On the Formation of Extended Galactic Disks by Tidally Disrupted Dwarf Galaxies

We explore the possibility that extended disks, such as that recently discovered in M31, are the result of a single dwarf (10–10M⊙) satellite merger. We conduct N-body simulations of dwarf NFW halos with embedded spheriodal stellar components on co-planar, prograde orbits in a M31-like host galaxy. As the orbit decays due to dynamical friction and the system is disrupted, the stellar particles relax to form an extended, exponential disk-like structure that spans the radial range 30– 200 kpc. The disk scale-length Rd correlates with the initial extent of the stellar component within the satellite halo: the more embedded the stars, the smaller the resulting disk scale-length. If the progenitors start on circular orbits, the kinematics of the stars that make up the extended disk have an average rotational motion that is 30 − 50 km/s lower than the host’s circular velocity. For dwarf galaxies moving on highly eccentric orbits (e ≃ 0.7), the stellar debris exhibits a much lower rotational velocity. Our results imply that extended galactic disks might be a generic feature of the hierarchical formation of spiral galaxies such as M31 and the Milky Way. Subject headings: stellar dynamics – methods: N-body simulations– methods: semi-analytical – galaxies: kinematics and dynamics – galaxies: halos – galaxies: dwarfs