The R- and Theta-relief method applied to the face-on galaxy M51 - Spoke and ring structures in the nuclear disk

We propose a method of pattern recognition, called the R-and-Theta-relief method, for studying fine strucutres in the face-on galaxies, particularly in nuclear disks. This method enhances radial adn azimuthal strucutres burried in a galactic disk. By applying the method to a B-band CCD image of face-on galaxy M51, we show that the nuclear disk is superimposed by multiple rings and radial spoke structures. These peculiar features may be related to dynamical resonances and/or to an ejection activity near the nucleus. Amplitudes of B-band intensity of 100-pc scale structures are an order of magnitudes greater in the central 1 kpc region than in the outer major arm regions, which indicates a much higher rate of star formation in the central regions than in arms.