Sub-parsec radio cores in nearby Seyfert galaxies

TL;DR

This study investigates the small-scale radio properties of Seyfert galaxy cores using VLBI, revealing diverse structures, high water maser detection rates, and insights into emission mechanisms and their relation to X-ray properties.

Contribution

It provides the first comprehensive VLBI survey of local Seyfert galaxy cores, highlighting heterogeneity and new correlations with maser emission and black hole characteristics.

Findings

Majority of VLBI cores show elongated or complex structures.

Water maser detection rate is higher than previous surveys.

Type 1 and Type 2 nuclei have similar VLBI properties, except brightness temperature.

Abstract

We present a census of sub-pc scale properties of the VLBI cores in a complete sample of local Seyfert galaxies. Seventeen out of 23 sources with a VLA detection are detected also with VLBI at 1.7 GHz and/or 5 GHz, with an average monochromatic radio luminosity log [P (5\, GHz) W Hz^{-1} = 19.4. Radio cores are of heterogeneous nature, the majority of them showing elongated structures or accompanied by extra components, broad ranges of brightness temperatures (10^{5}-10^{10} K) and spectral indices (from steep to highly inverted). Interestingly, the detection rate (26%) of water maser emission is considerably higher than that found in previous surveys (around 10%), suggesting that distance biases could significantly affect our knowledge of the actual occurrence of this phenomenon. The VLBI observational properties of type 1 and type 2 nuclei are similar except for the T_{B}, which is on average higher in type 1. These results suggest that both thermal and non-thermal emission are common in low luminosity AGN, with a prevalence of free-free processes among type 2 cores, likely associated to molecular gas. Though limited by the low number statistics, we find no significant correlation between the VLBI radio luminosity and the nuclear X-ray luminosity, the latter appears to be more connected to the tens of pc scales VLA radio emission, rather than to the sub-pc scales, particularly in the most X-ray luminous sources. The X-ray radio-loudness parameter R_{X} = L (6cm)/L(2-10 keV), is on average very low (< log R_X > = -4.8), with comparatively higher R_{X} found for sources with the largest black hole masses and the lowest Eddington ratios, although the radio power does not appear to depend on the accretion rate.