The eMERLIN and EVN view of FR0 radio galaxies
  We present the results from high-resolution observations carried out with the
eMERLIN UK-array and the European VLBI network (EVN) for a sample of 15 FR0s,
i.e. compact core-dominated radio sources associated with nearby early-type
galaxies (ETGs) which represent the bulk of the local radio galaxy population.
The 5-GHz eMERLIN observations available for 5 objects exhibit sub-mJy core
components, and reveal pc-scale twin jets for 4 out of 5 FR0s once the eMERLIN
and JVLA archival visibilities data are combined. The 1.66-GHz EVN observations
available for 10 FR0s display one- and two-sided jetted morphologies and
compact cores. The pc-scale core emission contributes, on average, to about one
tenth of the total extended radio emission, although we note a increasing core
contribution for flat/inverted-spectrum sources. We found an unprecedented
linear correlation between the pc-scale core luminosity
($\sim$10$^{21.3}$-10$^{23.6}$ W Hz$^{-1}$) and [O III] line luminosity,
generally considered as proxy of the accretion power, for a large sample of
LINER-type radio-loud low-luminosity active nuclei, all hosted in massive ETGs,
which include FR0s and FRIs. This result represents further evidence of a
common jet-disc coupling in FR0s and FRIs, despite they differ in kpc-scale
radio structure. For our objects and for other FR0 samples reported in the
literature, we estimate the jet brightness sidedness ratios, which typically
range between 1 and 3. This parameter roughly gauges the jet bulk Lorentz
factor $\Gamma$, which turns out to range between 1 and 2.5 for most of the
sample. This corroborates the scenario that FR0s are characterized by
mildly-relativistic jets, possibly as a result of lower spinning black holes
(BHs) than the highly-spinning BHs of relativistic-jetted radio galaxies, FRIs.
