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Astron. Astrophys. 353, 72-76 (2000)

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1. Introduction

An object of intense astrophysical interest is our closest active radio galaxy, Centaurus A (NGC 5128), only 3.25 Mpc distant. Its complex and intruiging optical structure was noted as early as 1847 by Sir John Herschel, working at the Cape of Good Hope. In the Hubble Atlas , Centaurus A is classified as the possible merger of an elliptical galaxy with a spiral galaxy, following the earlier suggestion by Baade & Minkowski (1954) that NGC 5128 represents two galaxies in collision.

Centaurus A has been imaged with the Infrared Space Observatory (ISO, Kessler et al. 1996) at wavelengths of 7 and 15 µm (Mirabel et al. 1999), to reveal the emission of very small dust grains and of macromolecules. The galaxy has also been optically imaged at the prime-focus of the 4-m Cerro Tololo reflector, to probe the large dust grain distribution.

Indeed, the smallest dust grains (radii 0.01 [FORMULA] and less) which may transiently be very hot (up to [FORMULA] 1000 K) and readily observed by the Infrared Space Observatory in the mid-infrared, contribute little to the extinction at optical wavelengths. Mid-infrared ISOCAM images (Cesarsky et al. 1996) do not trace the distribution of the large ([FORMULA] 0.1 µm) grains, which however dominate the extinction in visible light.

As reviewed by Greenberg & Li (1996), the large (tenth micron) grains are essentially always cold; they would need to be placed at a remarkably short distance of only 0.1 parsec from the intense radiation of an O5 star before the ice would evaporate. Indeed, these large grains are typically too cold (T [FORMULA] 20 K in the diffuse ISM, Block 1996) to be observed in emission shortward of 100 [FORMULA], so these dust grains were systematically missed by the Infrared Astronomy Satellite IRAS (see Sauvage & Thuan 1994). Yet it is these grains which we see in any Atlas photograph of a dusty galaxy.

In order to probe the distribution of dust grains of all sizes and temperatures, we have developed a unified method by combining optical with ISOCAM mid-infrared images. An example of this technique has already been applied to the Whirlpool Galaxy M 51 (Block et al. 1997). Such a methodology shows the large, cold dust grains in extinction and the very small hot grains and macromolecules in emission (negative extinction), so that optical minus mid-infrared imaging enhances all populations of dust grains and of macromolecules (Block et al. 1997).

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999