This is an artist’s view of a dust torus surrounding the accretion disk and the central black hole in active galactic nuclei. Credit: NASA E/PO – Sonoma State University, Aurore Simonnet |
By
combining the light of three powerful infrared telescopes, an
international research team has observed the active accretion phase of a
supermassive black hole in the center of a galaxy tens of millions of
light years away, a method that has yielded an unprecedented amount of
data for such observations. The resolution at which they were able to
observe this highly luminescent active galactic nucleus (AGN) has given
them direct confirmation of how mass accretes onto black holes in
centers of galaxies.
“This
three-telescope interferometry is a major milestone toward directly
imaging the growth phase of supermassive black holes,” said Sebastian
Hoenig, a postdoctoral researcher at the UC Santa Barbara Department of
Physics, and one of the astrophysicists who utilized this technique to
observe the AGN at the center of galaxy NGC 3783. The observation was
led by Gerd Weigelt, a director of the Max Planck Institute for Radio
Astronomy in Bonn, Germany.
Hoenig
described their findings as a ring of hot dust that marks the
transition from a more-distant mixture of gas and dust in a toroidal
(doughnut-shaped) structure, to a gaseous disk closer to the black hole.
The dusty part, he said, is interesting because it dominates the
infrared emission of active galactic nuclei and can be easily observed.
However,
observing the ring of hot dust in NGC 3783 was a challenge for the
astrophysicists. Not only is the ring distant and faint, but the ability
of individual infrared telescopes to resolve distances between actively
accreting objects is also highly limited. Even the largest
optical/infrared telescopes in the world, the Keck telescopes, were not
powerful enough, though they can show objects in the infrared comparable
to about the size of a football field at the distance of the moon.
“In
order to spatially resolve the accretion process onto supermassive
black holes in nearby galaxies, we have to be at least a factor of ten
better,” said Hoenig. To achieve that angular resolution in a single
telescope, it would have to be 130 meters in diameter.
This is the Very Large Telescope Interferometer at the ESO/Paranal Observatory in Chile. Credit: Sebastian Hoenig |
However,
by using the AMBER interferometry instrument to simultaneously combine
the light from three 8-meter telescopes at the Very Large Telescope
Interferometer (VLTI) at the Paranal Observatory in Chile, the research
team was able to achieve the angular resolution needed to observe the
hot dust ring. The Paranal Observatory is operated by the European
Southern Observatories (ESO).
The
combination of the light from the three telescopes was no small feat,
as the tiny differences in the arrival of light in the individual
telescopes have to undergo constant correction with an accuracy of a few
micrometers—roughly ten times smaller than the thickness of a hair,
according to Hoenig.
“The
ESO VLTI provides us with a unique opportunity to improve our
understanding of active galactic nuclei,” said lead researcher Weigelt.
“It allows us to study fascinating physical processes with unprecedented
resolution over a wide range of infrared wavelengths. This is needed to
derive physical properties of these sources.”
Up
next for the research team, which also includes astrophysicists from
the universities of Florence, Grenoble, and Nice, will be the continued
accumulation of information from additional observations toward a highly
detailed image of the active galactic nucleus at galaxy NGC 3783.
“Our
main interest is to learn how supermassive black holes in the centers
of galaxies are fueled, so that they grow to the enormous million to
billion solar mass objects we see today,” said Hoenig.
