A POSSIBLE "SUPER-SUPERNOVA" REMNANT IN NGC 6946

Abstract

Westerlund and Mathewson (1966) have presented rather convincing evidence that Constellation III of the Large Magellanic Cloud (Nail and Shapley 1953) is the result of a large-scale (super-supernova) explosion possibly of the type hypothesized by Shklovskii (1960). Hay ward (1964) has noted several similar formations in photographs of galaxies printed in the Hubble Atlas. This note describes a probable formation of this type in NGC 6946. Plate I shows two photographs of a section of the galaxy NGC 6946. One of the photographs (V) was taken on Kodak 103a-D emulsion with a Schott GG 11 filter; the other represents an Ha composite, made by subtracting a negative of the V plate from a plate taken on 103a-E emulsion with a Schott RG 1 filter. On the composite, Ha emission objects show up as black regions. In the center is the feature that appears to be similar to the SSN loop of the LMC. The notable features of this object are: that stars are arranged in a bright circular loop, 17 in diameter; that this loop shows evidence of being made up of two or perhaps three somewhat nonconcentric parts; that these loops are rich in extremely bright blue stars, readily resolved on blue plates with the 120-inch telescope; that the loops, bright in the continuum, are not rich in H n regions, as are other bright star clouds in the galaxy; that a small, bright H ii region, No. 33 in a catalog of Ha regions in NGC 6946 (Hodge 1966), can be seen at the center of the loop; and that a larger, faint H h region (No. 34 in the catalog) lies just outside the stellar loop. All of these features find their counterparts in the Constellation III features, identified by Westerlund and Mathewson as an SSN loop in the LMC. Figure 1 graphically compares them.