Using the Low Frequency Array (LOFAR), an international team led by Leiden University has produced the sharpest and most sensitive radio image ever made of the sky at low radio frequencies. The image reveals nearly 63,000 radio sources, including active galactic nuclei and star-forming galaxies, in a region of the sky around twenty-five times larger than the full Moon.
The researchers focused on ELAIS-N1, a well-studied area of the northern sky that contains many faint and distant galaxies. The new image is twenty times sharper than the previous most sensitive LOFAR image of this region. To put this into perspective, the resolution is high enough to distinguish a coin from a distance of around ten kilometres. At this level of detail, the complete LOFAR image contains 8 billion pixels.
Processing vast amounts of data into a single image
The data for this image were collected over 200 hours and totalled 400 terabytes. Until recently, it was impossible to process such a large volume of radio telescope data at this resolution into a single 8-billion-pixel image. However, improved calibration and data-processing techniques developed by the team and their partners have reduced computing costs by more than a factor of ten in recent years.
'The entire dataset could be processed in just a few months on Spider, SURF's powerful data-processing platform in the Netherlands,' says Jurjen de Jong, who led the study and is a postdoctoral researcher at Leiden Observatory and ASTRON.
The data have been made publicly available to the astronomical community. Much of the scientific value of this dataset lies in combining it with observations of the same region of the sky made by other telescopes, for example at optical, infrared and X-ray wavelengths. This allows astronomers to determine properties such as the distance and mass of each radio source in the image.
'With this sharp and sensitive view of the sky at low radio frequencies, we can study how radio jets interact with their surroundings.'
Radio jets powered by black holes
One of the most common objects in the new image is an active galactic nucleus. These compact, bright regions are powered by matter spiralling into a supermassive black hole. This process can produce powerful streams of particles known as jets.
LOFAR is particularly well suited to detecting these jets because of its high sensitivity to faint and extended radio emission.
In low-resolution images, distant galaxies and radio jets often appear as indistinct blobs, making it difficult to tell, for example, whether they consist of multiple galaxies. It is also hard to determine whether the radio emission comes from a jet or from star formation within a galaxy. LOFAR can distinguish between these different components, and many jets can now be seen in remarkable detail.
'With this sharp and sensitive view of the sky at low radio frequencies, we can study how radio jets interact with their surroundings. This allows us to see how these processes have evolved throughout cosmic history,' says Reinout van Weeren, Associate Professor at Leiden Observatory and co-author of the paper.
Other radio sources visible in the images are small galaxies where stars are actively forming. Because radio waves pass through dust, which can hide star formation from optical and infrared telescopes, radio observations provide an unobstructed view of these processes in distant galaxies. This allows astronomers to study how galaxies have evolved, from the earliest examples in the Universe to those we see today.
More sensitive and wider-field images with LOFAR2.0
According to the researchers, this work demonstrates the potential of LOFAR2.0, the telescope upgrade programme currently being carried out by ASTRON and its European partners. The upgrade will make it possible to obtain more sensitive, high-resolution images over larger areas of the sky.
De Jong says that LOFAR2.0 shows that the telescope will remain competitive in the era of the Square Kilometre Array, the next-generation radio observatory currently under construction in South Africa and Australia.
This video takes you on a journey through the ELAIS-N1 field as observed by LOFAR. Credit: De Jong et al.
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About LOFAR
LOFAR, short for Low Frequency Array, was designed and built by ASTRON, the Dutch Institute for Radio Astronomy. The observatory has facilities for observations, data processing and data storage in several countries, which are jointly managed by LOFAR ERIC.
LOFAR stations are spread across Europe, with distances of up to 1,980 kilometres between them. The telescope's central core is located in the Dutch province of Drenthe.
The results have been accepted for publication in Astronomy & Astrophysics: 'Pushing the frontiers of low-frequency radio astronomy: An ultra-deep sub-arcsecond LOFAR image of ELAIS-N1' by De Jong et al. A preprint of the paper can be found here.
This press release originally appeared on astronomie.nl.
