On the Accretion Origin of a Vast Extended Stellar Disk around the Andromeda Galaxy

We present the discovery of an inhomogenous, low surface brightness, extended disklike structure around the Andromeda galaxy (M31) based on a large kinematic survey of more than 2800 stars with the Keck DEIMOS multiobject spectrograph. The stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out to R ~ 70 kpc. The constituent stars have velocities close to the expected velocity of circular orbits in the plane of the M31 disk and typically have a velocity dispersion of ~30 km s-1. The color range on the upper red giant branch shows a large spread indicative of a population with a significant range of metallicity. The mean metallicity of the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 ± 0.2. The morphology of the structure is irregular at large radii and shows a wealth of substructures that must be transitory in nature and are almost certainly tidal debris. The presence of these substructures indicates that the global entity was formed by accretion. This extended disk follows smoothly on from the central parts of M31 disk out to ~40 kpc with an exponential density law with a scale length of 5.1 ± 0.1 kpc, which is similar to that of the bright inner disk. However, the population possesses similar kinematic and abundance properties over the entire region where it is detected in the survey. We estimate that the structure accounts for approximately 10% of the total luminosity of the M31 disk, and given the huge scale, contains ~30% of the total disk angular momentum. This finding indicates that at least some galactic stellar disks are vastly larger than previously thought and are formed, at least in their outer regions, primarily by accretion.