Interferometry Advances Black Hole Observations

A new technique that combines the light of three powerful infrared telescopes allows astronomers to see objects in that wavelength 130 million light-years away without having to build singularly massive and expensive telescopes specifically designed for that purpose.

An international collaboration, including researchers from the University of California, Santa Barbara, and the Max Planck Institute for Radio Astronomy in Bonn, Germany, used the AMBER interferometry instrument to combine the light from three 8-m telescopes at the Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile.

This is an artist's view of a dust torus surrounding the accretion disk and the central black hole in active galactic nuclei. [Image: NASA/EPO — Sonoma State University, Aurore Simonnet (http://epo.sonoma.edu/)]

The initial observations using the Keck Telescope in Hawaii, which is capable of resolving in the infrared objects the size of a football field from a distance of more than 200,000 miles away, were not powerful enough to resolve the accretion ring surrounding the black hole in the center of galaxy NGC 3783.

According to Sebastian Hoenig, a postdoctoral researcher at the UC Santa Barbara, they would need a telescope that was 10 times more powerful than Keck. To achieve that angular resolution, such a telescope would require a diameter of 130 m. The largest telescopes being built right now are only 30-40 m.

However, using the interferometry instrument, the researchers achieved the angular resolution needed to see the hot ring of dust, called an active galactic nucleus. To achieve such a resolution, the telescopes had to be constantly corrected, to an accuracy of a few micrometers, for the difference in the arrival of light among the three apertures.

The Very Large Telescope (VLT) at the European Southern Observatories' Cerro Paranal observing site. Located in the Atacama Desert of Chile, the site is more than 2600 m above sea level, providing incredibly dry, dark viewing conditions. The VLT is the world’s most advanced optical instrument, consisting of four unit telescopes with main mirrors 8.2 m in diameter and four movable 1.8-m-diameter auxiliary telescopes. The telescopes can work together in groups of two or three to form a giant interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes. (Image: Iztok Boncina/ESO)

The overarching goal of the researchers is to learn how the supermassive black holes that lie in the center of galaxies are fueled.

"The [European Southern Observatories'] VLTI provides us with a unique opportunity to improve our understanding of active galactic nuclei," said lead researcher Gerd 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 nucleus at galaxy NGC 3783.

For more information, visit: www.ucsb.edu