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

Abstract

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 ($L_{[\mathrm{O}\mathrm{III}]}$) 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 ($L_{\mathrm{core}}$) is found to scale with BH mass ($M_{\mathrm{BH}}$) and [O~III] luminosity. Below $M_{\mathrm{BH}}\sim$10${}^{6.5}$ M${}_{\odot }$, stellar processes from non-jetted HII galaxies dominate with $L_{\mathrm{core}}\propto M_{\mathrm{BH}}^{0.61\pm 0.33}$ and $L_{\mathrm{core}}\propto L_{[\mathrm{O}\mathrm{III}]}^{0.79\pm 0.30}$. Above $M_{\mathrm{BH}}\sim$10${}^{6.5}$ M${}_{\odot }$, accretion-driven processes dominate with $L_{\mathrm{core}}\propto M_{\mathrm{BH}}^{1.5-1.65}$ and $L_{\mathrm{core}}\propto L_{[\mathrm{O}\mathrm{III}]}^{0.99-1.31}$ for active galaxies: radio-quiet/loud LINERs, Seyferts and jetted HII galaxies always display (although low) signatures of radio-emitting BH activity, with $L_{1.5\mathrm{GHz}}\gtrsim$10${}^{19.8}$ W Hz${}^{-1}$ and $M_{\mathrm{BH}}\gtrsim 10^7$ 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]

Accpeted for publication on MNRAS (27 pages, 9 Figures, 3 Tables, Appendix A)