Determining the Co-Rotation Radii of Spiral Galaxies Using Spiral Arm
  Pitch Angle Measurements at Multiple Wavelengths
  The spiral arms spanning disk galaxies are believed to be created by density
waves that propagate through galactic disks. We present a novel method of
finding the co-rotation radius where the spiral arm pattern speed matches the
velocities of the stars within the disk. Our method uses an image-overlay
technique, which involves tracing the arms of spiral galaxies on images
observed in different wavelengths. Density wave theory predicts that spiral
arms observed from different wavelengths show a phase crossing at the
co-rotation radius. For the purpose of this study, 20 nearby galaxies were
analyzed in four different wavelengths with pitch angle measurements performed
by two independent methods. We used optical wavelength images ($B$-band 440nm),
two infrared wavelength images provided by Spitzer (3.6$\mu$m and 8$\mu$m) and
ultraviolet images from GALEX (1350\AA, 1750\AA). The results were compared and
verified with other records found in the literature. We then found rotation
curve data for six of our galaxies and used our co-rotation radii estimates to
measure the time that would elapse between star formation and moving to their
observed positions in the $B$-band spirals. The average time lapse for this
motion was found to be $\sim$ 50 Myr. The success of this new method of finding
the co-rotation radius confirms density wave theory in a very direct way.
