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-luminosityactive galactic nuclei (LLAGNs). The radio emission in most of these nucleidoes 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 radiospectra are indicative of either synchrotron self-absorbed emission from jets,convection-dominated accretion flows (CDAFs) with L >= 10^{-5} L_Edd, or ADAFswith strong (p >= 0.6) outflows. The jet interpretation is favored by threefactors: a) the detection of pc-scale radio extensions, morphologicallyreminiscent of jets, in the five nuclei with the highest peak radioflux-density; b) the domination of parsec-scale jet radio emission overunresolved `core' emission in the three best-studied nuclei; and c) the lack ofany clear correlation between radio spectral shape and black hole mass as wouldbe expected from the dependence of the radio turnover frequency on black holemass in ADAF and CDAF models. A jet domination of nuclear radio emissionimplies significantly lower accretion rates in ADAF-type models than earlierestimated from core radio luminosities.Comment: To appear in Astrophysical Journal Letters, October 1, 200