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(2) at 5 GHz to observe all galaxies with nuclear radi o flux densities above 3.5 mJy found in a VLA survey at 15 GHz of a sample of nearby LINER galaxies. All galaxies were detected revealing high brightn ess temperature (T-b greater than or similar to 10(8) K) radio sources. Fre e-free emission is unlikely since it greatly overpredicts the soft X-ray lu minosities. We infer the presence of active galactic nucleus (AGN)-like, no nthermal radio emission most likely powered by underfed black holes. Togeth er with our VLA sample we estimate from our observations that at least one- half of LINER galaxies host genuine AGNs. We find no evidence for highly in verted radio cores as predicted in the advection-dominated accretion flow m odel: the (nonsimultaneous) spectral indices are on average around alpha = 0.0. In the two brightest sources we detect some extended emission, which a ppears to originate in jets in at least one of these galaxies. Together wit h the spectral indices this suggests that the nuclear emission at centimete r radio waves is largely dominated by emission from radio jets, very simila r to the situation in more luminous AGNs. The energy released in these jets could be a significant fraction of the energy budget in the accretion flow .