Keck Observatory’s Interferometer Takes Closer look at Supermassive Black Holes

m. kishimoto, mpifr

UKIRT infrared images of the four target galaxies are shown in near-infrared color. Observations with the Keck Interferometer have resolved the inner structure of the bright nucleus in all the four galaxies. The inferred ring-like structure obtained for NGC 4151 at the top-left is depicted in the top-right panel. The ring radius is 0.13 light years, corresponding to an extremely small ~0.5 milli-arcsecond angular size on the sky. The distance to each galaxy is indicated in millions of light-years, together with the redshift (z) of each galaxy.

MAUNA KEA, HI—Astronomers at the W. M. Keck Observatory are using a technique called interferometry to provide new information about central black hole systems in galaxies.

Makoto Kishimoto, of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and an international team of collaborators successfully observed four active galactic nuclei systems with the Keck Interferometer in May 2009. For the first time, the team resolved a QSO (or quasi-stellar object), an energetic galaxy with an active galactic nucleus that lies at a distance of more than a billion light years from Earth.

Being able to observe the central accreting material in such a distant object is “due to the huge effort of the Observatory staff to improve the sensitivity of the Keck Interferometer,” said Kishimoto. The team also made follow up observations of the target galaxies with the United Kingdom Infrared Telescope. The results appear in the December issue of Astronomy & Astrophysics.

The active cores of galaxies are thought to be powered by accreting supermassive black holes. Many cores show very intense radiation across the electromagnetic spectrum. Sometimes the nucleus exhibits a central jet in radio wavelengths, while the black hole’s accreting gas and dust are especially strong in the optical and infrared wavelengths.

Using the intense light coming from the active nuclei of galaxies, astronomers want to “directly see what exactly is going on in the vicinity of accreting supermassive black holes, how the black hole is eating up the surrounding gas, and how the strong jet is being launched,” Kishimoto, the paper’s lead author, said.

But to observe such a distant object sharply enough in infrared wavelengths requires the use of a telescope having a diameter of 100 meters or more. Instead of building such a large telescope, which is currently impossible, a more practical way is to combine the light from two or more telescopes that are roughly 100 meters apart, he explained. This type of instrument, called a long-baseline interferometer, is possible with Keck.

The twin 10-meter Keck telescopes are separated by a distance of 85 meters and are routinely used as an interferometer. When the telescopes work together, astronomers are able to detect an interference pattern to infer what the black hole vicinity looks like, Kishimoto said.

In 2003, astronomer Mark Swain of the Jet Propulsion Laboratory (JPL) along with the Keck Interferometer development team from JPL and Keck Observatory used the interferometer to observe the material accreting around a supermassive black hole called NGC 4151. The observations provided the first direct clue of the inner region of a supermassive black hole system, said Robert Antonucci of the University of California, Santa Barbara and coauthor on the new paper.

“The results looked puzzling in 2003. But with the new data and with more external information, we are quite sure of what we are seeing,” Kishimoto said. According to the team’s results, the Keck interferometer has just begun to resolve the outer region of an active galactic nucleus’s accreting gas where co-existing dust grains are hot enough to sublimate, or transition directly from a solid to a gas, he explained.

Using independent measurements of the radius of the dust sublimation region—which come from the analysis of the variability of the optical and infrared light—the astronomers have started to probe how the accreting material is distributed away from the black hole, Kishimoto said.

Being able to isolate and study the light from extremely close to the black hole itself, where the matter is actually being “swallowed,” lets “us see gas clouds less than a light year from a supermassive black hole, helping us to figure out just how a black hole obtains its ‘food’,” Antonucci said.

These observations, and even more sensitive ones of the future, provide astronomers with a more detailed understanding of how a galaxy’s central black hole system works, Kishimoto said.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.