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

Abstract

We investigate photometric properties of spiral arms and stellar complexes/associations inside these arms in the grand design NGC 628 (M 74) galaxy. We analyze GALEX ultraviolet, optical U BVRI, 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 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 M 74. It is unclear why about two third of galaxies with beaded arms host these "beads" (star complexes) in one arm only.

A model for the Lin‐Shu type density‐wave structure of our Galaxy: Line‐of‐sight and transverse‐longitudinal velocities of 242 optically visible open clusters

“…Proceeding from the fact that the arms of the most spiral galaxies are trailing (Pasha & Smirnov 1982) we should postulate that in the Galaxy p < 0. As one can see, the geometrical form of a spiral arm is represented, as most other authors have done (Seigar & James 1998;Levine et al 2006;Hou et al 2009;Gusev & Efremov 2014;Vallée 2014), by a single logarithmic function or a spiral of constant pitch, r = r 0 e (ϕ−ϕ0) tan p . The Galaxy rotates nonuniformly and the angular speed is Ω(r) = v circ /r, and for the Sun, denote Ω 0 = v circ,0 /r 0 (hereafter the subscript "0" denotes values corresponding to the position of the Sun), λ = 2πr 0 tan p/m is the radial distance between spiral arms (the scale of irregularity of the velocity field in the Galactocentric direction) at the solar location and 2πr 0 /m is the angular distance between them (the scale of irregularity in the azimuthal direction).…”

“…It is interesting to note that in many disk galaxies, e.g. in the spiral galaxies M101, NGC 1300, NGC 3675, NGC 4378, and NGC 4725 (Kormendy & Norman 1979;Zaritsky & Rix 1997;van Eymeren et al 2011; see also Gusev & Efremov 2014), there appears to be a deviation from rotational symmetry.…”

Studies of Star-forming Complexes in the Galaxies NGC 628, NGC 2976, and NGC 3351

“…NGC 628 is a classic spiral galaxy viewed almost side-on, enabling us to isolate the largest number of SFCs. A homogeneous distribution of stellar complexes is observed along its long arm, while this distribution is not homogeneous along its short arm [21]. This does not have a unique explanation, but it would seem to be unusual for a seemingly ordinary spiral galaxy.…”

Propagation of star formation at sub-kiloparsec scales

“…As a result, we must observe older star formation re-gions in the central part of a star complex. NGC 628 is just an example of a galaxy with numerous star complexes (Gusev & Efremov 2013).…”