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The spiral arms spanning disk galaxies are believed to be created by densitywaves that propagate through galactic disks. We present a novel method offinding the co-rotation radius where the spiral arm pattern speed matches thevelocities of the stars within the disk. Our method uses an image-overlaytechnique, which involves tracing the arms of spiral galaxies on imagesobserved in different wavelengths. Density wave theory predicts that spiralarms observed from different wavelengths show a phase crossing at theco-rotation radius. For the purpose of this study, 20 nearby galaxies wereanalyzed in four different wavelengths with pitch angle measurements performedby 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) andultraviolet images from GALEX (1350\AA, 1750\AA). The results were compared andverified with other records found in the literature. We then found rotationcurve data for six of our galaxies and used our co-rotation radii estimates tomeasure the time that would elapse between star formation and moving to theirobserved positions in the $B$-band spirals. The average time lapse for thismotion was found to be $\sim$ 50 Myr. The success of this new method of findingthe co-rotation radius confirms density wave theory in a very direct way.

Determining the Co-Rotation Radii of Spiral Galaxies Using Spiral Arm Pitch Angle Measurements at Multiple Wavelengths