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 H i 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 Hα 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 superbubble 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.