Kinematics and simulations of the stellar stream in the halo of the
  Umbrella Galaxy
  We study the dynamics of faint stellar substructures around the Umbrella
Galaxy, NGC 4651, which hosts a dramatic system of streams and shells formed
through the tidal disruption of a nucleated dwarf elliptical galaxy. We
elucidate the basic characteristics of the system (colours, luminosities,
stellar masses) using multi-band Subaru/Suprime-Cam images. The implied stellar
mass-ratio of the ongoing merger event is about 1:50. We identify candidate
kinematic tracers (globular clusters, planetary nebulae, H ii regions), and
follow up a subset with Keck/DEIMOS spectroscopy to obtain velocities. We find
that 15 of the tracers are likely associated with halo substructures, including
the probable stream progenitor nucleus. These objects delineate a kinematically
cold feature in position-velocity phase space. We model the stream using single
test-particle orbits, plus a rescaled pre-existing N-body simulation. We infer
a very eccentric orbit with a period of roughly 0.35 Gyr and turning points at
approximately 2-4 and 40 kpc, implying a recent passage of the satellite
through the disc, which may have provoked the visible disturbances in the host
galaxy. This work confirms that the kinematics of low surface brightness
substructures can be recovered and modeled using discrete tracers - a
breakthrough that opens up a fresh avenue for unraveling the detailed physics
of minor merging.
