Corotation Resonance of Non-barred Spiral Galaxies

Abstract

One of the main assumptions of density wave theory is that it predicts the existence of corotation resonances in spiral galaxies. We have used the multi-band photometric method, as introduced by Puerari & Dottori (1997), to determine the location of the corotation resonance in a sample of 19 non-barred spiral galaxies. The method is based on the Fourier analysis of azimuthal profiles of images of galaxies. Another assumption of density wave theory is the presence of a stellar azimuthal age gradi- ent across spiral arms. This age gradient is formed because of shock-induced star formation in a stellar density wave. The direction of the age gradient switches at the corotation radius. We performed Fourier transformations on the azimuthal profiles of galaxy images and calculated the phase angle for each waveband, B, V, R and I. Then, we plotted the phase angle of every waveband as a function of radius. The radius at which the phase angles cross represents the location of the corotation resonance. We found that 15 out of 19 galaxies have a single phase crossing which denotes the location of a single CR while the remaining 4 galaxies were found to have a corotation region, where more than one phase crossing occurred within a ra- dial range of less than 5 arcseconds. Finally, swing amplification does not predict an age gradient across spiral arms and therefore it does not predict the existence of the corotation resonance. We have found evidence of such age gradients and corotation radii, and we can therefore rule out swing amplification models in favor of quasi-static density wave models. Keywords: galaxies: kinematics and dynamics - galaxies: spiral - galaxies: structure.