The Magellanic System: the puzzle of the leading gas stream
  The Magellanic Clouds (MCs) are the most massive gas-bearing systems falling
into the Galaxy at the present epoch. They show clear signs of interaction,
manifested in particular by the Magellanic Stream, a spectacular gaseous wake
that trails from the MCs extending more than 150 degree across the sky. Ahead
of the MCs is the "Leading Arm" usually interpreted as the tidal counterpart of
the Magellanic Stream, an assumption we now call into question. We revisit the
formation of these gaseous features in a first-infall scenario, including for
the first time a Galactic model with a weakly magnetised, spinning hot corona.
In agreement with previous studies, we broadly recover the location and the
extension of the Stream on the sky. In contrast, we find that the formation of
the Leading Arm -- that is otherwise present in models without a corona -- is
inhibited by the hydrodynamic interaction with the hot component. These results
hold with or without coronal rotation or a weak, ambient magnetic field. Since
the existence of the hot corona is well established, we are led to two possible
interpretations: (i) the Leading Arm survives because the coronal density
beyond 20 kpc is a factor of 10 lower than required by conventional spheroidal
coronal x-ray models, consistent with recent claims of rapid coronal rotation;
or (ii) the `Leading Arm' is cool gas trailing from a frontrunner, a satellite
moving ahead of the MCs, consistent with its higher metallicity compared to the
trailing stream. Both scenarios raise issues that we discuss.
