A Morphological Method to Determine Corotation Radii in Spiral Galaxies

Shock-induced star formation in a stellar density wave scenario produces an azimuthal gradient of ages across the spiral arms that has opposite signs on either side of the corotation resonance (CR). We present a method based on the Fourier analysis of azimuthal profiles to locate the CR and determine the arm character (trailing or leading) in spiral galaxies. Basically, we compare the behavior of the phase angle of the two-armed spiral in blue and infrared colors that pick out, respectively, young and older disk stellar population. We illustrate the method using theoretical leading and trailing spirals. We have also applied the method to the spiral galaxies NGC 7479, for which we confirm the reported leading arms, and NGC 1832. In these galaxies, we find two and three CRs, respectively.