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Round and arc-shaped formations are known in some galaxies, the Bubble complex (the Hodge object) in NGC 6946 being the most remarkable. The rim of the complex has the form of a regular arc in which part of a hexagonal structure is embedded. A similar morphology is recognized in the NGC 7421 galaxy in the part of its rim which is leading in the galaxy motion through the intergalactic gas, as suggested by the bow-shock appearance of the H I halo of the galaxy. A hexagonal shape is also found in the NGC 4676A galaxy. The H II radial velocities across the Bubble complex are compatible with its retrograde rotation and drift, which are characteristic of solitary vortices known in nonlinear hydrodynamics. The drift motion may explain the location of the Bubble complex at the tip of the largest elliptical H I hole in NGC 6946. The hexagonal vortices in the atmospheres of Jupiter and Saturn are within the gas streams, which seems to be suggestive as well. It is conjectured that the hexagonal rims of stellar systems might be relicts of flat segments of the shock wave produced by the ram pressure. The giant stellar arcs in NGC 300 and M33 are associated with the high-energy X-ray sources P42 and X-4 respectively and these might be the relics of hypernovae.
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Keywords: Galaxies; Individual (NGC 6946; M83. NGC 4449; NGC 7421; NGC 4676 and LMC); Large-scale star formation; Ram pressure and Hypernovae; Spiral arms; Stellar complexes; Vortices

Document Type: Research Article

Affiliations: Sternberg Astronomical Institute, Moscow University, Moscow 119899, Russia

Publication date: 2003-06-01

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