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 radio 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 brightness temperature (T-b greater than or similar to 10(8) K) radio sources. Free-free emission is unlikely since it greatly overpredicts the soft X-ray luminosities. We infer the presence of active galactic nucleus (AGN)-like, nonthermal radio emission most likely powered by underfed black holes. Together 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 inverted radio cores as predicted in the advection-dominated accretion flow model: the (nonsimultaneous) spectral indices are on average around alpha = 0.0. In the two brightest sources we detect some extended emission, which appears to originate in jets in at least one of these galaxies. Together with the spectral indices this suggests that the nuclear emission at centimeter radio waves is largely dominated by emission from radio jets, very similar 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.