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 ± 3 km 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 −1.3 × 10 11 M ⊙ , or M 125 > 5.4 × 10 11 M ⊙ at the 99 per cent confidence level. This is the first galaxy in which it has been possible to measure the halo mass distribution by such direct dynamical means over such a large distance range. The resulting orbit shows that if M32 or NGC 205 is connected with the stream, they must either trail or lag the densest region of the stream by more than 100 kpc. Furthermore, according to the best‐fitting orbit, the stream passes very close to M31, causing its demise as a coherent structure and producing a fan of stars that will pollute the inner halo, thereby confusing efforts to measure the properties of genuine halo populations. Our data show that several recently identified planetary nebulae, which have been proposed as evidence for the existence of a new companion of M31, are likely members of the Andromeda stream.