  We propose that the accretion of a dwarf spheroidal galaxy provides a common
origin for the giant southern stream and the warp of M31. We run about 40 full
N-body simulations with live M31, infalling galaxies with varying masses and
density profiles, and cosmologically-plausible initial orbital parameters.
Excellent agreement with a full range of observational data is obtained for a
model in which a dark-matter-rich dwarf spheroidal, whose trajectory lies on
the thin plane of corotating satellites of M31, is accreted from its turnaround
radius of about 200 kpcs into M31 at approximately 3 Gyrs ago. The satellite is
disrupted as it orbits in the potential well of the galaxy and forms the giant
stream and in return heats and warps the disk of M31. We show that our
cosmologically-motivated model is favoured by the kinematic data over the
phenomenological models in which the satellite starts its infall from a close
distance of M31. Our model predicts that the remnant of the disrupted satellite
resides in the region of the North-Eastern shelf of M31. The results here
suggest that the surviving satellites of M31 that orbit on the same thin plane,
as the disrupted satellite once did, could have all been accreted from an
intergalactic filament.
