Origin of radio emission from nearby low-luminosity active galactic nuclei

We use the observational data in radio, optical, and X-ray wave bands for a sample of active galactic nuclei (AGNs) with measured black hole masses to explore the origin of radio emission from nearby low-luminosity active galactic nuclei (LLAGNs). The maximal luminosity of an advection-dominated accretion flow (ADAF) can be calculated for a given black hole mass, as there is a critical accretion rate above which the ADAF is no longer present. We find that the radio luminosities are higher than the maximal luminosities expected from the ADAF model for most sources in this sample. This implies that the radio emission is predominantly from the jets in these sources. The radio emission from a small fraction of the sources (15/60; referred to as radio-weak sources) in this sample can be explained by the ADAF model. However, comparing the observed multiband emission data with the spectra calculated for the ADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find that neither ADAF nor ADIOS models can reproduce the observed multiband emission simultaneously, with reasonable magnetic field strengths, for these radio-weak sources. A variety of other possibilities are discussed, and we suggest that the radio emission is probably dominated by jet emission even in these radio-weak LLAGNs.