A high-frequency radio survey of low-luminosity active galactic nuclei

ABSTRACT We investigate the high-frequency radio spectra of twenty low-luminosity activegalactic nuclei (LLAGNs) with compact radio cores. Our millimetre survey withthe Nobeyama Millimetre Array (NMA) and analyses of submillimetre archivaldata that had been obtained with the Submillimetre Common User BolometerArray (SCUBA) on the James Clerk Maxwell Telescope (JCMT) reveal the fol-lowing properties. At least half of the LLAGNs show inverted spectra between 15and 96 GHz; we use published data at 15 GHz with the Very Large Array (VLA)in a 0.15-arcsec resolution and our measurements at 96 GHz with the NMA ina 7-arcsec resolution. The inverted spectra are not artificially made due to theirunmatched beam sizes, because of little diffuse contamination from dust, Hii re-gions, or extended jets in these LLAGNs. Such high-frequency inverted spectraare apparently consistent with a ‘submillimetre bump’, which is predicted by anadvection-dominated accretion flow (ADAF) model. We find a strong correlationbetween the high-frequency spectral index and low-frequency core power mea-sured with very-long-baseline-interferometry (VLBI) instruments. The invertedspectra were found exclusively in low-core-power sources, while steep spectrawere in high-core-power ones with prominent pc-scale jets. This suggests thatthe ADAF and nonthermal jets may coexists. The flux ratios between disc andjet seem to be different from LLAGN to LLAGN; disc components can be seenin nuclear radio spectra only if the jets are faint.Key words: galaxies: active – galaxies: Seyfert – radio continuum: galaxies –submillimetre.