The Nature of the Ultraluminous Oxygen‐Rich Supernova Remnant in NGC 4449

Abstract

Optical images and spectra, both ground based and taken by Hubble Space Telescope (HST), of the young, luminous O-rich supernova remnant in the irregular galaxy NGC 4449 are presented. HST images of the remnant and its local region were obtained with the ACS/WFC using filters F435W, F555W, F814W (B, V, and I, respectively), F502N ([O III]), F658N (Hα + [N II]), F660N ([N II]), and F550M (line-free continuum). These images show an unresolved remnant (FWHM < 0.05'') located within a rich cluster of OB stars which itself is enclosed by a nearly complete interstellar shell seen best in Hα + [N II] emission approximately 8'' × 6'' (150 pc × 110 pc ) in size. The remnant and its associated OB cluster are isolated from two large nearby H II regions. The ACS [O III] image shows the remnant may be partially surrounded by a clumpy ring of emission approximately 1'' (~20 pc) in diameter. Recent ground-based spectra of the remnant reveal (1) the emergence of broad, blueshifted emission lines of [S II] λλ6716, 6731, [Ar III] λ7136, and [Ca II] λλ7291, 7324 which were not observed in spectra taken in 1978-1980; (2) faint emission at 6540-6605 A centered about Hα and [N II] λλ6548, 6583 with an expansion velocity of 500 ± 100 km s−1; and (3) excess emission around 4600-4700 A suggestive of a Wolf-Rayet population in the remnant's star cluster. We use these new data to re-interpret the origin of the remnant's prolonged and bright luminosity and propose that the remnant is strongly interacting with dense, circumstellar wind loss material from a 20 M☉ progenitor star.