Strong Evidence for the Density-wave Theory of Spiral Structure in Disk
  Galaxies
  The density-wave theory of galactic spiral-arm structure makes a striking
prediction that the pitch angle of spiral arms should vary with the wavelength
of the galaxy's image. The reason is that stars are born in the density wave
but move out of it as they age. They move ahead of the density wave inside the
co-rotation radius, and fall behind outside of it, resulting in a tighter pitch
angle at wavelengths that image stars (optical and near-infrared) than those
that are associated with star formation (far-infrared and ultraviolet). In this
study we combined large sample size with wide range of wavelengths, from the
ultraviolet to the infrared to investigate this issue. For each galaxy we used
an optical wavelength image (B-band: 445 nm) and images from the Spitzer Space
Telescope at two infrared wavelengths (infrared: 3.6 and 8.0 {\mu}m) and we
measured the pitch angle with the 2DFFT and Spirality codes. We find that the
B-band and 3.6 {\mu}m images have smaller pitch angles than the infrared 8.0
{\mu}m image in all cases, in agreement with the prediction of density-wave
theory. We also used images in the ultraviolet from Galaxy Evolution Explorer,
whose pitch angles agreed with the measurements made at 8 {\mu}m.
