Evidence for Jet Domination of the Nuclear Radio Emission in Low-luminosity Active Galactic Nuclei

We present simultaneous, sub-arcsecond (<=50pc) resolution 5 GHz, 8.4 GHz, and 15 GHz VLA observations of a well-defined sample of sixteen low-luminosity active galactic nuclei (LLAGNs). The radio emission in most of these nuclei does not show the rising spectrum (0.2 <= s <= 1.3, L_nu propto nu^{s}) expected from thermal electrons in an advection dominated accretion flow (ADAF) with or without weak to moderately-strong outflows. Rather, the flat radio spectra are indicative of either synchrotron self-absorbed emission from jets, convection-dominated accretion flows (CDAFs) with L >= 10^{-5} L_Edd, or ADAFs with strong (p >= 0.6) outflows. The jet interpretation is favored by three factors: a) the detection of pc-scale radio extensions, morphologically reminiscent of jets, in the five nuclei with the highest peak radio flux-density; b) the domination of parsec-scale jet radio emission over unresolved `core' emission in the three best-studied nuclei; and c) the lack of any clear correlation between radio spectral shape and black hole mass as would be expected from the dependence of the radio turnover frequency on black hole mass in ADAF and CDAF models. A jet domination of nuclear radio emission implies significantly lower accretion rates in ADAF-type models than earlier estimated from core radio luminosities.