A search for molecular absorption in the tori of active galactic nuclei

We describe a search for molecular absorption at millimetre wavelengths associated with dusty molecular tori in active galactic nuclei (AGN). The sample observed consists of 11 flat-spectrum radio sources known to have red optical to infra-red colours plus two steep-spectrum narrow-line radio galaxies. Spectra of the sources were obtained in the 3-, 2-and 1.3-millimetre bands at frequencies corresponding to common molecular transitions of CO, HCO + , HCN and CS at the AGN redshift. The observations were thus sensitive to absorption taking place either in dusty molecular tori surrounding the AGN nucleus, or in molecular clouds in the AGN host galaxy. No absorptions were detected in any of the sources. We calculated upper limits to the column density in molecular absorption, using an excitation temperature of 10 K, to be