Alexander, D.M. 1,2, Efstathiou, E. 3, Hough, J.H. 1, Aitken, D. 1, Lutz, D. 4, Roche, P. 5, & Sturm, E. 4
1 Department of Physical sciences, University of Hertfordshire, Hatfield, Hertfordshire, UK
2 International School for Advanced Studies, SISSA, Trieste, Italy
3 Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London, UK
4 Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany
5 Department of Astrophysics, Oxford University, Keble Road, Oxford, UK
We present ISOSWS, ISOPHOT-S and 8 to 13 micron observations of
Centaurus A that show prominent PAH and Silicate features. These and
other data are used to construct a model for the infrared continuum.
We find that in a small nuclear aperture (
arcseconds, 
60 pc)
the
SED is characteristic of emission from a starburst and AGN torus; in
larger apertures an additional component of cirrus emission is required.
The torus model is that proposed by Efstathiou, Hough and Young (1995) for NGC1068, using a tapered disc. The cirrus spectrum is calculated with the code of Efsthathiou, Rowan-Robinson and Siebenmorgen (1998) which includes that emission from large classical particles, small graphite particles and PAHs. The starburst spectrum is also calculated with the above code and is assumed to arise from an ensemble of optically thick giant molecular clouds, centrally illuminated by hot stars.
Based on our model, the torus diameter is estimated to be 3.6 pc and
the best inclination angle of the torus is 45 degrees. This result has
implications for the detectability of tori and 
masers in low power
AGN and in particular for the use of the IRAS 60/25 micron flux ratio
as an indicator of torus inclination.