A Very Sharp View on Far-Distant Quasars
September 20, 2002
The combination of radio telescopes on both sides of the Atlantic Ocean results in a giant virtual radio telescope with several thousand kilometers in diameter. Radio sources (Quasars and Active Galactic Nuclei) at distances of up to a few billion light years have been observed at an unprecendently high frequency of 147 GHz (corresponding to 2 mm wavelength), thus giving a new record in angular resolution: 18 micro arcseconds (0.000018"). This compares to a spatial resolution of only 3 cm at the distance of the moon!
The angular resolution of a telescope, which is its ability to clearly separate two nearby points on the sky, can be improved either by increasing the diameter of the mirror or by observing at shorter wavelengths (corresponding to higher frequencies). In Very Long Baseline Interferometry (VLBI) radio telescopes on different continents and which are separated by up to several thousands of kilometers are combined via computers. This technique forms a virtual radio telescope with a diameter as large as the diameter of the Earth and the ability to make extremely sharp radio astronomical images. In recent years radio astronomers now extended this technique from the long centimeter wavelengths to the shorter millimeter wavelengths and achieved a considerable improvement in angular resolution.
Successful astronomical observations at short millimeter wavelengths needs special telescopes in special places. The radio telescopes have to have large parabolic mirrors with surfaces as accurate as only a few tens of microns (1 micron corresponds to 0.001 millimeter). These 10 to 30 meter large mirror surfaces also have to be thermally stable and often are made out of carbon fiber. Since in the millimeter radio band the signal from cosmic objects is strongly absorbed by the water vapor in the lower part of the Earth's atmosphere, the radio telescopes observing at millimeter wavelengths have to be located on high altitude mountains.
After several earlier millimeter-VLBI pilot observations with telescopes just located on one continent (eg. within Europe or within the U.S.), astronomers now have achieved for the first time a signal detection at short millimeter wavelengths also on the longer intercontinental baselines across the Atlantic Ocean. The new VLBI observations were performed in April 2002 at 2 mm wavelength (147 GHz). The following radio telescopes participated with success in this experiment: In Arizona, the 12-m Steward Observatory telescope on Kitt Peak and the 10-m Heinrich-Hertz telescope on Mt. Graham. The Heinrich-Hertz telescope is operated jointly by Steward Observatory in Tucson, AZ, and the Max-Planck-Institut for Radioastronomy in Bonn, Germany. In Europe, the 30-m IRAM telescope located on the Pico Veleta mountain, near the city of Granada in Spain, and the 14-m antenna of the Metsahovi Observatory in Finland participated. The distances between these globally distributed telescopes is 200 km on the Arizona baseline, 3100 km on the baseline between Finland and Spain, and most relevant for the new record in angular resolution, the 8500 km on the Arizona- Spain baseline.
The synthesized 'transatlantic' 2 mm wavelength telescope has an angular resolution of 18 micro arcsecond (0.000018") and was used to observe the active nuclei of Quasars at several billion light years distance from the Earth. "If, in our imagination, we would point a telescope of such an angular resolution towards the Moon, we would be able to see details on a scale of a golf ball", say scientists from the Max-Planck-Institute for Radio Astronomy (MPIfR) in Bonn. When pointed towards far away Active Galaxies and Quasars, it becomes possible to obtain a very detailed view of the region very near the accretion disk, which surrounds the billion solar masses black hole, expected at the center of these galaxies. Worldwide VLBI observations at millimeter wavelengths provide the significant advantage of an unprecedented sharp view into these Quasar Nuclei, which at longer (centimeter) wavelengths are obscured and not directly observable.
A significant role in the success of the recent observations is due to the efforts made by the Steward Observatory, Arizona, and the MIT-Haystack Observatory, Massachussetts. The Steward Observatory, with help from NRAO - Tucson, re-opened the 12-m Kitt Peak telescope which in previous years had successfully participated in world-wide VLBI observations at the longer wavelength of 3mm. Likewise, for the 10-m HHT telescope, on Mount Graham, a Helium cooled receiver for operation at 2 mm wavelength was especially built for this experiment. The VLBI equipment for the two Arizona telescopes (electronics and data aquisition terminals) and an accurate hydrogen-maser atomic clock for the HHT telescope was provided by the MIT-Haystack Observatory and was brought from the East coast to the two telescopes. "This was a major effort", says Shep Doeleman from Haystack observatory. After the observation, the recorded magnetic VLBI tapes containing tens of Terabytes of data were shipped to the so-called correlators, special super-computers in which the 'virtual world-wide' radio-telescope is formed. The final data reduction was done in parallel, at the two correlator centers of the MIT-Haystack Observatory in Westford, MA., USA, and at the Max-Planck-Institut for Radioastronomy in Bonn, Germany.
The reported experiment opens the possibility to establish a regular network for VLBI observations at 2 mm and shorter wavelengths. For the time being, keystones in this network are the American telescopes at Kitt Peak and Mount Graham, together with the IRAM 30-m telescope in Spain and in the near future the IRAM 6 element phased interferometer at Plateau de Bure in France. While these telescopes are located at similar Northern geographic latitudes, thus simulating a virtual telescope of large East-West dimension, it is extremely important to also include the 15-m telescope of SEST, Chile, and the future mm-wavelength interferometer. In the next few years, the APEX ('Atacama Path Finder Experiment') antenna, one of the prototype antennae for ALMA, could also be used for millimeter VLBI. The addition of these telescopes will extend the dimension of the virtual telescope in the direction North - South, thus resembling more of a conventional telescope with 'circular aperture'.
Scientifically, there are fascinating prospects for the future: detailed images of the innermost structures of distant Quasars and Active Galactic Nuclei will be possible. Central sources in nearby galaxies can be resolved down resolutions of a few light days, regions not much larger than the size of planetary systems. Such observations will help to understand processes of energy release in the central regions of the most luminous objects in the Universe. If the central radio source of our Milky Way Galaxy (Sagittarius A*, a Black Hole of approximately 3 Million solar masses) can be detected with VLBI at 2 mm or even shorter wavelengths, structures very close to the Schwarzschild horizon of the central super-massive Black Hole could become visible. "It might even be possible to observe relativistic effects like the distortion of spacetime in the direct neighbourhood of supermassive Black Holes within the next 10-20 years", says Thomas Krichbaum from MPIfR. He and Sheperd Doeleman from Haystack Observatory, presented the new results at the annual European VLBI network (EVN) conference in Bonn, Germany.
The achievements mentioned above were obtained in a truely international collaboration. Scientists and engineers from the following institutes made this possible: The Max-Planck-Institute for Radio Astronomy (MPIfR) in Bonn/Germany, the MIT-Haystack Observatory in Westford, MA, USA, the Steward Observatory, in Tucson, AZ, USA, IRAM, in Granada, Spain and Grenoble, France, the Metsähovi Radio Observatory in Finland, and the ESO-Swedish operated SEST-telescope.