The extraordinary extragalactic supernova remnant in NGC 4449

Spectrophotometry of an unusual emission region in the irregular galaxy NGC 4449 is presented that confirms that it is a supernova remnant (SNR) similar to Cas A, and a simple overall picture of the interaction between the SNR and the H II region in which it is embedded is described. Broad forbidden emission lines of O I at 6300 and 6363 A, of O II at 7320 and 7330 A, and of O III at 4959 and 5007 A are found, and the following are inferred: an expansion at 3500 km/s, electron temperature of about 40,000 K where oxygen is doubly ionized, a density exceeding 100,000/cu cm in the region where oxygen is once ionized, and about 0.1 solar masses of oxygen present in the gas that is presently cooling. In addition, the absence of hydrogen in fast-moving, oxygen-rich knots suggests that nucleosynthesis has been important in the star that created this remnant.