  Our situation as occupants of the Milky Way (MW) Galaxy, bombarded by the
Sagittarius dwarf galaxy, provides an intimate view of physical processes that
can lead to the dynamical heating of a galactic disc. While this evolution is
instigated by Sagittarius, it is also driven by the intertwined influences of
the dark matter halo and the disc itself. We analyse an N-body simulation
following a Sagittarius-like galaxy interacting with a MW-like host to
disentangle these different influences during the stages of a minor merger. The
accelerations in the disc plane from each component are calculated for each
snapshot in the simulation, and then decomposed into Fourier series on annuli.
The analysis maps quantify and compare the scales of the individual
contributions over space and through time: (i) accelerations due to the
satellite are only important around disc passages; (ii) the influence around
these passages is enhanced and extended by the distortion of the dark matter
halo; (iii) the interaction drives disc asymmetries within and perpendicular to
the plane and the self-gravity of these distortions increase in importance with
time eventually leading to the formation of a bar. These results have
interesting implications for identifying different influences within our own
Galaxy. Currently, Sagittarius is close enough to a plane crossing to search
for localized signatures of its effect at intermediate radii, the distortion of
the MW's dark matter halo should leave its imprint in the outer disc and the
disc's own self-consistent response is sculpting the intermediate and inner
disc.
