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

We present the discovery of an inhomogenous, low-surface brightness, extendeddisk-like structure around the Andromeda galaxy (M31) based on a largekinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. Thestellar structure spans radii from 15 kpc out to ~40 kpc, with detections outto R ~ 70 kpc. The constituent stars lag the expected velocity of circularorbits in the plane of the M31 disk by ~40 kms and have a velocity dispersionof ~30 kms. The color range on the upper RGB shows a large spread indicative ofa population with a significant range of metallicity. The mean metallicity ofthe 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 awealth of substructures which must be transitory in nature, and are almostcertainly tidal debris. The presence of these substructures indicates that theglobal entity was formed by accretion. This extended disk follows smoothly onfrom the central parts of M31 disk with an exponential density law ofscale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. Thepopulation possesses similar kinematic and abundance properties over the entireregion where it is detected in the survey. We estimate that the structureaccounts for approximately 10% of the total luminosity of the M31 disk, andgiven the huge scale, contains ~30% of the total disk angular momentum. Thisfinding indicates that at least some galactic stellar disks are vastly largerthan previously thought and are formed, at least in their outer regions,primarily by accretion. [abridged]Comment: 20 pages, 30 figures, ApJ submitte