Monster Telescope Combo Spies Feeding Black Hole

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Astronomers are finally getting a look at the extreme processes inside and around black holes. By combining the light from three powerful infrared telescopes, an international team has observed the active gas and dust accretion around a supermassive black hole in the center of a galaxy tens of millions of light-years away.

Resolving these features isn’t just confirmation of how mass accretes onto black holes in centers of galaxies, it’s how the image was taken is a major step in our Earth-bound exploration of the cosmos.

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The team, led by Gerd Weigelt, a director of the Max Planck Institute for Radio Astronomy in Bonn, Germany, resolved the inner ring of debris in the inner region of the active galaxy NGC 3783.

They used the AMBER interferometry instrument of the ESO’s Very Large Telescope Interferometer in Chile to combine the infrared light from three telescopes. Sebastian Hoenig, a postdoctoral researcher at the UC Santa Barbara Department of Physics, called the method “a major milestone toward directly imaging the growth phase of supermassive black holes.”

Interferometry is an imaging method that uses two or more — in this case, three — separate points on a telescope array to observe an object. The light from the individual telescopes in combined or “interfered” to create a complete picture.

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Since each individual image contains high-resolution information, the combined image can give astronomers stunning detail. With separate vantage points, the clarity of the final image is similar to the clarity of a telescope if its diameter were the same as the distance between the two points. In other words, this technique gives astronomers a spectacular view without the impossibly large hardware.

This method was necessary to see such a small object — the ring-shaped distribution of hot dust called a torus in the inner region of the active galaxy NGC 3783. The dust torus has an angular radius of only 0.7 milliarcseconds in the sky. That’s 5 million times smaller than one degree. To resolve something this small, astronomers would need a telescope with a mirror at least 100 meters in diameter. As we don’t have the technology to build such a large telescope, interferometry was the best bet.

This method was able to achieve an angular resolution equivalent to the resolution of a telescope with a diameter of 130 meters, 15 times higher than one of the VLTI telescopes alone. Each telescope has a mirror 8 meters (26 ft) in diameter.

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The torus marks the transition from a more-distant mixture of gas and dust in a toroidal or doughnut-shaped structure to the gaseous disk closer to the black hole. It’s easy to observe because it dominates the infrared emission of active galactic nuclei. Astronomers suspect that this dust torus is the central black hole’s fuel.

Black holes often have millions of times the mass of our sun and are surrounded by hot and bright gaseous disks called accretion disks that spew out radiation as material falls into them. The dust tori surrounding the accretion disks are most likely the reservoir of the material that flows through the accretion disks and finally feeds the growing black holes.

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 about and 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. And since there’s a supermassive black hole in the center of our galaxy, anything astronomers learn about black holes in distant galaxies will help them explain our own.

Image: Artist’s impression of the dusty torus surrounding a black hole. Credit: ESO

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