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Gaseous substructures such as feathers and spurs dot the landscape of spiralarms in disc galaxies. One of the candidates to explain their formation is thewiggle instability of galactic spiral shocks. We study the wiggle instabilityusing local 2D hydrodynamical isothermal non-self gravitating simulations. Wefind that: (1) Simulations agree with analytic linear stability analysis onlyunder stringent conditions. They display surprisingly strong non-linearcoupling between the different modes, even for small mode amplitudes ($\sim1\%$). (2) We demonstrate that the wiggle instability originates from acombination of two physically distinct mechanisms: the first is theKelvin-Helmholtz instability, and the second is the amplification ofinfinitesimal perturbations from repeated shock passages. These two mechanismscan operate simultaneously, and which mechanism dominates depends on theunderlying parameters. (3) We explore the parameter space and study theproperties of spurs/feathers generated by the wiggle instability. The wiggleinstability is highly sensitive to the underlying parameters. The featherseparation decreases, and the growth rate increases, with decreasing soundspeed, increasing potential strength and decreasing interarm distance. (4) Wecompare our simulations with a sample of 20 galaxies in the HST Archival Surveyof Spiral Arm Substructure of La Vigne et al. and find that the wiggleinstability is able to reproduce the typical range of feather separations seenin observations. It remains unclear how the wiggle instability relates tocompeting mechanisms for spur/feather formation such as the magneto-jeansinstability and the stochastic accumulation of gas due to correlated supernovafeedback.

The Physical Origin and the Properties of Arm Spurs/Feathers in Local Simulations of the Wiggle Instability