We investigate photometric properties of spiral arms and stellar complexes/associations inside these arms in the grand design NGC 628 (M74) galaxy.We analyze GALEX ultraviolet, optical UBVRI, and H-alpha? surface photometry data, including those obta
ined 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 center. 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 center. It is well expressed in CO (H_2) 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 center), 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.
The origin of the giant stellar arcs in the LMC remains a controversial issue, discussed since 1966. No other stellar arc is so perfect a segment of a circle, moreover, there is another similar arc nearby. Many hypotheses were advanced to explain the
se arcs, and all but one of these were disproved. It was proposed in 2004 that origin of these arcs was due to the bow shock from the jet, which is intermittently fired by the Milky Way nucleus and during the last episode of its activity the jet was pointed to the LMC. Quite recently evidence for such a jet has really appeared. We suppose it was once energetic enough to trigger star formation in the LMC, and if the jet opening angle was about 2{deg}, it could push out HI gas from the region of about 2 kpc in size, forming a cavity LMC4, but also squeezed two dense clouds, which occurred in the same area, causing the formation of stars along their surfaces facing the core of the MW. In result, spherical segments of the stellar shells might arise, visible now as the arcs of Quadrant and Sextant, the apices of which point to the center of the MW. This orientation of both arcs can be the key to unlocking their origin. Here we give data which confirm the above hypothesis, amongst which are radial velocities of stars inside and outside the larger one of the LMC arcs. The probability is low that a jet from an AGN points to a nearby galaxy and triggers star formation there, but a few other examples are now known or suspected.
