LeMMINGs. I. The eMERLIN legacy survey of nearby galaxies. 1.5-GHz
  parsec-scale radio structures and cores
  We present the first data release of high-resolution ($\leq0.2$ arcsec)
1.5-GHz radio images of 103 nearby galaxies from the Palomar sample, observed
with the eMERLIN array, as part of the LeMMINGs survey. This sample includes
galaxies which are active (LINER and Seyfert) and quiescent (HII galaxies and
Absorption line galaxies, ALG), which are reclassified based upon revised
emission-line diagrams. We detect radio emission $\gtrsim$ 0.2 mJy for 47/103
galaxies (22/34 for LINERS, 4/4 for Seyferts, 16/51 for HII galaxies and 5/14
for ALGs) with radio sizes typically of $\lesssim$100 pc. We identify the radio
core position within the radio structures for 41 sources. Half of the sample
shows jetted morphologies. The remaining half shows single radio cores or
complex morphologies. LINERs show radio structures more core-brightened than
Seyferts. Radio luminosities of the sample range from 10$^{32}$ to 10$^{40}$
erg s$^{-1}$: LINERs and HII galaxies show the highest and the lowest radio
powers respectively, while ALGs and Seyferts have intermediate luminosities. We
find that radio core luminosities correlate with black hole (BH) mass down to
$\sim$10$^{7}$ M$_{\odot}$, but a break emerges at lower masses. Using [O III]
line luminosity as a proxy for the accretion luminosity, active nuclei and
jetted HII galaxies follow an optical fundamental plane of BH activity,
suggesting a common disc-jet relationship. In conclusion, LINER nuclei are the
scaled-down version of FR I radio galaxies; Seyferts show less collimated jets;
HII galaxies may host weak active BHs and/or nuclear star-forming cores; and
recurrent BH activity may account for ALG properties.
