Dusty surprise around giant black hole

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Over the last twenty years, astronomers have found that almost all galaxies have a huge black hole at their centre.

Some of these black holes are growing by drawing in matter from their surroundings, creating in the process the most energetic objects in the Universe: active galactic nuclei (AGN). The central regions of these brilliant powerhouses are ringed by doughnuts of cosmic dust [1] dragged from the surrounding space, similar to how water forms a small whirlpool around the plughole of a sink. It was thought that most of the strong infrared radiation coming from AGN originated in these doughnuts.

But new observations of a nearby active galaxy called NGC 3783, harnessing the power of the Very Large Telescope Interferometer (VLTI) at ESO’s Paranal Observatory in Chile [2], have given a team of astronomers a surprise. Although the hot dust — at some 700 to 1000 degrees Celsius — is indeed in a torus as expected, they found huge amounts of cooler dust above and below this main torus [3].

As Sebastian Hönig (University of California Santa Barbara, USA and Christian-Albrechts-Universität zu Kiel, Germany), lead author of the paper presenting the new results, explains, “This is the first time we’ve been able to combine detailed mid-infrared observations of the cool, room-temperature dust around an AGN with similarly detailed observations of the very hot dust. This also represents the largest set of infrared interferometry for an AGN published yet.”

The newly-discovered dust forms a cool wind streaming outwards from the black hole. This wind must play an important role in the complex relationship between the black hole and its environment. The black hole feeds its insatiable appetite from the surrounding material, but the intense radiation this produces also seems to be blowing the material away. It is still unclear how these two processes work together and allow supermassive black holes to grow and evolve within galaxies, but the presence of a dusty wind adds a new piece to this picture.

In order to investigate the central regions of NGC 3783, the astronomers needed to use the combined power of the Unit Telescopes of ESO’s Very Large Telescope. Using these units together forms an interferometer that can obtain a resolution equivalent to that of a 130-metre telescope.

Another team member, Gerd Weigelt (Max-Planck-Institut für Radioastronomie, Bonn, Germany), explains, “By combining the world-class sensitivity of the large mirrors of the VLT with interferometry we are able to collect enough light to observe faint objects. This lets us study a region as small as the distance from our Sun to its closest neighbouring star, in a galaxy tens of millions of light-years away. No other optical or infrared system in the world is currently capable of this.”

These new observations may lead to a paradigm shift in the understanding of AGN. They are direct evidence that dust is being pushed out by the intense radiation. Models of how the dust is distributed and how supermassive black holes grow and evolve must now take into account this newly-discovered effect.

Hönig concludes, “I am now really looking forward to MATISSE, which will allow us to combine all four VLT Unit Telescopes at once and observe simultaneously in the near- and mid-infrared — giving us much more detailed data.” MATISSE, a second generation instrument for the VLTI, is currently under construction.

Notes

[1] Cosmic dust consist of silicate and graphite grains — minerals also abundant on Earth. The soot from a candle is very similar to cosmic graphite dust, although the size of the grains in the soot are ten or more times bigger than typical grain sizes of cosmic graphite grains.

[2] The VLTI is formed from a combination of the four 8.2-metre VLT Unit Telescopes, or the four moveable 1.8-metre VLT Auxiliary Telescopes. It makes use of a technique known as interferometry, in which sophisticated instrumentation combines the light from several telescopes into one observation. Although it usually does not produce actual images, this technique dramatically increases the level of detail that can be measured in the resulting observations, comparable to what a space telescope with a diameter of over 100 metres would measure.

[3] The hotter dust was mapped using the AMBER VLTI instrument at near-infrared wavelengths and the newer observations reported here used the MIDI instrument at wavelengths between 8 and 13 microns in the mid-infrared.

More information

This research was presented in a paper entitled “Dust in the Polar Region as a Major Contributor to the Infrared Emission of Active Galactic Nuclei”, by S. Hönig et al., to appear in the Astrophysical Journal on 20 June 2013.

The team is composed of S. F. Hönig (University of California in Santa Barbara, USA [UCSB]; Christian-Albrechts-Universität zu Kiel, Germany), M. Kishimoto (Max-Planck-Institut für Radioastronomie, Bonn, Germany [MPIfR]), K. R. W. Tristram (MPIfR), M. A. Prieto (Instituto de Astrofísica de Canarias, Tenerife, Spain), P. Gandhi (Institute of Space and Astronautical Science, Kanawaga, Japan; University of Durham, United Kingdom), D. Asmus (MPIfR), R. Antonucci (UCSB), L. Burtscher (Max-Planck-Institut für extraterrestrische Physik, Garching, Germany), W. J. Duschl (Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Germany) and G. Weigelt (MPIfR).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

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