Imaging and spectroscopic observations of a strange elliptical bubble in
  the northern arm of the spiral galaxy NGC 6946
  NGC 6946, known as the Fireworks galaxy because of its high supernova rate
and high star formation, is embedded in a very extended HI halo. Its northern
spiral arm is well detached from the galactic main body. We found that this arm
contains a large (~300 pc in size) Red Ellipse, named according to a strong
contamination of the H-alpha emission line on its optical images. The ellipse
is accompanied by a short parallel arc and a few others still smaller and less
regular; a bright star cluster is seen inside these features. The complicated
combination of arcs seems to be unique, it is only a bit similar to some SNRs.
However, the long-slit spectral data obtained with the Russian 6-m telescope
did not confirm the origin of the nebula as a result of a single SN outburst.
The emission-line spectrum corresponds to the photoionization by young hot
stars with a small contribution of shock ionization. The most likely
explanation of the Red Ellipse is a superbbuble created by a collective
feedback of massive stars in the star cluster located in the NE side of the Red
Ellipse. However, the very regular elliptical shape of the nebulae seems
strange.
