From radio-quiet to radio-silent: low-luminosity Seyfert radio cores

A strong effort has been devoted to understand the physical origin of radio emission from low-luminosity AGNs (LLAGNs), but a comprehensive picture is still missing. We used high-resolution (\le 1 arcsec), multifrequency (1.5, 5.5, 9, and 14 GHz) NSF's Karl G. Jansky Very Large Array (VLA) observations to characterize the state of the nuclear region of ten Seyfert nuclei, which are the faintest members of a complete, distance-limited sample of 28 sources. With the sensitivity and resolution guaranteed by the VLA-A configuration, we measured radio emission for six sources (NGC 3185, NGC 3941, NGC 4477, NGC 4639, NGC 4698, and NGC 4725), while for the remaining four (NGC 0676, NGC 1058, NGC 2685, and NGC 3486) we put upper limits at tens μJy beam^-1 level, below the previous 0.12 mJy beam^-1level of Ho & Ulvestad (2001), corresponding to luminosities down to L \le 10^{19} W Hz^-1 at 1.5 GHz for the highest RMS observation. Two sources, NGC 4639 and NGC 4698, exhibit spectral slopes compatible with inverted spectra (α \le 0, S_{ν } ∝ {ν }^{-α }), hint for radio emission from an optically thick core, while NGC 4477 exhibits a steep (+0.52 ± 0.09) slope. The detected sources are mainly compact on scales ≤arcseconds, predominantly unresolved, except NGC 3185 and NGC 3941, in which the resolved radio emission could be associated to star formation processes. A significant X-ray - radio luminosities correlation is extended down to very low luminosities, with slope consistent with inefficient accretion, expected at such low Eddington ratios. Such sources will be one of the dominant Square Kilometre Array (SKA) population, allowing a deeper understanding of the physics underlying such faint AGNs.