Regular chains of star formation complexes in spiral arms of NGC 628

We investigate photometric properties of spiral arms and stellar complexes/associations inside these arms in the grand design NGC 628 (M74) galaxy. We analyse GALEX ultraviolet, optical UBVRI and Hα surface photometry data, including those obtained with 1.5 m telescope at the Maidanak Observatory. In the longer arm, the large and bright stellar complexes are located at regular intervals along the arm, but only farther from the galaxy centre. They are joined with the narrow lane of dust, visible only in the infrared bands. The usual dust lane along the stellar arm inner side is seen there only at distances closer to the galaxy centre. It is well expressed in CO (H2) image. We have found that the second, short arm hosts two dust lanes, the strong and wide at the inner side, and narrow and irregular along its outer edge. This outer dust lane is well seen in IR images only. The shorter arm contains no star complexes at all. Gradients of age and luminosity of stars across both arms are missing (again excepting the parts of arms located closer to the centre), which is confirmed by our photometric cuts across both arms. The drastic difference in the morphology of the two symmetric arms (grand design type) of a galaxy has now been confirmed by objective measurements in the case of M74. It is unclear why about two third of galaxies with beaded arms host these 'beads' (star complexes) in one arm only.