sgr streams.

The results of this kinematic survey are compared to the predictions of $N$-body models of the destruction of Sgr in Galactic halos of various shapes. We find that the observed radial velocity trends along the streams are best reproduced by the triaxial halo model. Amongst the axisymmetric models, the prolate halo provides the best match to the observations, while the spherical and oblate models can be ruled out.