Density waves and star formation in grand-design spirals

Context. H II regions in the arms of spiral galaxies are indicators of recent star-forming processes. They may have been caused by the passage of the density wave or simply been created by other means near the arms. The study of these regions may give us clues to clarify the controversy about the existence of a triggering scenario, as proposed in the density-wave theory. Aims. If the passage of the density wave contributes significantly to the formation of H II regions in the arms of grand-design spirals, we can find a relationship in their position relative to the arms. Methods. Using Hα direct imaging, we characterize the H II regions from a sample of three grand-design galaxies: NGC 5457, NGC 628, and NGC 6946. Broad-band images in R and I were used to determine the position of the arms. The H II regions found to be associated with arms were selected for the study. The age and star formation rate of these H II regions was obtained using measurements on the Hα line. The distance between the current position of the selected H II regions and the position they would have if they had been created in the centre of the arm was calculated. A parameter, T, which measures whether a region was created in the arm or in the disc, was defined. Results. With the help of the T parameter we determine that the majority of regions were formed some time after the passage of the density wave, and the regions are located behind the zone they should have occupied (in the direction of the rotation of the galaxy) had they been formed in the centre of the arm. Moreover, the relative efficiency of the star formation rate between arm and interarm was calculated for all the galaxies. Conclusions. The large number of regions created after the passage of the arm may be explained by the effect of the density wave, which helps to create the star-forming regions after its passage. There is clear evidence of triggering for NGC 5457 and a co-rotation radius is proposed. A more modest triggering seems to exist for NGC 628, and insignificant evidence of triggering is found for NGC 6946.