Using the European radio telescope LOFAR, an international team of astronomers involving astronomers from Paris Observatory - PSL and CNRS obtain the most sensitive images of the deep Universe ever made at low frequencies. The description of these images and the first scientific results are the subject of a series of fourteen articles published in a special issue of the scientific journal Astronomy & Astrophysics, on April 7, 2021.
By repeatedly observing the same regions of the sky and combining the data, images corresponding to a very long exposure were created.
In this way, the international scientific team was able to detect the faint glow of light resulting from stars that exploded into supernovas, in tens of thousands of galaxies present in the distant universe.
The deepest LOFAR image ever made, in the region of the sky known as Elais-N1, which is one of the three fields studied in this study.
The image comes from a single LOFAR pointing observed repeatedly for a total duration of 164 hours. More than 80 000 sources are detected, including spectacular large-scale emission from massive black holes, but most of the sources are distant galaxies (see boxes)
The LOFAR radio telescope}}
LOFAR is a giant radio telescope spread across Europe, observing a very faint and low energy light, invisible to the human eye. This light is created by ultra energetic particles whose speed is close to that of light. Thus LOFAR allows astrophysicists to study star explosions (supernova), collisions of galaxy clusters and active black holes, which accelerate these particles in shocks or jets. These sources are so distant that their light has sometimes traveled for billions of years to reach us, thus testifying to the life of our Universe when it was still young.
The image on the left shows a small part of one of the deep fields observed in the optical domain (visible light). By superimposing the radio image of this same part of the sky (in orange in the right frame), we can observe new astrophysical phenomena. Here LOFAR allows to detect the presence of ultra energetic particles accelerated by supermassive black holes in huge jets propagating outside the galaxies.
The deep radio images produced by LOFAR total hundreds of hours of observations, and are so sensitive that they are dominated in number by galaxies like our own Milky Way. These galaxies emit faint light at low frequencies (radio range) produced by stars that have exploded in supernovas. The combination of the unprecedented sensitivity of this image and its large field of view - about 300 times the size of the full moon - has made it possible to detect tens of thousands of galaxies in the early universe, when they were still forming. Supernovas resulting from active star formation are usually enveloped in a veil of dust. In the radio domain, it is possible to see through the dust and build an unbiased understanding of star formation. The deep LOFAR images have provided more accurate measurements of the number of new stars forming in this ancient and distant period of the Universe.
The deep images produced by the collaboration are so sensitive that the majority of the objects detected in radio (in orange) are distant star-forming galaxies (point-like and faint objects). Nevertheless, active black holes can be observed as in the upper left part of the image
This remarkable dataset has allowed a wide range of additional scientific investigations, for example studying radio emission from massive black holes or galaxy cluster collisions. Unexpected results have also been obtained. In particular, repeated observations have allowed the detection of sources with varying luminosity. The red dwarf star CR Draconis has shown radio emission bursts that strongly resemble those coming from Jupiter, possibly driven by a star-planet interaction or by the rapid rotation of the star.
Huge numerical challenge
LOFAR does not produce sky maps directly. The signals from its 70,000 antennas must be combined to produce these deep images. More than 4 petabytes of raw data - equivalent to about one million DVDs - have been acquired and processed. The low-frequency images of our Universe are diffusely hidden within the large amount of data that LOFAR has observed. This weak signal could be extracted, thanks to recent mathematical and algorithmic advances and the use of large computer clusters.
Multi-wavelength data
The target fields were chosen because they were the best-studied fields in the Northern Hemisphere sky. This allowed the team to collect optical, infrared, and submillimeter data for galaxies detected at low frequencies. The comparison of these radio images with data obtained with other telescopes at other wavelengths was vital for the interpretation of the LOFAR images.
LOFAR
LOFAR is the first telescope of its kind in the world. 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 United Kingdom. The French part of the network is located in Nançay, in the Cher department, within the radio astronomy station of the Observatoire de Paris (Observatoire de Paris - PSL / CNRS / University of Orléans). In its "high band" configuration, LOFAR observes at frequencies of about 150 MHz - between the FM and DAB radio bands. LOFAR is unique in its ability to produce high quality images of the sky at wavelengths in the meter range. These deep-field images are a testament to its capabilities and a treasure trove for future discoveries.
LOFAR
The LOFAR international telescope is a European radio antenna array, with its core located in Exloo, The Netherlands. LOFAR operates by combining the signals of more than 70,000 individual antenna dipoles, across the Netherlands and in the European partner countries. They are linked together by a high-speed fiber optic network. Powerful computers are used to process their signals to synthesize a European radio antenna that spans 1300 kilometers. The LOFAR international telescope is unique in its sensitivity, wide field of view and resolution. The LOFAR data archive is already the largest collection of astronomical data in the world.
LOFAR was designed, built and is currently operated by ASTRON, the Netherlands Institute for Radio Astronomy. France, Germany, Ireland, Italy, Latvia, the Netherlands, Poland, Sweden and the United Kingdom are all partner countries of the LOFAR international telescope.
