Modelling the inner disc of the Milky Way with manifolds. I - A first
  step
  We study the bar-driven dynamics in the inner part of the Milky Way by using
invariant manifolds. This theory has been successfully applied to describe the
morphology and kinematics of rings and spirals in external galaxies, and now,
for the first time, we apply it to the Milky Way. We compute the orbits
confined by the invariant manifolds of the unstable periodic orbits located at
the ends of the bar. We discuss whether the COBE/DIRBE bar and the Long bar
compose a single bar or two independent bars and perform a number of
comparisons which, taken together, argue strongly in favour of the former. More
specifically, we favour the possibility that the so-called COBE/DIRBE bar is
the boxy/peanut bulge of a bar whose outer thin parts are the so-called Long
bar. This possibility is in good agreement both with observations of external
galaxies, with orbital structure theory and with simulations. We then analyse
in detail the morphology and kinematics given by five representative Galactic
potentials. Two have a Ferrers bar, two have a quadrupole bar and the last one
a composite bar. We first consider only the COBE/DIRBE bar and then extend it
to include the effect of the Long bar. We find that the large-scale structure
given by the manifolds describes an inner ring, whose size is similar to the
near and far 3-kpc arm, and an outer ring, whose properties resemble those of
the Galactic Molecular Ring. We also analyse the kinematics of these two
structures, under the different galactic potentials, and find they reproduce
the main over-densities found in the galactic longitude-velocity CO diagram.
Finally, we consider for what model parameters, the global morphology of the
manifolds may reproduce the two outer spiral arms. We conclude that this would
necessitate either more massive and more rapidly rotating bars, or including in
the potential an extra component describing the spiral arms.
