Radio structures of the nuclei of nearby Seyfert galaxies and the nature of the missing diffuse emission

TL;DR

This study uses high-resolution radio observations to analyze the nuclear structures of nearby Seyfert galaxies, revealing unresolved cores, diffuse emission, and missing flux at parsec scales, which suggests complex jet and environment interactions.

Contribution

It provides detailed high-resolution radio imaging of Seyfert nuclei, highlighting the presence of diffuse emission and flux discrepancies, advancing understanding of their nuclear structures.

Findings

Most nuclei are unresolved at VLA resolution.

VLBA reveals missing flux at parsec scales, indicating extended low-surface brightness components.

Steep-spectrum sources often have undetected flux likely from disrupted jets.

Abstract

We present archival high spatial resolution VLA and VLBA data of the nuclei of seven of the nearest and brightest Seyfert galaxies in the Southern Hemisphere. At VLA resolution (~0.1 arcsec), the nucleus of the Seyfert galaxies is unresolved, with the exception of MCG-5-23-16 and NGC 7469 showing a core-jet structure. Three Seyfert nuclei are surrounded by diffuse radio emission related to star-forming regions. VLBA observations with parsec-scale resolution pointed out that in MRK 1239 the nucleus is clearly resolved into two components separated by ~30 pc, while the nucleus of NGC 3783 is unresolved. Further comparison between VLA and VLBA data of these two sources shows that the flux density at parsec scales is only 20% of that measured by the VLA. This suggests that the radio emission is not concentrated in a single central component, as in elliptical radio galaxies, and an additional low-surface brightness component must be present. A comparison of Seyfert nuclei with different radio spectra points out that the ``presence'' of undetected flux on milli-arcsecond scale is common in steep-spectrum objects, while in flat-spectrum objects essentially all the radio emission is recovered. In the steep-spectrum objects, the nature of this ``missing'' flux is likely due to non-thermal AGN-related radiation, perhaps from a jet that gets disrupted in Seyfert galaxies because of the denser environment of their spiral hosts.