[Previous] | [Session 72] | [Next]
J.S. Ulvestad (NRAO), L.C. Ho (Carnegie Observatories, Pasadena)
We have used the Very Large Array to image radio continuum emission in a sample of 52 Seyfert nuclei selected from the optical spectroscopic catalog of Ho, Filippenko, & Sargent. The observations were made at 6 cm in the B array and at 20 cm in the A array, yielding matched angular resolutions of ~1'', corresponding to 100 pc at the typical distance of 20 Mpc. A statistical sample of 45 northern Seyferts (20 type 1 and 25 type 2) having total magnitudes BT \leq 12.64 has been constructed. The space density of weak Seyferts in this statistical sample is (1.25±0.38) \times 10-3~Mpc-3, considerably higher than found for other Seyfert samples. Statistical tests have been carried out to compare the properties of this sample with expectations based on unified schemes for active galactic nuclei (AGNs). Host galaxy types, radio spectra, and radio source sizes are uncorrelated with Seyfert type, as predicted by the unified schemes.
Approximately half the detected galaxies have flat or inverted radio spectra. Surprisingly, Seyfert 1 galaxies are found to have somewhat stronger radio sources than Seyfert 2 galaxies among the objects with the weakest nuclear activity. We suggest that these properties may be accounted for if low-luminosity AGNs have qualitatively diffferent properties than their higher luminosity cousins. One possibility is that a minimum level of activity is required for a radio source to emerge from the vicinity of the central power source. Below this level, Seyfert radio sources might be affected by free-free absorption associated with the nuclear torus or a compact narrow-line region, accounting for both the weakness of the radio emission and the preponderance of flat spectra. Another possibility is that the low-luminosity Seyferts have highly sub-Eddington accretion rates, with an inverted-spectrum ADAF core component diluted by a steep-spectrum jet.
NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
The author(s) of this abstract have provided an email address for comments about the abstract: julvesta@nrao.edu