  Within the standard hierarchical structure formation scenario, Milky Way-mass
dark matter haloes have hundreds of dark matter subhaloes with mass $\gtrsim
10^8 \, {\rm M_{\odot}}$. Over the lifetime of a galactic disc a fraction of
these may pass close to the central region and interact with the disc. We
extract the properties of subhaloes, such as their mass and trajectories, from
a realistic cosmological simulation to study their potential effect on stellar
discs. We find that massive subhalo impacts can generate disc heating, rings,
bars, warps, lopsidedness as wells as spiral structures in the disc.
Specifically, strong counter-rotating single-armed spiral structures form each
time a massive subhalo passes through the disc. Such single-armed spirals wind
up relatively quickly (over $1-2$ Gyrs) and are generally followed by
co-rotating two-armed spiral structures that both develop and wind up more
slowly. In our simulations self-gravity in the disc is not very strong and
these spiral structures are found to be kinematic density waves. We demonstrate
that there is a clear link between each spiral mode in the disc and a given
subhalo that caused it, and by changing the mass of the subhalo we can modulate
the strength of the spirals. Furthermore, we find that the majority of
subhaloes interact with the disc impulsively, such that the strength of spirals
generated by subhaloes is proportional to the total torque they exert. We
conclude that only a handful of encounters with massive subhaloes is sufficient
for re-generating and sustaining spiral structures in discs over their entire
lifetime.
