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Direct observation of black hole accretion disc
Posted: December 9, 2009

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The Keck Interferometer (KI) and the UK Infrared Telescope (UKIRT) have joined forces to present some of the first infrared long-baseline interferometric measurements of nearby Active Galactic Nuclei.

The central region of many galaxies show very intense radiation in the form of a strong jet emanating from this central engine. These Active Galactic Nuclei (AGN) are thought to be powered by supermassive black holes accreting gas and dust. Four such AGN were studied by a team lead by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy in Bonn, and included NGC 4151, a relatively nearby galaxy 50 million light years away, and a distant quasar at a distance of more than a billion light years.

UKIRT infrared images of the four target galaxies.The inferred ring-like structure obtained for NGC4151 at the top-left is depicted in the top-right panel (the ring radius is 0.13 light-years, corresponding to an extremely small ~0.5 milli-arcsecond angular size on the sky). The distance to each galaxy is indicated in million light-years, together with the redshift (z) of each galaxy. Image: M. Kishimoto, galaxy images with United Kingdom Infrared Telescope (UKIRT).

The observations allowed the astronomers to probe the mechanics in the black hole vicinity, that is, learn how these galactic monsters consume their surrounding gas and how a strong jet is being emitted from the nucleus. In the case of NGC 4151, the observations revealed a ring-like emission from sublimating dust grains co-existing in the accreting gas, and offered some constraints into the shape and geometry of the accreting material.

“The Keck measurements give us a uniquely high-resolution look at the innermost regions of the active nucleus, and the UKIRT images are very powerful in disentangling the contributions of the host galaxy and the accretion disc in the interferometry data”, says Kishimoto. “While we have got the highest spatial resolution in the IR, this is still a relatively outer region of the central black hole system. We hope to achieve an even higher resolution using telescopes that are much further apart in order to get even closer to the centre, and we also hope to observe many other supermassive black hole systems. When we do that, we will once again be calling on infrared imaging to help us disentangle the results.”

To make these kinds of measurements with a single telescope would require one with a diameter of 100 metres; but by combining the beams of two or more telescopes separated by several tens of kilometres or more, the interference pattern of the two beams can be used to infer what the black hole vicinity looks like. Arrays extended over several kilometres have already been used at radio wavelengths, but not yet in infrared or optical – these currently only employ two or three telescopes, such as the Keck Interferometer which comprises two 10 metre telescopes.

The UKIRT's wide field camera (WFCAM) enabled the astronomers to separate the bright nucleus of the galaxy from the disc of billions of stars in which it is embedded. “This is a slightly unusual use of UKIRT’s WFCAM, in that it capitalizes on some of the lesser-known strengths of the instrument,” says UKIRT Director Gary Davis. “The wide field of view allows us to observe entire galaxies up close, but it also gives the sharp images required to look at more distant objects. It is wonderful to see two observatories on Mauna Kea joining forces in this way: by combining UKIRT imaging with Keck interferometry we can get a full and detailed view of these esoteric objects.”

Using different, independent measurements of the radius of this dust sublimation region –which come from the analysis of the variabilities of the optical and infrared light – the team thinks that they have also possibly started to probe how the accreting material is distributed radially from the black hole that is, how compact or how extended the material distribution is, which will provide the focus for follow-up studies.