Unusual Emission-Line Regions in the Tidal Arm of NGC 7318B in Stephan's Quintet

We report the optical spectroscopic discovery of two unusual emission-line regions in the tidal arm of a galaxy, NGC 7318B, that belongs to Stephan's Quintet. They are associated spatially with the unusual radio continuum emission discovered between the two galaxies NGC 7318B and NGC 7319. Taking account of both the large velocity widths of the emission lines, ~900 km s^-1, and the strong [S II] λλ6717, 6731 emission with respect to Hα emission, we identify them as optical emission-line filaments of supernova remnants (SNRs) traced by the radio continuum and soft X-ray emission. Our narrowband Hα imaging shows that the unusual emission-line regions are indeed parts of the ionized bubble associated with one radio continuum peak. The required number of SNRs is estimated to be ~10⁵-10⁶ on the basis of the Hα, radio, and soft X-ray luminosities. Since there is no optical counterpart like a star cluster, it is suggested that ~10⁵-10⁶ supernova events occurred nearly simultaneously in this region several times 10⁶ yr ago.