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

TL;DR

This paper presents imaging and spectroscopic observations of a unique elliptical nebula in NGC 6946, suggesting it is a superbubble formed by stellar feedback rather than a supernova remnant.

Contribution

It provides detailed imaging and spectral analysis of a peculiar elliptical nebula, proposing a new interpretation as a superbubble caused by stellar feedback.

Findings

The nebula is a large (~300 pc) elliptical structure with associated arcs.

Spectral data indicate photoionization by young stars, not a supernova remnant.

The shape is unusual for a superbubble, prompting new questions about its formation.

Abstract

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.