2012MNRAS.426..588B

We report on sensitive dual-frequency (1.7 and 5GHz) 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 < 22Mpc) 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 < ∼ 100µJy. In three sources, namely NGC3227, NGC3982 and NGC4138, radio emission is detected at both 1.7 and 5GHz 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 T_B> 10^7.5K) and flat/intermediate spectral index (0.3 ≤ α ≤ 0.6, S(ν) ∼ ν^–α), 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 NGC4477 at 5GHz; keeping in mind the modest significance of this detection ( ∼ 5σ), 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.