Résumé :
Star complexes are the largest groups of OB-associations,
young clusters and isolated stars with age up to ~100 Myr and
average size ~ 600 pc. Inside complexes there is hierarchical
distribution of embedded star groupings (the larger the older),
formed by interplay of gravitation and turbulence, but complexes
themselves were formed by the gravitational instability in the
galactic gas disks. The regular spacing of complexes along
the grand design spiral arms is the best evidence for such formation;
the fragments of flocculent spirals are the sheared complexes
themselves. The clumps in the spiral arms were known since
the first good photos of galaxies but generally have been considered
the occasional agglomerations of genetically unconnected stars
and clusters - until Efremov, Elmegreen, et al. (1970s)
have presented evidences for the coherent formation of complexes
from gas superclouds.
However some complexes have peculiar shape and origin,
being probably triggered by the external pressure. The system
of the perfectly circular giant stellar arcs in the LMC
is especially provocative. Many ideas exist on their origin,
including the formation under pressure of the intermittent jet
from the MW core (Efremov 2003), but none might be
considered proven. Another strange complex is one in NGC 6946.
The complex is similar to a BCD galaxy, hosting a young supermassive
cluster (SSC), like many BCDs. The complex rim is again a part of the
perfect circle. The data on stars and gas there were accumulated
with the HST, 6m Russian telescope and the WSRT. The formation of
the complex and its SSC in result of the impact of HVC and/or dark
mini-halo is not excluded.
It is curious that both the LMC and NGC 6946 peculiar complexes
were first noted by Hodge (1967); since then nor he, neither anybody
else were able to find other features so striking.
