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

TL;DR

This study analyzes the distribution and properties of star formation complexes in the spiral arms of galaxy NGC 628, revealing regular spacing in one arm and differences in dust lanes and star complexes between the two arms.

Contribution

It provides detailed photometric analysis of spiral arm structures and star complexes in NGC 628, highlighting asymmetries and regularities in star formation patterns.

Findings

Star complexes are regularly spaced in the longer arm, especially farther from the galaxy center.

The shorter arm lacks star complexes and shows different dust lane structures.

Gradients of age and luminosity across arms are generally absent, except near the galaxy center.

Abstract

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 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 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.