A Critical Examination of Hypernova Remnant Candidates in M101. II. NGC 5471B

NGC 5471B has been suggested to contain a hypernova because of its extraordinarily bright X-ray emission. To assess its true nature, we have obtained high-resolution images in continuum bands and nebular lines with the Hubble Space Telescope, and high-dispersion long-slit spectra with the Kitt Peak National Observatory 4 m echelle spectrograph. The images reveal three supernova remnant (SNR) candidates in the giant H II region NGC 5471, the brightest one being the 77 × 60 pc shell in NGC 5471B. The Hα velocity profile of NGC 5471B can be decomposed into a narrow component (FWHM 41 km s-1) from the background H II region and a broad component (FWHM 148 km s-1) from the SNR shell. Using the brightness ratio of the broad to narrow components and the Hα flux measured from the WFPC2 Hα image, we derive a Hα luminosity of (1.4 ± 0.1) × 1039 ergs s-1 for the SNR shell. The [S II] λ6716/λ6731 doublet ratio of the broad velocity component is used to derive an electron density of ∼700 cm-3 in the SNR shell. The mass of the SNR shell is thus 4600 ± 500 M⊙. With a ∼330 km s-1 expansion velocity implied by the extreme velocity extent of the broad component, the kinetic energy of the SNR shell is determined to be 5 × 1051 ergs. This requires an explosion energy greater than 1052 ergs, which can be provided by one hypernova or multiple supernovae. Comparing to SNRs in nearby active star formation regions, the SNR shell in NGC 5471B appears truly unique and energetic. We conclude that the optical observations support the existence of a hypernova remnant in NGC 5471B.