An international team of astronomers has published the most sensitive images of the Universe ever taken at low radio frequencies, using the International Low Frequency Array (LOFAR). By observing the same regions of sky over and over again and combining the data to make a single very-long exposure image, the team has detected the faint radio glow of stars exploding as supernovae, in tens of thousands of galaxies out to the most distant parts of the Universe. A special issue of the scientific journal Astronomy & Astrophysics is dedicated to fourteen research papers describing these images and the first scientific results.
Published by the editorial team, 7 April 2021
Cosmic star formation
Philip Best, University of Edinburgh (UK), who led the deep survey, explained: “When we look at the sky with a radio telescope, the brightest objects we see are produced by massive black holes at the centre of galaxies. However, our images are so deep that most of the objects in it are galaxies like our own Milky Way, which emit faint radio waves that trace their on-going star-formation.”
“The combination of the high sensitivity of LOFAR and the wide area of sky covered by our survey – about 300 times the size of the full moon – has enabled us to detect tens of thousands of galaxies like the Milky Way, far out into the distant Universe. The light from these galaxies has been travelling for billions of years to reach the Earth; this means that we see the galaxies as they were billions of years ago, back when they were forming most of their stars.”
Isabella Prandoni, INAF Bologna (Italy), added: “Star formation is usually enshrouded in dust, which obscures our view when we look with optical telescopes. But radio waves penetrate the dust, so with LOFAR we obtain a complete picture of their star-formation.” The deep LOFAR images have led to a new relation between a galaxy’s radio emission and the rate at which it is forming stars, and a more accurate measurement of the number of new stars being formed in the young Universe.
The deepest LOFAR image ever made, in the region of sky known as ‘Elais-N1’, which is one of the three fields studied as part of this deep radio survey. This image arises from a single LOFAR pointing observed repeatedly for a total of 164 hours. Over 80,000 radio sources are detected; this includes some spectacular large-scale emission arising from massive black holes, but most sources are distant galaxies like the Milky Way, forming their stars. Credit: Philip Best & Jose Sabater, University of Edinburgh.
The remarkable dataset has enabled a wide range of additional scientific studies, ranging from the nature of the spectacular jets of radio emission produced by massive black holes, to that arising from collisions of huge clusters of galaxies. It has also thrown up unexpected results. For example, by comparing the repeated observations, the researchers searched for objects that change in radio brightness. This resulted in the detection of the red dwarf star CR Draconis. Joe Callingham, Leiden University and ASTRON (NL), noted that “CR Draconis shows bursts of radio emission that strongly resemble those from Jupiter, and may be driven by the interaction of the star with a previously unknown planet, or because the star is rotating extremely quickly.”
Huge computational challenge
LOFAR does not directly produce maps of the sky; instead the signals from more than 70,000 antennas must be combined. To produce these deep pictures, more than 4 petabytes of raw data – equivalent to about a million DVDs – were taken and processed. “The deep radio images of our Universe are diffusely hidden, deep inside the vast amount of data that LOFAR has observed.” said Cyril Tasse from Paris Observatory, University PSL (France). “Recent mathematical advances made it possible to extract these, using large clusters of computers.”
A video fly-through of part of the sky that was studied (Credit: Jurjen de Jong, Leiden University)
Just as important in extracting the science has been a comparison of these radio images with data obtained at other wavelengths. “The parts of the sky we chose are the best-studied in the Northern sky” explained Philip Best. This has allowed the team to assemble optical, near-infrared, far-infrared and sub-millimetre data for the LOFAR-detected galaxies, which has been crucial in interpreting the LOFAR results.
LOFAR is the world’s leading telescope of its type. It is operated by ASTRON, the Netherlands Institute for Radio Astronomy, and coordinated by a partnership of 9 European countries: France, Germany, Ireland, Italy, Latvia, the Netherlands, Poland, Sweden and the UK. In its ‘high-band’ configuration, LOFAR observes at frequencies of around 150 MHz – between the FM and DAB radio bands. “LOFAR is unique in its ability to make high-quality images of the sky at metre-wavelengths”, said Huub Röttgering, Leiden University, who is leading the overall suite of LOFAR surveys. “These deep field images are a testament to its capabilities and a treasure trove for future discoveries”.