LeMMINGs. III. The e-MERLIN Legacy Survey of the Palomar sample. Exploring the origin of nuclear radio emission in active and inactive galaxies through the [O III] -- radio connection

What determines the nuclear radio emission in local galaxies? We combine optical [O III] line emission, robust black hole (BH) mass estimates, and high-resolution e-MERLIN 1.5-GHz data, from the LeMMINGs survey, of a statistically-complete sample of 280 nearby, optically active (LINER and Seyfert) and inactive HII and Absorption line galaxies [ALG]) galaxies. Using [O III] luminosity ($L_{[\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}]}$) as a proxy for the accretion power, local galaxies follow distinct sequences in the optical-radio planes of BH activity, which suggest different origins of the nuclear radio emission for the optical classes. The 1.5-GHz radio luminosity of their parsec-scale cores ($L_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}$) is found to scale with BH mass ($M_{\mathrm{B}\mathrm{H}}$) and [O~III] luminosity. Below $M_{\mathrm{B}\mathrm{H}}\sim$10${}^{6.5}$ M${}_{\odot }$, stellar processes from non-jetted HII galaxies dominate with $L_{\rm core} \propto M_{\rm BH}^{0.61\pm0.33}$and$L_{\rm core} \propto L_{\rm [O~III]}^{0.79\pm0.30}$. Above $M_{\mathrm{B}\mathrm{H}}\sim$10${}^{6.5}$ M${}_{\odot }$, accretion-driven processes dominate with $L_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}\propto M_{\mathrm{B}\mathrm{H}}^{1.5-1.65}$ and $L_{\rm core} \propto L_{\rm [O~III]}^{0.99-1.31}$ for active galaxies: radio-quiet/loud LINERs, Seyferts and jetted HII galaxies always display (although low) signatures of radio-emitting BH activity, with $L_{\rm 1.5, GHz}\gtrsim$10$^{19.8}$WHz$^{-1}$and$M_{\rm BH}\gtrsim10^{7}$M$_{\odot}$, on a broad range of Eddington-scaled accretion rates ($\dot{m}$). Radio-quiet and radio-loud LINERs are powered by low-$\dot{m}$ discs launching sub-relativistic and relativistic jets, respectively. Low-power slow jets and disc/corona winds from moderately high to high-$\dot{m}$ discs account for the compact and edge-brightened jets of Seyferts, respectively. Jetted HII galaxies may host weakly active BHs. Fuel-starved BHs and recurrent activity account for ALG properties. [abridged]