Spiral patterns with straight arm segments

The phenomenon of 'rows', which are straight geometrical segments in the sp iral arms of galaxies, is studied. The Whirlpool nebula. Messier 51 (NGC 51 94) in Canes Venatici, is considered to be an example of a giant grand desi gn galaxy. Optical photographs, H alpha, ultraviolet and far-ultraviolet im ages, CO, 21-cm and synchrotron emission maps, and a K-s- band mosaic of M5 1 are studied. With this observational material, multiple rows can be recog nized in the spiral arms of the galaxy. The rows comprise a major part of t he arms. The lengths of the rows increase almost linearly with distance fro m the centre. They intersect one another at an (average) angle approximate to 2 pi/3. A possible physical explanation of the phenomenon of rows is dis cussed on the basis of the assumption that the formation of straight arm se gments might be due to the gas-dynamical effect of stability of flat shock fronts, and the tendency of a slightly curved shock front to become flat, A quantitative flattening criterion enables an explanation of the geometrica l properties of the arm patterns found in M51 and also in M101, A brief lis t of spirals with rows is given.