ИСТИНА

We investigated photometric properties of the two principal spiral arms and star formation regions inside these arms in the grand-design galaxy NGC 628 (M74). We also studied the chemical abundances and sizes of star formation regions in the arms. GALEX ultraviolet, optical UBVRI and Hα surface photometry and spectroscopic data were used. 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. The usual dust lane along the stellar arm inner side is seen there only at distances closer to the galaxy centre. 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. The shorter arm contains no large star complexes at all, it contains smaller star formation regions (stellar associations) only. 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 NGC 628. Our another result may imply the existence of a certain minimal distance between young star groups, measured in projection on to the arm central line, in both spiral arms. Most distances between adjacent groups are either this minimal one (Λ≈400 pc) or twice and four times this minimal distance. This value is the same in inner part of the longer arm, where bright complexes are absent, and in outer part of this arm, which is populated by bright complexes. The characteristic separations of the bright complexes in the longer arm are also a multiple of Λ (800 pc or 1600 pc). We identified and studied 30 brightest star formation regions in ultraviolet light located in the spiral arms of galaxy. We find that the star formation regions in the longer arm are systematically brighter and larger than the regions in the other (shorter) arm. The star formation regions in the longer arm have a higher star formation rate density than the regions in the shorter arm. The regions in the shorter arm show higher N/O ratio at a higher oxygen abundance, but they have lower ultraviolet and Hα luminosities. These findings can be explained if we assume that star formation regions in the shorter arm had a higher star formation rate in the past, but it is now lower than that in the opposite arm. Results of stellar evolutionary synthesis show that the brightest regions in the longer arm are slightly younger than those in the shorter arm (3.5±2.2 Myr versus 6.0±1.1 Myr). Our results demonstrate that there is a difference in inner structure and parameters of the interstellar medium between the spiral arms of NGC 628, one of which is long and hosts a regular chain of bright star formation complexes, while the other, shorter, one does not.