  Gaseous substructures such as feathers and spurs dot the landscape of spiral
arms in disc galaxies. One of the candidates to explain their formation is the
wiggle instability of galactic spiral shocks. We study the wiggle instability
using local 2D hydrodynamical isothermal non-self gravitating simulations. We
find that: (1) Simulations agree with analytic linear stability analysis only
under stringent conditions. They display surprisingly strong non-linear
coupling between the different modes, even for small mode amplitudes ($\sim
1\%$). (2) We demonstrate that the wiggle instability originates from a
combination of two physically distinct mechanisms: the first is the
Kelvin-Helmholtz instability, and the second is the amplification of
infinitesimal perturbations from repeated shock passages. These two mechanisms
can operate simultaneously, and which mechanism dominates depends on the
underlying parameters. (3) We explore the parameter space and study the
properties of spurs/feathers generated by the wiggle instability. The wiggle
instability is highly sensitive to the underlying parameters. The feather
separation decreases, and the growth rate increases, with decreasing sound
speed, increasing potential strength and decreasing interarm distance. (4) We
compare our simulations with a sample of 20 galaxies in the HST Archival Survey
of Spiral Arm Substructure of La Vigne et al. and find that the wiggle
instability is able to reproduce the typical range of feather separations seen
in observations. It remains unclear how the wiggle instability relates to
competing mechanisms for spur/feather formation such as the magneto-jeans
instability and the stochastic accumulation of gas due to correlated supernova
feedback.
