The faintest Seyfert radio cores revealed by VLBI

TL;DR

This study uses VLBI observations to detect and analyze the faintest radio cores in Seyfert galaxies, revealing diverse properties and underlying mechanisms at very low luminosities.

Contribution

First VLBI detection of sub-mJy Seyfert nuclei, showing diverse radio properties and suggesting multiple physical origins for faint radio cores.

Findings

Detected radio cores in four Seyfert galaxies at sub-mJy levels.

Observed a range of spectral indices and brightness temperatures indicating different emission mechanisms.

The undetected nucleus suggests an ADAF as a plausible explanation.

Abstract

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.