The LOFAR radio-telescope
Radio-telescopes detect very low energy radiation undetectable by the naked eye as well as any optical telescope. LOFAR is one of the largest radio-telescopes in the world. It in particular works at very low frequencies (from 10 to 250 megahertz) in a thus far practically unexplored energy range. It is made up of an array of one hundred thousand antennae distributed over Europe, and is directed by par ASTRON in the Low Countries.
The French part of the network is installed at Nançay, in the Cher region, within the radioastronomical observatory of the Paris Observatory (Observatoire de Paris - PSL/ CNRS / Université d’Orléans).
LOFAR produces gigantic quantities of data. Making an image using this antenna array involves inverting a gigantic set of thousands of equations. The Paris Observatory has played a central role in enabling the LOFAR data to be exploited.
The LoTSS survey
In this first sky map of the radio sky produced by LOFAR (baptized “LoTSS survey”), only 2,5% of the northern hemisphere has been published. This part already contains over three hundred thousand astrophysical objects : 90% of them were unknown till now. Some of these radio sources are so far that their radiation has been traveling for hundreds of thousands of years before reaching the LOFAR antennas.
Low frequency radio radiation is emitted by extremely energetic charged particles which are slowed down by magnetic fields. Thus, a very large fraction of this light is emitted via energetic and sometimes violent processes.
Super massive black holes
The origin of the super-massive black holes which are in the cores of all galaxies, and whose mass can be as high as billions of solar masses, is still a mystery. When the mater is sucked up by a black hole
When matter is sucked up by a super-massive black hole, jets of charged and very energetic particles are created and perturb the environment of the galaxy. Radio waves are then emitted. Radio observations of these objects enables one to study how super-massive black holes grow, and their role in the formation of galaxies.
LOFAR reveals, for example, that the super-massive black holes associated with the most massive galaxies are always active, and that matter has been falling ceaselessly within them for billions of years. These images, produced by LOFAR, also enable one to study how the black holes regularly perturb the dynamics of the intergalactic medium.
Clusters of galaxies
We have known for several years that the shock waves created by the huge motions of the intergalactic medium can accelerate particles up to the very high energies needed to emit radio waves. In particular, when galaxy clusters (made up of hundreds or thousands of galaxies) collide, they produce radio emission which can spread over thousands of light years. This emission is abundantly observed by LOFAR, and constitutes an exceptionally good method to study the large scale dynamics and structure of the Universe.
Next steps
LOFAR has opened a new observational window on the deep Universe. The 26 papers published in the special issue of the journal Astronomy et Astrophysics concern only the first two percent of the sky map. Over and above the new scientific discoveries contained in this preliminary data, the novelty is the richness of these images. They contain new astrophysical objects whose nature has yet to be understood.
There will be many new scientific discoveries. The objective of the team is to create sensitive high resolution images of the whole of the northern hemisphere sky, which will have overall 15 million radio sources. It will take several years to fully exploit the total 48 petaoctets of data, which represents the equivalent of almost 40 Eiffel tower’s worth of DVDs. The final image should be available towards the year 2024. Since this energy domain has hardly been explored, the scientific impact of this huge survey is hard to assess. The project is eagerly supported by the French astronomical community. The construction of SKA in the southern hemisphere should start in 2020. French astronomers have built a trial model on the site of the Nançay radlio-astronomical station : named NenuFAR (New Extension in Nançay Upgrading LOFAR), together with LOFAR, it will prepare the French astronomical community for the use of SKA.