The image may seem totally normal, and it appears to contain thousands of stars, but in reality they are supermassive black holes. Surprisingly it is a map of the sky that shows more than 25,000 supermassive black holes. Each black hole is located in a different distant galaxy.
An international group of astronomers published in early 2021 in the journal Astronomy & Astrophysics, the map of the sky that shows 25,000 black holes, positioning it as the most detailed ever produced in the field of so-called low radio frequencies. The map is the result of 256 hours of observations of the northern sky, using data from the LOw Frequency ARray (LOFAR) in Europe. This interferometric network consists of about 20,000 radio antennas, distributed in 52 stations spread across nine European countries.
“This is the result of many years of working with incredibly difficult data. “We have had to invent new methods to convert radio signals into images of the sky,” research director Francesco de Gasperin said in a statement.
Currently, LOFAR is the largest radio telescope operating at the lowest frequencies that can be observed from Earth. Unlike single-dish telescopes, LOFAR is a multipurpose sensor network, with innovative computing and networking infrastructure that can handle extremely large volumes of data.
Because this is not a space telescope, but rather makes its observations from the surface, it has significant challenges to overcome. The main one is the ionosphere that surrounds the Earth – a layer of free electrons that clouds telescopes – makes it difficult to track black holes.
“It’s similar to trying to see the world while submerged in a pool. When you look up, the waves of the pool water bend the light rays and distort your vision,” explains co-author Reinout van Weeren (Leiden Observatory).
The frequencies that penetrate the ionosphere can vary depending on atmospheric conditions. In order to successfully complete the task, the team used supercomputers that ran algorithms to correct ionospheric interference every four seconds. Throughout the 256 hours that LOFAR was observing the sky, there were many corrections.
When you hear “25 thousand supermassive black holes,” chances are a huge number and a lot of space come to mind, and they do. However, the map covers only 4% of the northern half of the sky. 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.
More detailed research may allow the study of more than 1 million low-frequency radio spectra, providing unique insights into physical models for galaxies, active nuclei, galaxy clusters, and other fields of research. “This experiment represents a unique attempt to explore the ultra-low-frequency sky with high angular resolution and depth,” the paper’s authors wrote.
The study is detailed in an article published in Astronomy & Astrophysics.
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