Radio sources in low-luminosity active galactic nuclei. I. VLA detections of compact, flat-spectrum cores

We report a high-resolution (0."2), 15 GHz survey of a sample of 48 low-luminosity active galactic nuclei with the Very Large Array.(5) Compact radio emission has been detected above a flux density of 1.1 mJy in 57% (17 of 30) of low-ionization nuclear emission-line region (LINER) nuclei and low-luminosity Seyfert galaxies. The 2 cm radio power is significantly correlated with the emission-line ([O I] lambda 6300) luminosity. Using radio fluxes at other frequencies from the literature, we find that at least 15 of the 18 detected radio cores have a flat to inverted spectrum (alpha greater than or equal to -0.3, S-v proportional to v(alpha)). While the present observations are consistent with the radio emission originating in star-forming regions (the brightness temperatures are greater than or equal to 10(2.5-4.5) K), higher resolution radio observations of 10 of the detected sources, reported in an accompanying paper, show that the cores are very compact (less than or similar to1 pc), of high brightness temperature (T-b greater than or similar to 10(8) K), and probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our results suggest that at least 50% of low-luminosity Seyfert galaxies and LINERs in the sample are accretion powered, with the radio emission presumably coming from jets or advection-dominated accretion flows. We have detected only 1 of 18 "transition" (i.e., LINER + H II) nuclei observed, indicating that their radio cores are significantly weaker than those of "pure" LINERs. Compact 2 cm radio cores are found in both type 1 (i.e., with broad H alpha) and type 2 (without broad H alpha) nuclei. There is weak evidence, limited in significance by small numbers, that low-luminosity active galactic nuclei with compact radio cores exhibit radio ejecta preferentially aligned along the rotation axis of the galaxy disk. If this result were confirmed by a larger sample, it would lend support to the idea that the misalignment of accretion disks with the galaxy stellar disk in more luminous Seyfert galaxies is a result of radiation-pressure-induced warping of their accretion disks.