THE OPTICAL COUNTERPART OF THE RADIO-SOURCE CLOSE TO THE SEYFERT-2 NUCLEUS OF NGC 5953=ARP91B

Spectroscopic and morphological observations of the nuclear region of the Seyfert 2 galaxy NGC 5953 ≡ Arp 91 B are presented. It is shown that an extended emitting region, with features typical of a supergiant H II region, is located to the west of the nucleus within a radio structure which is emitting as a nonthermal source with spectral index α = 0.6 in the 1.4-5.0 GHz range. The faintness of the [O III] λλ 4959, 5007 emission lines in the spectrum of this region is discussed in terms of a state of low ionization, and high abundances of heavy elements in the emitting gas. In addition, the possibility that the ionization is produced by the Seyfert 2 nucleus and that its luminosity has undergone an abrupt decrease in luminosity has been taken into account. The nonthermal radio emission is tentatively interpreted as the result of frequent supernova blasts and the consistency of this interpretation with the optical data is discussed. Another extended emitting region, located close to the nucleus to its east, in a position symmetrically opposite to the western region, is revealed by our spectra. Its line ratios and fluxes are typical of a very luminous giant H II region. A bright stellar "knot," located 3" west of the nucleus, just within the emitting region associated with the extended radio source, is a foreground star of spectral type F8-G2 and magnitude V ∼ 16.