Letter to the Editor A MORPHOLOGICAL METHOD TO DETERMINE CO-ROTATION RADII IN SPIRAL GALAXIES

Shock induced star formation in a stellar density wave scenario produces an azimuthal gradient of ages across the spiral arms which has opposite signs on either side of the corotation resonance (CR). We present a method based on the Fourier analysis of azimuthal pro les, 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 which 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 NGC7479, for which we con rm the reported leading arms, and NGC1832. In these galaxies we nd two and three CRs respectively. Subject headings: galaxies: spiral { galaxies: kinematics and dynamics { galaxies: individual (NGC1832, NGC7479) { method: analytical