Scott Chapman from the California Institute of Technology and Rodrigo Ibata from the Observatoire de Strasbourg in France have led a team of astronomers in a project to map out the detailed motions of stars in the outskirts of the Andromeda galaxy. Their recent observations with the Keck telescopes show that the tenuous sprinkle of stars that extend outward from the galaxy are actually part of the main disk itself. This means that the spiral disk of Andromeda is three times larger in diameter than previously estimated.

Velocities of 10,000 stars in outer regions of Andromeda were measured with Keck/DEIMOS. More than half of these stars some distance from the disk which were once thought to be merely the "halo" of stars in the region and not part of the disk itself were used to provide evidence that there is a vast extended stellar disk that extends to greater than 70kpc in radius. These stars are clearly moving at velocities that demonstrate they are in orbit around the center of the galaxy.

An INT2.5m telescope (located in the Canary Islands) colour-coded map of M31 is shown below, highlighting the irregular "halo" of stars (yellow) which loops and bubbles around the 'classical' disk of M31 (red). The Keck velocity measurements have helped to determined  the nature of the inhomogenous halo (the fact that the outer fringes of the disk are clumpy and blobby). The outer regions of Andromeda are clearly dominated by a huge rotating disk, but at the same time must be the result of satellite galaxies long ago slamming together, giving rise to this inhomogeneous "messy" morphology. This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies - giant rotating disks do not arise from the accretion of small galaxy fragments.