Investigating the Origins of Spiral Structure in Disk Galaxies through a
  Multiwavelength Study
  The density-wave theory of spiral structure proposes that star formation
occurs in or near a spiral-shaped region of higher density that rotates rigidly
within the galactic disk at a fixed pattern speed. In most interpretations of
this theory, newborn stars move downstream of this position as they come into
view, forming a downstream spiral which is tighter, with a smaller pitch angle
than that of the density wave itself. Rival theories, including theories which
see spiral arms as essentially transient structures, may demand that pitch
angle should not depend on wavelength. We measure the pitch angle of a large
sample of galaxies at several wavelengths associated with star formation or
very young stars (8.0 {\mu}m, H-{\alpha} line and 151 nm in the far-UV) and
show that they all have the same pitch angle, which is larger than the pitch
angle measured for the same galaxies at optical and near-infrared wavelengths.
Our measurements in the B band and at 3.6 {\mu}m have unambiguously tighter
spirals than the starforming wavelengths. In addition, we have measured in the
u-band, which seems to fall midway between these two extremes. Thus, our
results are consistent with a region of enhanced stellar light situated
downstream of a starforming region.
