Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores

We report a high-resolution (02), 15 GHz survey of a sample of 48 low-luminosity active galactic nuclei with the Very Large Array. 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] λ6300) 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 (α ≥ -0.3, S_ν ∝ ν^α). While the present observations are consistent with the radio emission originating in star-forming regions (the brightness temperatures are ≥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 (≲1 pc), of high brightness temperature (T_b ≳ 10⁸ 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 Hα) and type 2 (without broad Hα) 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.