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NGC 6946, known as the Fireworks galaxy because of its high supernova rateand high star formation, is embedded in a very extended HI halo. Its northernspiral arm is well detached from the galactic main body. We found that this armcontains a large (~300 pc in size) Red Ellipse, named according to a strongcontamination of the H-alpha emission line on its optical images. The ellipseis accompanied by a short parallel arc and a few others still smaller and lessregular; a bright star cluster is seen inside these features. The complicatedcombination 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 telescopedid 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 hotstars with a small contribution of shock ionization. The most likelyexplanation of the Red Ellipse is a superbbuble created by a collectivefeedback of massive stars in the star cluster located in the NE side of the RedEllipse. However, the very regular elliptical shape of the nebulae seemsstrange.

Imaging and spectroscopic observations of a strange elliptical bubble in the northern arm of the spiral galaxy NGC 6946