The Halo Shape and Evolution of Polar Disc Galaxies
  We examine the properties and evolution of a simulated polar disc galaxy.
This galaxy is comprised of two orthogonal discs, one of which contains old
stars (old stellar disc), and the other, containing both younger stars and the
cold gas (polar disc) of the galaxy. By exploring the shape of the inner region
of the dark matter halo, we are able to confirm that the halo shape is a oblate
ellipsoid flattened in the direction of the polar disc. We also note that there
is a twist in the shape profile, where the innermost 3 kpc of the halo flattens
in the direction perpendicular to the old disc, and then aligns with the polar
disc out until the virial radius. This result is then compared to the halo
shape inferred from the circular velocities of the two discs. We also use the
temporal information of the simulation to track the system's evolution, and
identify the processes which give rise to this unusual galaxy type. We confirm
the proposal that the polar disc galaxy is the result of the last major merger,
where the angular moment of the interaction is orthogonal to the angle of the
infalling gas. This merger is followed by the resumption of coherent gas
infall. We emphasise that the disc is rapidly restored after the major merger
and that after this event the galaxy begins to tilt. A significant proportion
of the infalling gas comes from filaments. This infalling gas from the filament
gives the gas its angular momentum, and, in the case of the polar disc galaxy,
the direction of the gas filament does not change before or after the last
major merger.
