  The 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 a form of a regular arc in which a part of a hexagonal
structure is embedded. A similar morphology is recognized in NGC 7421 galaxy,
in that part of its rim which is leading in the galaxy motion through the
intergalactic gas, as suggested by the bow-shock appearance of the HI halo of
the galaxy. A hexagonal shape is also found in NGC4676A galaxy. The HII radial
velocities across the Bubble complex are compatible with its retrograde
rotation and drift, which are characteristic for solitary vortices known in
nonlinear hydrodynamics. The drift motion may explain the location of the
Bubble complex at the tip of the largest elliptical HI hole in NGC 6946. The
hexagonal vortices in the atmospheres of Jupiter and Saturn are within the gas
streams what seems to be suggestive as well. It is conjectured that the
hexagonal rims of stellar systems might be relics 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 energetic X-ray sources P42 and X-4 respectively (fig.
11), and these might be the relics of Hypernovae.
