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Giant radio galaxies are thought to be massive ellipticals powered by accretion of interstellar matter onto a supermassive black hole. Interactions with gas rich galaxies may provide the interstellar matter to feed the active galactic nucleus (AGN). To power radio lobes that extend up to distances of hundreds of kiloparsecs, gas has to be funneled from kiloparsec size scales down to the AGN at rates of ~ 1 Modot yr-1 during geq 108 years. Therefore, large and massive quasi-stable structures of gas and dust should exist in the deep interior of the giant elliptical hosts of double lobe radio galaxies. Recent mid-infrared observations with ISO revealed for the first time a bisymmetric spiral structure with the dimensions of a small galaxy at the centre of Centaurus A [17]. The spiral was formed out of the tidal debris of accreted gas-rich object(s) and has a dust morphology that is remarkably similar to that found in barred spiral galaxies (see Figure 5). The observations of the closest AGN to Earth suggest that the dusty hosts of giant radio galaxies like CenA, are "symbiotic" galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

Figure 5

Figure 5. The ISO 7µm emission (dark structure; [17]) and VLA 20 cm continuum in contours [1], overlaid on an optical image from the Palomar Digital Sky Survey. The emission from dust with a bisymmetric morphology at the centre is about 10 times smaller than the overall size of the shell structure in the elliptical and lies on a plane that is almost parallel to the minor axis of its giant host. Whereas the gas associated to the spiral rotates with a maximum radial velocity of 250 km s-1, the ellipsoidal stellar component rotates slowly approximately perpendicular to the dust lane [27]. The synchrotron radio jets shown in this figure correspond to the inner structure of a double lobe radio source that extends up to 5° ( ~ 300 kpc) on the sky. The jets are believed to be powered by a massive black hole located at the common dynamic center of the elliptical and spiral structures.

The barred spiral at the centre of CenA has dimensions comparable to that of the small Local Group galaxy Messier 33. It lies on a plane that is almost parallel to the minor axis of the giant elliptical. Whereas the spiral rotates with maximum radial velocities of ~ 250 km s-1, the ellipsoidal stellar component seems to rotate slowly (maximum line-of-sight velocity is ~ 40 km s-1) approximately perpendicular to the dust lane. The genesis, morphology, and dynamics of the spiral formed at the centre of CenA are determined by the gravitational potential of the elliptical, much as a usual spiral with its dark matter halo. On the other hand, the AGN that powers the radio jets is fed by gas funneled to the center via the bar structure of the spiral. The spatial co-existence and intimate association between these two distinct and dissimilar systems suggest that Cen A is the result from a cosmic symbiosis.

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