High-resolution VLA observations of FR0 radio galaxies: properties and
  nature of compact radio sources
  We present the results of Karl G. Jansky Very Large Array (VLA) observations
to study the properties of FR0 radio galaxies, the compact radio sources
associated with early-type galaxies which represent the bulk of the local
radio-loud AGN population. We obtained A-array observations at 1.5, 4.5, and
7.5 GHz for 18 FR0s from the FR0CAT sample: these are sources at $z<0.05$,
unresolved in the FIRST images and spectroscopically classified as low
excitation galaxies (LEG). Although we reach an angular resolution of $\sim$0.3
arcsec, the majority of the 18 FR0s is still unresolved. Only four objects show
extended emission. Six have steep radio spectra, 11 are flat cores, while one
shows an inverted spectrum. We find that 1) the ratio between core and total
emission in FR0s is $\sim$30 times higher than in FRI and 2) FR0s share the
same properties with FRIs from the nuclear and host point of view. FR0s differ
from FRIs only for the paucity of extended radio emission. Different scenarios
were investigated: 1) the possibility that all FR0s are young sources
eventually evolving into extended sources is ruled out by the distribution of
radio sizes; 2) similarly, a time-dependent scenario, where a variation of
accretion or jet launching prevents the formation of large-scales radio
structures, appears to be rather implausible due to the large abundance of
sub-kpc objects 3) a scenario in which FR0s are produced by mildly relativistic
jets is consistent with the data but requires observations of a larger sample
to be properly tested.
