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 (10⁹-10¹⁰ M_☉) satellite merger. We conduct N-body simulations of dwarf NFW halos with embedded spheroidal stellar components on coplanar, prograde orbits in an 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 R_d 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^-1 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.