The "invisible" Universe offers a spectacle far vaster than what our direct vision allows us to appreciate. A recent mapping effort has catalogued over 13 million cosmic objects and events, revealing a radically different sky when observed in radio waves.
Dubbed LoTSS-DR3, this project relies on the LOFAR network, the world's largest low-frequency radio telescope. By scanning the sky in these waves, astronomers distinguish luminous jets emitted by supermassive black holes, colliding galaxies, and exploding stars. This approach profoundly alters our perception of space.
Nested at the heart of large galaxies, supermassive black holes become active when they attract matter. This process generates powerful jets that extend far beyond their host galaxy. By detecting the radio waves produced by these jets, scientists can study how this energy influences the evolution of surrounding galaxies. Martin Hardcastle from the University of Hertfordshire notes that this allows them to examine various developmental stages of these objects.
Beyond black holes, the observation campaign has captured signals from galaxy mergers and supernovae. These violent events accelerate particles to speeds close to that of light, emitting detectable radio waves.
For closer structures, the data provides information on the composition of our own galaxy, the Milky Way. Marijke Haverkorn from Radboud University observes that LOFAR allows the mapping of internal magnetic fields with exceptional precision.
The continuation of this work looks promising with the arrival of LOFAR 2.0. This upgrade will double the observation speed and improve data resolution.
How radio telescopes work
Radio telescopes, like LOFAR, capture electromagnetic waves that are invisible to the human eye. These waves come from cosmic sources such as black holes or exploding stars, and their detection requires large antennas or arrays. By combining signals from several antennas, astronomers create detailed images of the sky, revealing structures that visible light does not show. This technology has evolved since the first radio telescopes, now enabling large-scale mapping.
Unlike optical telescopes, radio instruments operate day and night, as radio waves pass through Earth's atmosphere without being blocked by clouds or dazzled by sunlight. They operate in specific frequency bands, often low ones, to study phenomena like magnetic fields or accelerated particles. LOFAR, for example, uses a network of antennas spread across several countries to increase sensitivity and resolution.
The collected data is processed by powerful computers to remove interference and reconstruct images. This process allows for the mapping of millions of objects in a single study, as shown by LoTSS-DR3. Advances in computing have made it possible to analyze huge volumes of data, opening new perspectives in astronomy.
The use of radio waves complements other observation methods, such as infrared or X-rays. Each wavelength reveals distinct aspects of the Universe, offering a more complete view. Radio telescopes thus play a key role in our understanding of cosmic processes, from the nearest to the farthest.