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Astronomers using the European Southern Observatory's (ESO) Very Large Telescope have released images showing in unprecedented detail how matter spirals toward the black hole at the centre of a galaxy, in this case NGC 1097.
"This is the first time that a detailed view of the channelling process of matter, from the main part of the galaxy down to the very end in the nucleus is released," says Almudena Prieto (Max-Planck Institute, Heidelberg, Germany), lead author of the paper describing these results.
"These observations provide astronomers with new insights on how supermassive black holes lurking inside galaxies get fed" adds co-author Dr Witold Maciejewski from the University of Oxford.
Located about 45 million light-years away in the southern constellation Fornax (the Furnace), NGC 1097 is a relatively bright, spiral galaxy seen face-on. An image of NGC 1097 and its small companion, NGC 1097A, was taken in December 2004 with the VIMOS instrument on ESO's Very Large Telescope (VLT). In this image, available as ESO PR Photo 35d/04, NGC 1097 has a strongly elongated, non-circular feature called a bar, and a prominent ring inside the bar.
NGC 1097 is a very moderate example of a galaxy with an active nucleus: it emits more energy than can be accounted for through standard stellar emission. The additional emission is thought to arise from matter (gas and stars) falling into oblivion in a central black hole. However, NGC 1097 possesses a comparatively faint nucleus, and the black hole in its centre must be on a very strict "diet": only a small amount of gas and stars are apparently being swallowed by the black hole at any given moment.
Astronomers have been trying for a long time to understand how the matter is "gulped" down towards the black hole. Directly watching the feeding process requires very high spatial resolution at the centre of galaxies. This can be achieved with adaptive optics [1].
Thus, astronomers [2] obtained images of NGC 1097 with the adaptive optics NACO instrument attached to Yepun, the fourth Unit Telescope of ESO's Very Large Telescope (VLT). These new images probe with unprecedented detail the presence and extent of material in the very proximity of the nucleus. The resolution achieved with the images is about 0.15 arcsecond, corresponding to about 30 light-years across. For comparison, this is only 8 times the distance between the Sun and its nearest star, Proxima Centauri.
The newly released NACO near-infrared images show that the prominent ring in the centre of NGC 1097 consists of more than 300 regions of star formation, a factor four larger than previously known from Hubble Space Telescope images. These regions can be seen as white spots all over the ring in ESO PR Photo 33a/05. At the centre of the ring, a moderate active nucleus is located. Details from the nucleus and its immediate surroundings are however outshone by the overwhelming stellar light of the galaxy seen as the bright diffuse emission inside the ring.
The astronomers therefore applied a masking technique that allowed them to suppress the stellar light (see ESO PR Photo 33b/05). This unveils a bright nucleus at the centre, but mostly a complex central network of filamentary structures spiralling down to the centre.
"Our analysis of the VLT/NACO images of NGC 1097 shows that these filaments end up at the very centre of the galaxy", says co-author Juha Reunanen from ESO.
"This network closely resembles those seen in computer models", adds Witold Maciejewski from the University of Oxford, UK. "The nuclear filaments revealed in the NACO images are the tracers of cold dust and gas being channelled towards the centre to eventually ignite the AGN."
The astronomers also note that the curling of the spiral pattern in the innermost 300 lightyears seem indeed to confirm the presence of a super-massive black hole in the centre of NGC 1097. Such a black hole in the centre of a galaxy causes the nuclear spiral to wind up as it approaches the centre, while in its absence the spiral would be unwinding as it moves closer to the centre.