An international research team led by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy (Bonn, Germany) has unveiled some of the first IR long-baseline interferometric measurements of nearby active galactic nuclei, done with the Keck interferometric telescope in Hawaii. The measurements indicate a ring-like emission from sublimating dust grains, and the radius of the ring to yield insights into the morphology of the accreting material around the black hole in these nuclei.1
The nuclei of many galaxies show very intense radiation from x-ray to optical, IR, and radio, where the nucleus sometimes exhibits a strong jet. These active galactic nuclei are thought to be powered by accreting supermassive black holes. The accreting gas and dust are especially bright in the optical and IR regions.
Up to a billion light-years away
Makoto Kishimoto and his team successfully observed four such active galactic nuclei with the Keck Interferometer. Their target sources included NGC 4151 (a relatively nearby galaxy only 50 million light years away) and 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 to obtain up-to-date near-IR images of the galaxies.
Astronomers have been trying to directly see how a supermassive black hole eats up the surrounding gas and how the strong jet is launched. However, to spatially resolve such a distant object at IR wavelengths, a telescope with a diameter of the order of 100 m would be required. Instead of building such a huge telescope, a more practical way is to combine the beams from two or more telescopes spaced on the order of 100 m apart to detect an interference pattern and infer what the black-hole vicinity looks like. The two Keck telescopes, 10 m diameter each and spaced 85 m apart, form the so-called Keck interferometer (KI).
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 in the near IR, have been particularly difficult. (The difficulty intensifies as the observation wavelength grows shorter.)
Until recently, only one active galactic nucleus had been successfully observed with the KI. This galaxy, NGC 4151, is one of the brightest of these sources in the optical/IR wavelengths. The new, more-sensitive observations of four galaxies have led to quite a clear picture of what is being resolved: a ring-like emission of dust grains, co-existing in the accreting gas, and which are hot enough to be sublimating.
1. M. Kishimoto et al., Astronomy & Astrophysics 207, L57 (2009).