Over the past decade, improvements in radio interferometry and data processing have enabled surveys with unprecedented sensitivity and resolution. One of the most ambitious projects in this effort is the LOFAR Two-metre Sky Survey. The latest data release from this survey has now produced the most detailed map of the low-frequency radio sky ever assembled. Using observations from the Low Frequency Array telescope network, astronomers catalogued nearly 13.7 million radio sources across most of the northern sky.

The dataset represents the third major public release of the survey and currently forms the largest catalogue of radio sources available to researchers. The observations reveal an enormous population of energetic cosmic objects. Many sources correspond to galaxies hosting active supermassive black holes. Others trace star-forming galaxies, supernova remnants, and large-scale structures such as galaxy clusters.

A large-scale map of the radio universe

The LOFAR Two-metre Sky Survey covers a large fraction of the observable sky. The latest data release maps roughly 19,000 square degrees, which corresponds to nearly 88 percent of the northern celestial hemisphere. This coverage allows astronomers to examine the distribution of radio sources across a substantial part of the universe.

Within this region, researchers detected nearly 13.7 million individual radio sources. Many of these sources appear extremely faint and were not visible in earlier radio surveys. Consequently, the dataset greatly expands the known population of radio-emitting objects.

The observations focus on radio frequencies between 120 and 168 megahertz. These frequencies correspond to wavelengths of about two metres, which explains the name of the survey. Low-frequency observations are particularly sensitive to synchrotron radiation, a process that occurs when high-energy charged particles spiral through magnetic fields.

As a result, the radio sky looks very different from the optical sky. Optical telescopes primarily show stars and glowing gas clouds. Radio observations instead reveal jets, lobes, and diffuse structures produced by energetic particles and magnetic fields.

The telescope network behind the survey

The observations were obtained using the Low Frequency Array, a large radio interferometer distributed across Europe. Unlike traditional radio telescopes that rely on a single dish, this system consists of thousands of small antennas grouped into stations.

Each station records incoming radio signals from the sky. Computers then combine the signals from all stations to reconstruct detailed images through interferometric techniques. This approach allows the array to achieve high sensitivity and sharp angular resolution.

The telescope network currently includes 52 observing stations located in several European countries. Signals from these stations travel through high-speed data links to central processing facilities. There, powerful computing systems correlate the signals and produce the final observational data.

Because the stations are separated by large distances, the array effectively behaves like a giant telescope spanning thousands of kilometres. The wide separation between antennas improves the angular resolution of the observations and allows astronomers to detect fine structures within distant radio sources.

A decade of observations and data processing

Constructing the radio sky map required extensive observational and computational effort. The survey accumulated data over approximately 10.5 years of observations. During this period, astronomers collected nearly 12,950 hours of telescope data. The raw observations generated an enormous volume of information. In total, the dataset reached about 18.6 petabytes of raw data before processing.

Handling such large datasets presents significant technical challenges. Radio signals often experience distortions as they travel through Earth’s atmosphere and ionosphere. Furthermore, instrumental effects can introduce additional variations that must be corrected.

To address these issues, researchers developed sophisticated calibration techniques. Automated pipelines process the raw data and remove distortions step by step. These pipelines also combine observations from multiple stations to produce accurate images of the sky. Processing the survey required substantial computing resources. Scientists used roughly 20 million CPU core hours to calibrate and analyze the data.

Even after processing, the released scientific products remain extremely large. The final data release occupies approximately 590 terabytes of images, catalogues, and related datasets. Despite its size, the dataset is publicly accessible. Astronomers around the world can analyze the data and search for discoveries. This open approach ensures that the survey will continue to generate scientific results for many years.

A vast population of active supermassive black holes

One of the most significant outcomes of the survey is the identification of millions of galaxies hosting active supermassive black holes. These objects dominate the radio sky and account for a large fraction of the detected sources.

Nearly every large galaxy contains a black hole at its center. Most of the time, these black holes remain relatively inactive. However, when gas and dust begin to fall toward the black hole, the situation changes.

The infalling material forms a rotating accretion disk around the black hole. Within this disk, intense magnetic fields develop and accelerate charged particles to extremely high energies. Some of these particles escape along narrow beams emerging from the poles of the system.

These beams form powerful relativistic jets that extend far beyond the host galaxy. As the particles travel through magnetic fields, they emit synchrotron radiation that becomes visible in radio observations. Over time, the jets inflate large radio lobes surrounding the galaxy. These lobes may extend hundreds of thousands or even millions of light-years into intergalactic space. The LOFAR survey detects these structures in remarkable detail. Consequently, the dataset reveals a vast population of active galaxies distributed across the universe.

Clear skies!

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