The origin of radio emission in low-luminosity active galactic nuclei: Jets, accretion flows, or both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of 10(3)-10(4) times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures (T-b greater than or similar to 10(9) K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared with the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.