An international research team presents some of the first long-baseline interferometric measurements in the infrared towards nearby active galactic nuclei with the Keck Interferometric Telescope in Hawaii. The team, led by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy in Bonn, Germany, found the measurements to indicate a ring-like emission from sublimating dust grains and its radius to yield insights into the morphology of the accreting material around the black hole in these nuclei.

The nuclei of many galaxies show intense radiation from X-ray to optical, infrared, and radio where the nucleus sometimes exhibits a strong jet. Scientists think these active galactic nuclei (AGN) are powered by accreting supermassive black holes. The accreting gas and dust are especially bright in optical and infrared radiation.

In May 2009, Kishimoto and his team observed four such AGN with the Keck Interferometer at Hawaii. Their target sources included NGC 4151, a relatively nearby galaxy only 50 million light-years away, and also a distant quasar at redshift 0.108 (corresponding to a distance of more than a billion light-years). The United Kingdom Infrared Telescope (UKIRT) was used to follow up the Keck observations in order to obtain up-to-date near-infrared images of the galaxies.

Astronomers have been trying to see how the supermassive black hole is eating up the surrounding gas and how the strong jet is being launched around the black hole. However, to spatially resolve such a distant object at infrared wavelengths, a telescope having a diameter of 328 feet (100 meters) would be required. Instead of building such a huge telescope, a more practical way is to combine the beams from two or more telescopes that are far apart in order to detect an interference pattern of the two beams and infer what the black hole vicinity looks like.

"The technique we are using is very new and very demanding in terms of observing conditions and data analysis," said Robert Antonucci from the University of California at Santa Barbara.

In the future, there will be many telescopes, or a telescope array extended over several miles. Such arrays have already been used at radio wavelengths, but not yet at infrared or optical. Optical/infrared interferometry is still in an early stage — currently using two or three telescopes. A prototype array is formed by the two Keck telescopes of 33 feet (10 meters) diameter each.

While the Keck Interferometer has been used to observe many stars in our galaxy, it has been quite challenging to observe objects outside of our galaxy, especially supermassive black holes in the nuclei of other galaxies. This is simply because they are much fainter. Interferometric observations of such objects, especially at the shorter side of infrared wavelengths or near-infrared, has been particularly difficult. The difficulty is directly related to the size of the wavelength — e.g., in the radio wavelength that is much longer than infrared wavelengths, the interferometric technique already is used routinely.

Until recently, only one AGN has been observed successfully with the Keck Interferometer. This galaxy, NGC 4151, is one of the brightest of these sources in the optical/infrared wavelengths. The new, more sensitive observations of four galaxies have lead to a clear picture of what is being resolved — a ring-like emission of dust grains, co-existing in the accreting gas, which are hot enough to be sublimating.

Utilizing different, independent measurements of the radius of this dust sublimation region, which come from the analysis of the variabilities of the optical and infrared light, the team thinks that they possibly have started to probe how the accreting material is distributed radially from the black hole — how compact or how extended the material distribution is.

"While we have got the highest spatial resolution in the infrared, this is still a relatively outer region of the central black hole system," said Kishimoto. "We hope to achieve an even higher resolution using telescopes that are much further apart in order to get even closer to the center, and we also hope to observe many other supermassive black hole systems."