Dynamics of stream-subhalo interactions
  We develop a formalism for modelling the impact of dark matter subhaloes on
cold thin streams. Our formalism models the formation of a gap in a stream in
angle-frequency space and is able to handle general stream and impact geometry.
We analyse an $N$-body simulation of a cold stream formed from a progenitor on
an eccentric orbit in an axisymmetric potential, which is perturbed by a direct
impact from a $10^8 M_\odot$ subhalo, and produce a complete generative model
of the perturbed stream that matches the simulation well at a range of times.
We show how the results in angle-frequency space can be related to physical
properties of the gaps and that previous results for more constrained
simulations are recovered. We demonstrate how our results are dependent upon
the mass of the subhalo and the location of the impact along the stream. We
find that gaps formed far downstream grow more rapidly than those closer to the
progenitor due to the more ordered nature of the stream members far from the
progenitor. Additionally, we show that the minimum gap density plateaus in time
at a value that decreases with increasing subhalo mass.
