Physical properties of the nuclear region in Seyfert galaxies derived from observations with the European VLBI Network

We report on sensitive dual-frequency (1.7 and 5?GHz) European very long baseline interferometry Network observations of the central region of nine Seyfert galaxies. These sources are among the faintest and least luminous members of a complete sample of nearby (d < 22?Mpc) low-luminosity active galactic nuclei. We detect radio emission on milliarcsecond scale in the nuclei of four galaxies, while for the other five sources we set an upper limit of < 100?mu Jy. In three sources, namely NGC?3227, NGC?3982 and NGC?4138, radio emission is detected at both 1.7 and 5?GHz and it is resolved in two or more components. We describe the structural and spectral properties of these features; we find that in each of these three nuclei there is one component with high brightness temperature (typically TB > 107.5?K) and flat/intermediate spectral index (0.3 = a = 0.6, S(?) ?-a), accompanied by secondary steep spectrum extended components. In these cases, non-thermal emission from jets or outflows is thus the most natural explanation. A faint feature is detected in NGC?4477 at 5?GHz; keeping in mind the modest significance of this detection ( 5?s), we propose the hot corona as the origin of non-thermal emission, on the basis of the unrealistic magnetic field values required by synchrotron self-absorption. Finally, the five non-detected nuclei remain elusive and further observations on intermediate scales will be necessary to investigate their nature.