Radio Sources in Low‐Luminosity Active Galactic Nuclei. II. Very Long Baseline Interferometry Detections of Compact Radio Cores and Jets in a Sample of LINERs

We have used the VLBA at 5 GHz to observe all galaxies with nuclear radioflux densities above 3.5 mJy found in a VLA survey at 15 GHz of a sample ofnearby LINER galaxies. All galaxies were detected revealing high brightnesstemperature ($T_{b} \ga 10^8$ K) radio sources. Free-free emission is unlikelysince it greatly overpredicts the soft X-ray luminosities. We infer thepresence of AGN-like, non-thermal radio emission most likely powered byunder-fed black holes. Together with our VLA sample we estimate from ourobservations that at least half of LINER galaxies host genuine AGN. We find noevidence for highly inverted radio cores as predicted in the ADAF model: the(non-simultaneous) spectral indices are on average around $\alpha=0.0$. In thetwo brightest sources we detect some extended emission, which appears tooriginate in jets in at least one of these galaxies. Together with the spectralindices this suggests that the nuclear emission at centimeter radio waves islargely dominated by emission from radio jets, very similar to the situation inmore luminous AGN. The energy released in these jets could be a significantfraction of the energy budget in the accretion flow.