The local radio-loud AGN population is dominated by compact sources named
FR0s. These sources show features, for example the host type, the mass of the
supermassive black hole (SMBH), and the multi-band nuclear characteristics,
that are similar to those of FRI radio galaxies. However, in the radio band,
while FR0 and FRI share the same nuclear properties, the kiloparsec-scale
diffuse component dominant in FRI is missing in FR0s. With this project we
would like to study the parsec-scale structure in FR0s in comparison with that
of FRI sources. To this end we observed 18 FR0 galaxies with the VLBA at 1.5
and 5 GHz and/or with the EVN at 1.7 GHz and produced detailed images at
milliarcsec resolution of their nuclear emission to study the jet and core
structure. All sources have been detected but one. Four sources are unresolved,
even in these high-resolution images; jets have been detected in all other
sources. We derived the distribution of the jet-to-counter-jet ratio of FR0s
and found that it is significantly different from that of FRIs, suggesting
different jet bulk speed velocities. Combining the present data with published
data of FR0 with VLBI observations, we derive that the radio structure of FR0
galaxies shows strong evidence that parsec-scale jets in FR0 sources are mildly
relativistic with a bulk velocity on the order of 0.5c or less. A jet structure
with a thin inner relativistic spine surrounded by a low-velocity sheath could
be in agreement with the SMBH and jet launch region properties.Comment: 12 pages, 16 figures Accepted for the publication in Astronomy and
Astrophysic