The faintest Seyfert radio cores revealed by VLBI

Abstract

In this Letter, we report on dual-frequency European VLBI Network observations of the faintest and least luminous radio cores in Seyfert nuclei, going to sub-millijansky flux densities and radio luminosities around 10 19 WHz-1. We detect radio emission from the nuclear region of four galaxies (NGC4051, NGC4388, NGC4501, and NGC5033), while one (NGC5273) is undetected at the level of ∼ 100 μ Jy. The detected compact nuclei have rather different radio properties: spectral indices range from steep (α>0.7) to slightly inverted (α = -0.1), brightness temperatures vary from TB = 105K to larger than 107K, and cores are either extended or unresolved, in one case accompanied by lobe-like features (NGC4051). 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 NGC5273 nucleus, the presence of an advection-dominated accretion flow is consistent with the radio luminosity upper limit. © 2009. The American Astronomical Society. All rights reserved.