Feeding the central engine in giant radio galaxies

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 M⊙ yr −1 during ≥10 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 (Mirabel et al. 1999). 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. 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. 1. A barred spiral at the centre of Centaurus A The mid-infrared and submillimeter observations of the dust emission in CenA revealed a bisymmetric structure of 5 (∼ 5 kpc for a distance of 3.5 Mpc) in total length (Mirabel et al. 1999). Figure 1 shows that in contrast to the optical dark lanes which show a wide and somewhat chaotic distribution, the structure of the mid-infrared emission is remarkably thin, smooth and bisymmetric. The emitting dust is less extended and displaced from the most prominent optical dark lanes. This displacement is not due to major differences between the spatial distributions of the cold and very warm dust components. In a three dimensional tilted and warped disk, projection effects play an important role, and the optical appearance of the dark lanes in a luminous ellipsoidal system may be affected by relatively small amounts of cold dust in the outer parts of the bending disk that are located in the foreground side of the luminous ellipsoidal distribution of stars.