Johnny T. Butler 
A new sky map of the distant Universe created from data collected using the European LOFAR radio telescope has identified hundreds of thousands of unknown galaxies until today.
This map also allows to better identify celestial phenomena such as supermassive black holes, but also to better understand the evolution of clusters of galaxies.
An eye on the Universe
LOFAR (short for LOw Frequency ARray) is a network of 100,000 antennas in 50 observation stations in five European countries (the Netherlands, France, Sweden, the United Kingdom and Germany). LOFAR is the largest radio telescope in the world.
Dozens of institutions are involved in analyzing the huge amounts of data generated by the project, including the Observatoire de Paris in France.
This type of radio telescope observes light of very low energy, invisible to the human eye and any optical telescope.
It has the distinction of operating at very low frequencies (between 10 and 250 megahertz) in a field of energy essentially unexplored to date.
A first map
This map shows only 2.5% of the northern hemisphere sky, but still contains over 300 000 astrophysical objects, of which 90% of them were hitherto unknown.
These sources of radio radiation are so distant that their light has traveled billions of years before reaching the antennas of LOFAR.
The unpublished information extracted from the first data makes it possible to better understand the nature of celestial objects whose nature remains difficult to define,
Supermassive black holes
The origin of supermassive black holes in the heart of galaxies remains enigmatic. When the material is sucked by this type of black hole, jets of very energetic particles are formed and disturb the environment of the galaxy.
Radio waves are then emitted. The radio observation of these objects made it possible to study the growth processes of supermassive black holes, and their role in the formation of galaxies.
The present observations thus reveal that the supermassive black holes associated with the most massive galaxies are still active, and that matter has been falling incessantly in their interior for billions of years. Images produced by LOFAR also make it possible to study how black holes periodically upset the dynamics of the intergalactic environment.
Clusters of scanned galaxies
Astrophysicists have known for some years that the shock waves created by the gigantic gas movements of the intergalactic medium can accelerate particles to the very high energies necessary for the emission of radio waves. In particular, clusters of galaxies (which contain hundreds or thousands of galaxies) colliding together generate radio emissions that can span millions of light years.
LOFAR has observed these emissions in abundance, and the data collected make it possible to study the dynamics of the large-scale structure of the Universe.
Towards a complete map
The ultimate goal of scientists is to create a high-resolution map of the entire northern hemisphere, which will reveal about 15 million radio sources.
The goal could be reached around 2024, and may have a major impact on our understanding of the Universe.
In addition, the construction of another low frequency radio telescope (SKA) is due to start in the southern hemisphere in 2020.
The discoveries are described in a series of 26 articles published in a special issue of the journal Astronomy & Astrophysics.
