Bars & boxy/peanut bulges in thin & thick discs: I. Morphology and
  line-of-sight velocities of a fiducial model
  We explore trends in the morphology and line-of-sight (los) velocity of
stellar populations in the inner regions of disc galaxies, using N-body
simulations with both a thin (kinematically cold) and a thick (kinematically
hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin
disc component is $\sim$50\% stronger than the thick disc bar and is more
elongated, with an axis ratio almost half that of the thick disc bar. The thin
disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker
with a rather boxy shape. This leads to the signature of the b/p bulge in the
thick disc to be weaker and further away from the plane than in the thin disc.
Regarding the kinematics, we find that the los velocity of thick disc stars in
the outer parts of the b/p bulge can be \emph{larger} than that of thin disc
stars, by up to 40\% and 20\% for side-on and Milky Way-like orientations of
the bar respectively. This is due to the different orbits followed by thin and
thick disc stars in the bar-b/p region, which are affected by the fact that: i)
thin disc stars are trapped more efficiently in the bar - b/p instability and
thus lose more angular momentum than their thick disc counterparts and ii)
thick disc stars have large radial excursions and therefore stars from large
radii with high angular momenta can be found in the bar region. We also find
that the difference between the los velocities of the thin and thick disc in
the b/p bulge ($\Delta v_{los}$) correlates with the initial difference between
the radial velocity dispersions of the two discs ($\Delta \sigma$) . We
therefore conclude that stars in the bar - b/p bulge will have considerably
different morphologies and kinematics depending on the kinematic properties of
the disc population they originate from.
