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The faintest Seyfert radio cores revealed by VLBI

In this letter, we report on dual-frequency European VLBI Network (EVN) observations of the faintest and least luminous radio cores in Seyfert nuclei, going to sub-mJy flux densities and radio luminosities around 10^19 W/Hz. 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 ~100 microJy. 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 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 (ADAF) is consistent with the radio luminosity upper limit.