Radio Telescope Image Reveals 63,000 Sources, Sharpest Low-Frequency View of the Sky Ever
A Leiden-led team used LOFAR to image one patch of sky 20 times more sharply than before, and processed 400 terabytes of data in just a few months.
An international team led by Leiden University has produced the sharpest and most sensitive radio image of the sky ever made at low radio frequencies, using the Low Frequency Array, or LOFAR, telescope network in Europe.
The image covers ELAIS-N1, a well-studied patch of the northern sky containing many faint, distant galaxies. It spans about 25 times the area of the full moon and reveals almost 63,000 radio sources, including active galactic nuclei and star-forming galaxies. The image is 20 times sharper than the previous most sensitive LOFAR image of the same area. The team says that resolution would be enough to pick out a coin from about 10 kilometers, or 6 miles, away. The full image contains 8 billion pixels.
The data were gathered over 200 hours of observing and amounted to 400 terabytes. For years, processing that much data at such high resolution was impossible. Improved calibration and data-reduction methods developed by the team and collaborators have cut the cost by more than a factor of 10 in the past few years. "The entire data set could be processed on Spider, the high-performance data-processing platform hosted at SURF in the Netherlands, in just a few months," said Jurjen de Jong, who led the work and is a postdoctoral researcher at Leiden Observatory and ASTRON.
Much of the image's value lies in what astronomers can now tell apart. Many of the sources are active galactic nuclei, the luminous centers of galaxies powered by matter spiraling onto a supermassive black hole. These can drive powerful jetlike outflows, which LOFAR is especially good at detecting because of its sensitivity to faint, extended radio emission. In low-resolution images, distant galaxies and their jets are featureless blobs, and it is often hard to tell whether a source is one galaxy or several, or whether radio emission comes from a jet or from star formation. In the new image, components can be separated and massive jets traced in detail.
"With this sharp and sensitive view of our sky at low radio frequencies, we have a new window to study how these radio jets interact with their environment and how they evolve over cosmic time," said Reinout van Weeren, an associate professor at Leiden Observatory and co-author of the paper.
The other major population is small star-forming galaxies, which convert gas into stars. Because radio waves pass through the dust that hides star formation from optical and infrared telescopes, the observations give an unobscured picture of that activity in distant galaxies, from the earliest ones to the present universe.
The data have been made publicly available. Much of their scientific value will come from combining them with images of the same patch of sky at optical, infrared and X-ray wavelengths, which lets astronomers derive distance and mass for each radio source. The results have been accepted for publication in Astronomy & Astrophysics and are available on the arXiv preprint server.
The researchers say the work shows the promise of LOFAR2.0, the upgrade ASTRON is carrying out with European partners, which will allow more sensitive and wider high-resolution images. De Jong said the image shows LOFAR2.0 will stay competitive in the era of the Square Kilometre Array, the large radio observatory now under construction in South Africa and Australia.
