Details in structure of a distant quasar: First high-resolution image from LOFAR radio telescope array

This wavelength range has not been accessible to such detailed observations before, as the telescopes have to be spaced far apart. The first image showing fine details of the quasar 3C 196 observed at wavelengths between 4 and 10 m was achieved by using just a small fraction of the final LOFAR array that will cover large parts of Europe.

After first tests of the individual antennas, the observations now bring together eight stations of the "LOw Frequency ARray" (LOFAR), Five stations in the Netherlands were connected with three stations in Germany: Effelsberg near Bonn, Tautenburg near Jena and Unterweilenbach near Munich. All antennas were targeted at the quasar 3C 196, a strong radio source at a distance of several billions of light years. "We chose this object for the first tests, because we know its structure very well from observations at shorter wavelengths", explains Olaf Wucknitz (AIfA). "The goal was not to find something new but to see the same or similar structures also at very long wavelengths to confirm that the new instrument really works. Without the German stations, we only saw a fuzzy blob, no sub-structure. Once we included the long baselines, all the details showed up."

Observations at wavelengths covered by LOFAR are not new. In fact, the pioneers of radio astronomy started their work in the same range. However, they were only able to produce very rough maps of the sky and to measure just the positions and intensities of objects. "We are now returning to this long neglected wavelength range", says Michael Garrett, general director of ASTRON (The Netherlands), the research institute in charge of the international LOFAR project. "But this time we are able to see much fainter objects and, even more important, to image very fine details. This offers entirely new opportunities for astrophysical research."

"The high resolution and sensitivity of LOFAR mean that we are really entering uncharted territory, and the analysis of the data was correspondingly intricate", adds Olaf Wucknitz. "We had to develop completely new techniques. Nevertheless, producing the images went surprisingly smoothly in the end. The quality of the data is stunning." The next step for Wucknitz is to use LOFAR to study so-called gravitational lenses, where the light from distant objects is distorted by large mass concentrations. High resolution is required to see the interesting structures of these objects. This research would be impossible without the international stations.

The resolution of an array of radio telescopes, i.e. the size of the smallest structures that it can resolve and distinguish, depends directly on the separation between the telescopes. The larger these baselines are relative to the observed wavelength, the better the achieved resolution. Currently the German stations provide the first long baselines of the array and improve the resolution by a factor of ten over just using the Dutch stations.

"We want to use LOFAR to search for signals from very early epochs of the Universe", says Benedetta Ciardi from the Max Planck Institute for Astrophysics (MPA) in Garching. "Having a completely theoretical background myself, I never had thought that I would get excited over a radio image, but this result is really fascinating."

Further improvement should come very soon with observations at slightly shorter wavelengths, which will increase the resolution by another factor of four. In addition, the imaging quality will improve significantly with more stations coming online soon. The image of quasar 3C 196 therefore is just the first but very important step.

"The image quality of the final array depends crucially on the uniformity with which large areas are covered with stations", says Anton Zensus, director at the Max Planck Institute for Radioastronomy (MPIfR) and in charge of the VLBI research group at the institute. "The German stations are already an indispensable contribution to the international array. What we are still lacking, however, is a station in northern Germany to close the gap between our stations and the ones of our Dutch friends. This would increase the image quality a lot."