Possible Origin of Radio Emission from Nonthermal Electrons in Hot Accretion Flows for Low-Luminosity Active Galactic Nuclei

The two components of radio emission, above and below 86 GHz respectively, from the Galactic center source-Sgr A* can be naturally explained by the hybrid of thermal and nonthermal electrons in hot accretion flows (e.g., radiatively inefficient accretion flow, RIAF, e.g., Yuan et al. 2003). We further apply this model to a sample of nearby low-luminosity active galactic nuclei (LLAGNs), which are also believed to be powered by the RIAF. We selected the LLAGNs with only compact radio cores according to the high-resolution radio observations, and the sources observed with jets or jet-like features are excluded. We find that the radio emission of LLAGNs is severely underpredicted by pure RIAF model, and can be naturally explained by the RIAF model with a hybrid electron population consisting of both thermal and nonthermal particles. Our model can roughly reproduce the observed anti-correlation between the mass-corrected radio loudness and Eddington ratio for the LLAGNs in our sample. We further model the spectral energy distributions of each source in our sample, and find that roughly all sources can be well fitted if a small fraction of the steady state electron energy is ejected into the nonthermal electrons. The size of radio emission region of our model is around several thousand gravitational radii, which is also roughly consistent with the recent high-resolution VLBI observations for some nearby LLAGNs.