  Tidal interactions between disc galaxies and low mass companions are an
established method for generating galactic spiral features. In this work we
present a study of the structure and dynamics of spiral arms driven in
interactions between disc galaxies and perturbing companions in 3-D
N-body/smoothed hydrodynamical numerical simulations. Our specific aims are to
characterize any differences between structures formed in the gas and stars
from a purely hydrodynamical and gravitational perspective, and to find a
limiting case for spiral structure generation. Through analysis of a number of
different interacting cases, we find that there is very little difference
between arm morphology, pitch angles and pattern speeds between the two media.
The main differences are a minor offset between gas and stellar arms, clear
spurring features in gaseous arms, and different radial migration of material
in the stronger interacting cases. We investigate the minimum mass of a
companion required to drive spiral structure in a galactic disc, finding the
limiting spiral generation cases with companion masses of the order
$1\times10^9M_\odot$, equivalent to only 4% of the stellar disc mass, or 0.5%
of the total galactic mass of a Milky Way analogue.
