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