ar X iv : a st ro - p h / 02 02 04 7 v 1 1 F eb 2 00 2 A Critical Examination of Hypernova Remnant Candidates in M 101 . II . NGC 5471

NGC5471B has been suggested to contain a hypernova remnant 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 NGC5471, with the brightest one being the 77 × 60 pc shell in NGC5471B. The Hα velocity profile of NGC5471B can be decomposed into a narrow component (FWHM = 41 km s) from the background H ii region and a broad component (FWHM = 148 km s) 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 an Hα luminosity of (1.4 ± 0.1) × 10 ergs s 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 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 Visiting astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. Current address: Jet Propulsion Lab, MS 169-506, 4800 Oak Grove Dr., Pasadena, CA 91109; [email protected]