DETECTION OF A NUCLEAR RADIO-SOURCE IN NGC-7331 - A RELATIVELY RADIO-QUIET LINER

We have used the Very Large Array to observe the nearby spiral galaxy NGC 7331. The observations detected an unresolved radio source in the nucleus of this galaxy, with flux densities of 234 +/- 25 mu Jy and 12 1 +/- 16 mu Jy at 20 and 6 cm, respectively. This is equivalent (at th e distance of this galaxy) to luminosities of approximately 3 and 4 ti mes that of the Galactic center source Sgr A at 20 and 6 cm. The sourc e in NGC 7331, with a radio spectral index (S-mu proportional to mu(al pha)) of alpha = -0.6 +/- 0.1, sits in a region of very low gas densit y, surrounded by a ring of radio emission. Optical spectra show that N GC 7331 has a weak LINER nucleus. We compare the radio and optical pro perties of this nucleus with other LINER nuclei detected in the radio, and find that NGC 7331 has an unusually low radio to H alpha flux rat io. The combination of an unresolved radio source and a LINER nucleus spectrum suggests that this galaxy harbors a massive black hole. The r adio observations of NGC 7331 did not detect emission from the histori cal supernova 1959D with a 3 sigma upper limit of 75 mu Jy at 20 cm.