Radio Sources in Low-Luminosity Active Galactic Nuclei. III

This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at $D \leq 19$ Mpc. We first report new 2 cm (150 mas resolution using the VLA) and 6 cm (2 mas resolution using the VLBA) radio observations of the previously unobserved nuclei in our samples and then present results on the complete survey. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150 mas resolution. Higher (2 mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures $\ga$ $10^8\,$K. The five LLAGNs with the highest core radio fluxes also have pc-scale “jets”. Compact radio cores are almost exclusively found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad H α emission). Only a few “transition” nuclei have compact radio cores; those detected in the radio have optical emission-line diagnostic ratios close to those of LINERs/Seyferts. This indicates that some transition nuclei are truly composite Seyfert/LINER+$\ion{H}{II}$ region nuclei, with the radio core power depending on the strength of the former component. The core radio power is correlated with the nuclear optical “broad” H α luminosity, the nuclear optical “narrow” emission-line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. The scalings suggest that many of the radio-non-detected LLAGNs are simply lower power versions of the radio-detected LLAGNs. The ratio of core radio power to nuclear optical emission-line luminosity increases with increasing bulge luminosity for all LLAGNs. Also, there is evidence that the luminosity of the disk component of the galaxy is correlated with the nuclear emission-line luminosity (but not the core radio power). About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ~150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear ( ≤ 150 mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ~150 galaxies give: $\log P_{\rm 2~cm} = 1.31$( ± 0.16) log M MDO + 8.77 and $\log P_{\rm 2~cm} = 1.89$( ± 0.21$)\,\log\, L_{B}$(bulge$) -0.17$. Low accretion rates ( ≤ $10^{-2}{-}10^{-3}$ of the Eddington rate) are implied in both advection- and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs, with the nuclear radio emission originating in either the accretion inflow onto the massive black hole or from jets launched by this black hole–accretion disk system.