The M31/M33 tidal interaction: A hydrodynamic simulation of the extended
  gas distribution
  We revisit the orbital history of the Triangulum galaxy (M33) around the
Andromeda galaxy (M31) in view of the recent Gaia Data Release 2 proper motion
measurements for both Local Group galaxies. Earlier studies consider highly
idealised dynamical friction, but neglect the effects of dynamical mass loss.
We show the latter process to be important using mutually consistent orbit
integration and N-body simulations. Following this approach we find an orbital
solution that brings these galaxies to within ~50 kpc of each other in the
past, ~6.5 Gyr ago. We explore the implications of their interaction using an
N-body/hydrodynamical simulation with a focus on the origin of two prominent
features: 1) M31's Giant Stellar Stream; and 2) the M31-M33 HI filament. We
find that the tidal interaction does not produce a structure reminiscent of the
stellar stream that survives up to the present day. In contrast, the M31-M33 HI
filament is likely a fossil structure dating back to the time of the ancient
encounter between these galaxies. Similarly, the observed outer disc warp in
M33 may well be a relic of this past event. Our model suggests the presence of
a tidally induced gas envelope around these galaxies, and the existence of a
diffuse gas stream, the 'Triangulum stream', stretching for tens of kpc from
M33 away from M31. We anticipate upcoming observations with the recently
commissioned, Five-hundred-meter Aperture Spherical radio Telescope (FAST) that
will target the putative stream in its first years of operation.
