Fuel for the Black Hole

First investigations with the Very Large Telescope Interferometer reveal the dust torus around the black hole in a galactic nucleus

Black holes swallow everything that comes near them and are fuelled by gas and dust from their surroundings. An international research team led by Gerd Weigelt of the Max Planck Institute for Radio Astronomy in Bonn has now focused its attention on this reservoir of material. Using near-infrared interferometry, they observed the inner region of galaxy NGC 3783, which contains a black hole surrounded by a so-called “dust torus”. This torus apparently represents the reservoir of gaseous and dusty material that feeds the hot gas disk (“accretion disk”) and the supermassive black hole at the centre of the galaxy. The observations were carried out with the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO).

Extreme physical processes occur in the innermost regions of galactic nuclei.  Supermassive black holes were discovered in many galaxies. The masses of these black holes are often a millionfold larger than the mass of our sun. These central black holes are surrounded by hot and bright gaseous disks, called “accretion disks”. The emitted radiation from these accretion disks is probably generated by infalling material. To maintain the high luminosity of the accretion disk, fresh material has to be permanently supplied. The dust tori (see Fig. 1) surrounding the accretion disks are most likely the reservoir of the material that flows through the accretion disk and finally “feeds” the growing black hole.

Observations of these dust tori are very challenging since their sizes are very small. A giant telescope with a mirror diameter of more than 100 Meters would be able to provide the required angular resolution, but unfortunately telescopes of this size will not be available in the near future. This raises the question: Is there an alternative approach that provides the high resolution required?

The solution is to simultaneously combine (“interfere”) the light from two or more telescopes since these multi-telescope images, which are called interferograms, contain high-resolution information. In the reported NGC 3783 observations, the AMBER interferometry instrument was used to combine the infrared light from two or three telescopes of ESO’s Very Large Telescope Interferometer (VLTI, see Fig. 2). This interferometric method is able to achieve an extreme angular resolution that is proportional to the distance between the telescopes. Since the largest distance between the four telescopes of the VLTI is 130 Meters, an angular resolution is obtained that is as high as the theoretical resolution of a telescope with a mirror diameter of 130 Meters  a resolution that is 15 times higher than the resolution of one of the VLTI telescopes, which have a mirror diameter of 8 Meter.

“The ESO VLTI provides us with a unique opportunity to improve our understanding of active galactic nuclei,”, says Gerd Weigelt from the Max Planck Institute for Radio Astronomy in Bonn. “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.”

And Makoto Kishimoto emphasizes: “We hope to obtain more detailed information in the next few years by additional observations at shorter wavelengths, with longer baselines, and with higher spectral resolution. Most importantly, in a few years, two further interferometric VLTI instruments will be available, which can provide complementary information.”

To resolve the nucleus of the active galaxy NGC 3783, the research team recorded thousands of two- and three-telescope interferograms with the VLTI.  The telescope distances were in the range of 45 to 114 Meters. The evaluation of these interferograms allowed the team to derive the radius of the compact dust torus in NGC 3783.  A very small angular torus radius of 0.74 milli-arcsecond was measured, which corresponds to a radius of 0.52 light years. These near-infrared radius measurements, together with previously obtained mid-infrared measurements, allowed the team to derive important physical parameters of the torus of NGC 3783.

“The high resolution of the VLTI is also important for studying many other types of astrophysical key objects”, underlines Karl-Heinz Hofmann. “It is clear that infrared interferometry will revolutionize infrared astronomy in a similar way as radio interferometry has revolutionized radio astronomy.”

HOR / NJ