LOFAR telescope makes deeper images of universe than ever before
The results constitute an important step on the road to detecting the elusive signals of an era known as the Epoch of Reionization.
June 2, 2011
An international team led by astronomers at ASTRON and the Kapteyn Institute of the University of Groningen in the Netherlands have used the LOFAR telescope, designed and constructed by ASTRON, to make the deepest wide-field images of the sky in the relatively unexplored part of the spectrum around 150 megahertz. It reveals faint radio sources never seen before.
A very small part of the raw LOFAR image is shown of the field centered on the bright quasar 3C 196. It shows tens of discrete sources. The image has an angular resolution of 8 arcseconds. The image still needs to be de-convolved.
Photo by LOFAR/Pano Labropoulos (University of Groningen)
This tiny part of the LOFAR image centers on the North Celestial Pole. It shows at least seven discrete sources, some of them double or complex. The image has an angular resolution of 8 arcseconds but still needs to be de-convolved.
Photo by LOFAR/Sarod Yatawatta (University of Groningen)
The properties of the foregrounds that trouble our view of the distant universe are often discussed at astronomy conferences today. Two projects are central: Planck observations of the Cosmic Microwave Background and (searches for) redshifted 21cm line observations of an era known as the Epoch of Reionization (EoR). This phase in the universe is believed to have taken place in the period between about 400 and 800 million years after the Big Bang. During the EoR, the neutral hydrogen was slowly disappearing, probably as a result of the strong "ionizing" power of the first stars and quasars. Detecting the EoR is one of the hottest projects in astronomy today.
A group of astronomers based at ASTRON and the Kapteyn Institute, headed by Ger de Bruyn, Michiel Brentjens, Leon Koopmans, and Saleem Zaroubi, is in the race to first detect these signals. They lead a team of about a dozen members, including astronomers currently working in Germany, the United States, Canada, and Sweden. The results constitute an important step on the road to detecting the elusive signals. However, there is still a long way to go along this road.
Most of the antenna stations of the international LOFAR telescope have already been rolled out across the Netherlands and Europe. It has been taking data for a large number of astronomers after its official opening by Her Majesty Queen Beatrix of the Netherlands last year. The LOFAR data on which the images are based were obtained in a 6-hour synthesis on the night of January 29/30, 2011, and the evening of April 1, 2011, using 18 core stations and seven remote stations. Signals were recorded with the High Band Antennas and covered the frequency range from 115 to 163 MHz. After initial processing on the central LOFAR cluster, they were transferred and further processed on a cluster dedicated to the processing of data for the LOFAR Epoch-of-Reionization project. This cluster is also located at the Computing Centre of the University of Groningen.
Cutouts from a very small part of the giant images are shown in the associated figures. One of the fields is centered at the North Celestial Pole, which is special in the sense that nighttime observations can be obtained all year round. The second field was centered at the bright compact quasar 3C 196 in the constellation Lynx. The images, which have a resolution of 8 arcseconds, are already comparable to, or even slightly better, than the best published images taken with the Giant Meter-wavelength Radio Telescope (GMRT) in India. The images contain a large number of both very bright and very faint sources, spanning a so-called dynamic range of more than 200,000:1 in brightness between sources in the 3C 196 image.
This is an important record for the time being for LOFAR. The image quality, however, is still not perfect, and significant improvements can be expected in the months ahead using improved knowledge of the effects of the LOFAR station beams. Continued efforts are also needed to improve the software to deal with imaging artifacts and the ionosphere. These two fields and several others will be observed for about 100 nights to conclusively detect signals from the EoR.