THE FAINTEST SEYFERT RADIO CORES REVEALED BY VLBI

In this Letter, we report on dual-frequency European VLBI Network observations of the faintest and least luminous radio cores in Seyfert nuclei, going to sub-millijansky flux densities and radio luminosities around 10(19) W Hz(-1). We detect radio emission from the nuclear region of four galaxies (NGC 4051, NGC 4388, NGC 4501, and NGC 5033), while one (NGC 5273) is undetected at the level of similar to 100 mu Jy. The detected compact nuclei have rather different radio properties: spectral indices range from steep (alpha > 0.7) to slightly inverted (alpha = -0.1), brightness temperatures vary f from T(B) = 10(5) K to larger than 10(7) K, and cores are either extended or unresolved, in one case accompanied by lobe-like features (NGC 4051). In this sense, diverse underlying physical mechanisms can be at work in these objects: jet-base or outflow solutions are the most natural explanations in several cases; in the case of the undetected NGC 5273 nucleus, the presence of an advection-dominated accretion flow is consistent with the radio luminosity upper limit.