These aren’t ordinary stars. (LOFAR/LOL Survey)

The map, which will be published in the journal Astronomy & Astrophysics, is the most detailed sky map in the field of so-called low radio frequencies. For the cosmic mapping work, the astronomers used 52 stations with LOFAR antennas distributed in nine European countries.

To the untrained eye, the sky map appears to contain thousands of stars, but they are actually supermassive black holes. Each black hole is located in a different distant galaxy. Radio emissions are emitted by matter that was ejected as it approached the black hole. Research leader Francesco de Gasperin (formerly Leiden University, now Universität Hamburg, Germany) says of the study: “This is the result of many years of working with incredibly difficult data. We had to invent new methods to convert radio signals into images of the sky.”

From the bottom of the pool

Observations at long radio wavelengths are complicated by the ionosphere that surrounds the Earth. This shell of free electrons acts like a cloudy lens that moves constantly through the radio telescope. Co-author Reinout van Weeren (Leiden Observatory) explains: “It’s similar to when you’re trying to see the world while you’re submerged in a pool. When you look up, the waves in the pool water bend light rays and distort the view.”

All-Sky Map

The new map was created by combining 256 hours of northern sky observations. The researchers deployed supercomputers with new algorithms that correct for the effect of the ionosphere every four seconds. The scientific director of the Leiden Observatory, Huub Röttgering, is the last author of the publication. He is delighted with the results: “After many years of software development, it’s wonderful to see that this has really worked.” The map now covers 4 percent of the northern half of the sky. The astronomers plan to continue until they have mapped the entire northern sky. In addition to supermassive black holes, the map also provides information about the large-scale structure of the universe, among other things.