Structure of NGC 5128 (Centaurus A) at submillimetre wavelengths

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Abstract
We present new small-beam submillimetre maps and multi-aperture photometry at three epochs and several wavelengths of the nearest giant radio galaxy, NGC 5128 (Centaurus A). The nucleus appears as an unresolved central source (FWHM < 6 arcsec at 800 and at 450 µm), emitting 7.3 Jy at 800 µm. Between 1990 May and 1991 June the central source brightness appeared to decrease by ~ 15 per cent longwards of this wavelength. The emission from the central source is evidently nonthermal in origin, with a very flat spectrum at least down to 800 µm, and very possibly even down to 350 µm. The millimetre–submillimetre spectral shape supports the identification of the nucleus as a blazar by Bailey et al.; even for such an object it is amongst the flattest known at these frequencies. At 800 µm a diffraction-limited map (13.5 arcsec FWHM) shows a ridge of emission extending 30 arcsec in PA 145° on either side of the central source, parallel to the near-infrared polarization vector of the nucleus. This structure is also evident at 450 µm and appears to have a thermal spectrum. We interpret it as emission from warm dust in a circumnuclear torus such as proposed by Israel et al., with its symmetry axis parallel to the inner radio and X-ray jets and which may be involved in their confinement. Emission at both 800 and 450 µm also extends along the axis of the inner jets, again with an apparently thermal spectrum. On the 800-µm map at 20-arcsec resolution, clumpy low-level emission from the well-known warped dust lane is detected out to > 2 arcmin (3 kpc) from the nucleus. Stronger emission within ~ 60 arcsec of the nucleus appears to be associated with molecular material, some of which may be concentrated in a ring at this radius. Further out the emission appears to arise from a diffuse ‘cirrus’ dust component. An estimate of the total 800-µm flux from this component, combined with published estimates of its far-infrared and CO emission, implies an overall gas-to-dust ratio in Cen A of ~ 330, somewhat higher than the Galactic value, but lower than that previously derived from far-infrared and CO measures. This indicates that some of the missing dust in the latter determinations may be hidden in the form of cool (< 30 K) material.

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