Watched black holes while "eating"

Astronomers look deep into the heart of active galaxies

The Keck Interferometer on the Mauna Kea (Hawaii). The telescope consists of two separate 10-meter telescopes in separate domes that are 85 meters apart. Future interferometer facilities of this type will consist of a large telescope network spanning several kilometers. Such interferometers have been around for many years in the radio, but not in the infrared astronomy. Here, the technology is at a very early stage, which is why usually only two or three telescopes for interferometry can be used. A prototype for this are the two Keck telescopes, each with a 10-meter mirror diameter; They are operated with a mutual distance of 85 meters. © Keck Observatory, Hawaii
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Deep into the heart of active galaxies, astronomers have been watching with a telescope on Hawaii. They managed for the first time to spatially resolve several radiation sources. The near infrared observations show an annular structure consisting of dust and gas. The measured ring radii provide new insights into the material that flows into the central black hole, the researchers report in the journal "Astronomy & Astrophysics".

The nuclei of many galaxies shine not only in the optical light, but also in the range of the X-ray, infrared and radio radiation. In addition, often a matter jet - a so-called jet - shoots out of the center. It is believed that this energy is generated by the influx of matter onto extremely massive, central black holes.

In search of the origin of the matter jet

The scientists around Makoto Kishimoto from the Max Planck Institute for Radio Astronomy in Bonn are interested in the details: they want to watch a massive black hole at their meal and find out exactly where the high-energy matter jet from the central area starts.

In May 2009, the team observed four of these active galactic nuclei (AGN) using the Keck interferometer in Hawaii. Subsequently, on the United Kingdom Infrared Telescope (UKIRT) timely recordings of the respective galaxies in the near infrared region were obtained. Among the objects studied were NGC 4151, a relatively close galaxy 50 million light-years away, but also the Quasar IRAS 13349 + 2438 at a distance of more than a billion light-years.

"This was only possible thanks to the great effort of the Keck employees, who have greatly improved the sensitivity of their low-radiation instruments, " says Max Planck researcher Kishimoto, first author of the publication. display

Infrared images of the four galaxies studied. With the Keck interferometer, the inner region of the luminous galaxy nuclei could be resolved in details. The resulting ring-shaped structure for the galaxy NGC 4151 is shown in the top right-hand panel. While the entire extent of this galaxy covers several 10, 000 light-years, the radius of the inner ring is only 0.13 light-years. The distance of each of the four galaxies is given in millions of light years, along with the corresponding redshift z in the spectrum. M. Kishimoto, based on galaxy footage with UKIRT

Black holes as a big challenge

In order to dissolve such distant objects in the infrared, telescopes of 100 meters in diameter and more would be required. But it is easier to superimpose the beam lengths of two or more adjacent individual telescopes. Information about the surroundings of the black holes can be obtained from the interference patterns thus obtained.

While this Keck interferometer is routinely used to observe stars in our Milky Way galaxy, investigations of much farther objects, especially the massive black holes in the centers of distant galaxies, make one unequally greater challenge: they radiate many times weaker than nearby stars. The observation of such objects with interferometers, especially in the short-wave infrared near-infrared - are particularly difficult.

Until recently, astronomers have successfully observed only one active galaxy nucleus using the Keck interferometer. This galaxy, NGC 4151, is one of the brightest AGNs in visible and infrared wavelengths. The new measurements of four of these galaxies now provide a fairly reliable picture of what can be resolved with the interferometer: a ring-shaped structure that is distinct from the near-infrared radiation of dust rners.

Soon better interferometer?

With the evaluation of different, mutually independent measurements of the ring radius, the research team is on the track of the spatial extent around the central black hole. What is the radial distribution, how compact or expanded is the dust torus? "Although we have the highest spatial resolution in the infrared so far, we are still investigating a relatively remote region around the central black hole" says Kishimoto.

The scientist hopes to get closer to the central source with future interferometers. "In addition, we plan to investigate a number of other active galaxies with massive black holes in the center."

(idw - Max Planck Society, 09.12.2009 - DLO)