Energy Source for Shingle-like Structures in Spiral-arms

The filaments or shingles observed in neighbouring spiral-arms were found in a previous paper to have a wavelength of ˜ 1.3 kpc. The life-time of the shingles was estimated at ˜ 10⁸ years. The energy source to regenerate the shingles is sought in terms of a JEANS -type instability modified by a two stream instability between population I and disk material. From the dispersion relation it is found that an instability of this type occurs at a critical-wavelength of 1.9 kpc.