Stellar Bars in Counter-Rotating Dark Matter Halos: The Role of Halo
  Orbit Reversals
  Disk galaxies can exchange angular momentum and baryons with their host dark
matter (DM) halos. These halos possess internal spin, `lambda', which is
insignificant rotationally but does affect interactions between the baryonic
and DM components. While statistics of prograde and retrograde spinning halos
in galaxies is not available at present, the existence of such halos is
important for galaxy evolution. In the previous works, we analyzed dynamical
and secular evolution of stellar bars in prograde spinning halos and the DM
response to the bar perturbation, and found that it is modified by the resonant
interactions between the bar and the DM halo orbits. In the present work, we
follow the evolution of stellar bars in retrograde halos. We find, that this
evolution differs substantially from evolution in rigid unresponsive halos,
discussed in the literature. First, we confirm that the bar instability is
delayed progressively along the retrograde `lambda' sequence. Second, the bar
evolution in the retrograde halos differs also from that in the prograde halos,
in that the bars continue to grow substantially over the simulation time of 10
Gyr. The DM response is also substantially weaker compared to this response in
the prograde halos. Third, using orbital spectral analysis of the DM orbital
structure, we find a phenomenon we call the orbit reversal --- when retrograde
DM orbits interact with the stellar bar, reverse their streaming and
precession, and become prograde. This process dominates the inner halo region
adjacent to the bar and allows these orbits to be trapped by the bar, thus
increasing efficiency of angular momentum transfer by the Inner Lindblad
Resonance. We demonstrate this reversal process explicitly in a number of
examples.
