Artist's impression of the surroundings of the supermassive black hole in NGC 3783
Wide-field view of the region around galaxy NGC 3783
The active galaxy NGC 3783 in the constellation of Centaurus
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ESO’s Very Large Telescope Interferometer
has gathered the most detailed observations ever of the dust around the
huge black hole at the centre of an active galaxy. Rather than finding
all of the glowing dust in a doughnut-shaped torus around the black
hole, as expected, the astronomers find that much of it is located above
and below the torus. These observations show that dust is being pushed
away from the black hole as a cool wind — a surprising finding that
challenges current theories and tells us how supermassive black holes
evolve and interact with their surroundings.
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”.
Links
Contacts
Sebastian HönigUniversity of California Santa Barbara
USA
Tel: +49 431 880 4108
Cell: +49 176 9995 0941
Email: shoenig@physics.ucsb.edu
Poshak Gandhi
University of Durham
United Kingdom
Email: poshak.gandhi@durham.ac.uk
Gerd Weigelt
Max-Planck-Institut für Radioastronomie
Bonn, Germany
Email: weigelt@mpifr.de
Wolfgang Duschl
Christian-Albrechts-Universität zu Kiel
Kiel, Germany
Email: wjd@astrophysik.uni-kiel.de
