SEARCH FOR CO ABSORPTION FROM A MOLECULAR TORUS IN CYGNUS-A

We have observed millimeter wavelength spectra from the narrow-line ra dio galaxy Cygnus A in a search for CO(1-0) absorption from a hypothes ized dusty molecular torus surrounding the central nucleus. We find no evidence for CO absorption. This result may be consistent with a mode l in which the torus is composed of small, opaque clouds with a coveri ng factor of roughly unity along lines of sight that pass through the torus; the clouds must be small compared to the size of the compact ra dio source. If the CO is thermally excited, the space between the clou ds must be very clear of molecular gas, perhaps implausibly so. Howeve r, it has been suggested to us [M. Rees, private communication (1993)] that the presence of the strong compact radio source could result in a very high rotation temperature for molecules in a compact torus, pos sibly accounting for the lack of absorption. In the absence of such no nthermal excitation effects, our observations argue against tori compo sed of diffuse molecular gas, or ones where the clouds have a covering factor much greater than 1 through the torus.