Taking measure of the Andromeda halo: A Kinematic analysis of the giant stream surrounding M31

We present a spectroscopic survey of the giant stellar stream found in the halo of the Andromeda galaxy. Taken with the DEIMOS multi-object spectrograph on the Keck2 telescope, these data display a narrow velocity dispersion of 11 ± 3k m s −1 , with a steady radial velocity gradient of 245 km s −1 over the 125-kpc radial extent of the stream studied so far. This implies that the Andromeda galaxy possesses a substantial dark matter halo. We fit the orbit of the stream in different galaxy potential models. In a simple model with a composite bulge, disc and halo, where the halo follows a universal profile that is compressed by the formation of the baryonic components, we find that the kinematics of the stream require a total mass inside 125 kpc of M 125 = 7.5 +2.5