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

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