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 ^ { -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 \sim 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 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 .