Astron. Astrophys. 324, 51-64 (1997)

6. Summary

We have obtained new high signal-to-noise molecular absorption spectra towards the Centaurus A radio core. This has allowed the identification of up to 17 components in the spectrum. The components associated with the systemic velocity (LV) correspond to intervening material in the accreted gaseous disk of Cen A, well outside the circumnuclear ring in non-circular motions. It is not possible at present to derive their exact distances from the center. The high velocity (HV) redshifted components are interpreted in terms of a nuclear ring, at about 100 pc distance, with non-circular motions. The spectrum indicate that only the central component has relatively moderate opacities, while the HV components have low opacities. From the comparison between the , HCN, HNC and CS spectra, we deduce that the absorbing gas is diffuse and cold on average. Abundances are compatible with Galactic values.

Comparison with the spectrum obtained 7 years ago by Eckart et al. (1990) revealed no time variations at a level 10%. Changes were expected from similar Galactic experiments. We therefore suggest that this constrains the apparent size of the mm continuum source to be larger than 500 AU (or 0.2mas), unless the absorbing gas is made up of a very large number of very small (a few tens of AU) clumps. In the latter case variations tend to average out over time. The core at 8.4 GHz appears as a point source ( 2mas) in VLBI experiments (Jones et al. 1996). The source size is therefore constrained between 0.2 and 2 mas. The larger size might be only apparent, enlarged through interstellar scattering by the ionised gas in Cen A itself.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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