Incredibly, this image above doesn’t show a starry sky. Instead, every white dot is a supermassive black hole at the core of a different galaxy. This extraordinary ensemble was created not by visible light but from radio waves emitted by matter orbiting these distant and huge black holes, and is the most detailed sky map in low radio frequencies yet.
The radio observations were collected by the LOw-Frequency ARray (LOFAR), a system of 52 stations spread over nine European countries: Germany, The Netherlands, Poland, France, the UK, Sweden, Ireland, Latvia, and Italy. Together they are the largest combined radio telescope in the world.
"This is the result of many years of work on incredibly difficult data. We had to invent new methods to convert the radio signals into images of the sky,” lead author Dr Francesco de Gasperin from the University of Hamburg, said in a statement.
The data collected covers 4 percent of the northern half of the sky and is just the beginning of an ambitious program to map the entire northern sky. As reported in Astronomy & Astrophysics, the program will help answer questions on hugely different scales from the magnetospheres of exoplanets to the distributions of galaxies in the universe.
The observations are conducted in what is known as low radio frequencies. Long radio wavelengths are greatly affected by the ionosphere, the layer of electrically charged particles that surround our planet. The free electrons that make that layer are not good for these radio observations as their motion affects the quality of what astronomers can observe.
“It's similar to when you try to see the world while immersed in a swimming pool. When you look up, the waves on the water of the pool deflect the light rays and distort the view,” co-author Reinout van Weeren from the Leiden Observatory explained.
Correcting the effect of the ionosphere was not an easy task. The researchers had to use supercomputers that could do exactly that every 4 seconds. Given this map took 256 hours of observations, you can imagine what a challenge it was.
“After many years of software development, it is so wonderful to see that this has now really worked out,” said Huub Röttgering, scientific director of the Leiden Observatory.
Black holes by definition do not emit light, so it is incredible that this map allows us to see something that is quintessentially invisible to us. Seeing 25,000 of them all at the same time is absolutely extraordinary.