Dependence of Spiral Arms Pitch Angle on Wavelength as a Test of Density
  Wave Theory
  Large-scale galactic shocks, predicted by density wave theory, trigger star
formation (SF-arms) downstream from the potential of the oldest stars (P-arms),
resulting in a color jump from red to blue across spiral arms in the direction
of rotation, while aging of these newly formed young stars induces the opposite
but coexisting classic age gradient further downstream from the SF-arms. As the
techniques for measuring pitch angle are intensity-weighted, they trace both
the SF-arms and P-arms and are not sensitive to the classic age gradient.
Consequently, the measured pitch angle of spiral arms should be systematically
smaller in bluer bandpasses compared to redder bandpasses. We test these
predictions using a comprehensive sample of high-quality optical ($BVRI$)
images of bright, nearby spiral galaxies acquired as part of the
Carnegie-Irvine Galaxy Survey, supplemented by $Spitzer$ 3.6 $\mu m$ data to
probe evolved stars and $GALEX$ ultraviolet images to trace recent star
formation. We apply one-dimensional and two-dimensional techniques to measure
the pitch angle of spiral arms, paying close attention to adopt consistent
procedures across the different bandpasses to minimize error and systematic
bias. We find that the pitch angle of spiral arms decreases mildly but
statistically significantly from the reddest to the bluest bandpass,
demonstrating conclusively that young stars trace tighter spiral arms than old
stars. Furthermore, the correlation between the pitch angle of blue and red
bandpasses is non-linear, such that the absolute value of pitch angle offset
increases with increasing pitch angle. Both effects can be naturally explained
in the context of the density wave theory for spiral structure.
