The image above may look like a fairly normal image of the night sky, but what you’re looking at is much more special than the bright stars. Each of these white dots is an active supermassive black hole.
And each of these black holes is devouring material in the heart of a galaxy millions of light-years away; this is how they might be specified.
With a total of 25,000 such points, astronomers have created the most detailed map to date of black holes at low radio frequencies, a success that took years and a European-sized radio telescope to compile.
“This is the result of many years of work on incredibly difficult data,” explained astronomer Francesco de Gasperin of the University of Hamburg in Germany. “We had to invent new methods to turn radio signals into images of the sky.”
(LOFAR / LOL survey)
When they are not long, the black holes do not give off any detectable radiation, which makes them much harder to find. When a black hole actively accretes on the material, which makes it roll from a disk of dust and gas that surrounds it as water surrounds a drain, the intense forces involved generate radiation through multiple lengths of wave that we can detect through the immensity of space.
What makes the above image so special is that it covers ultra-low radio wavelengths, as detected by the LOw Frequency ARray (LOFAR) in Europe. This interferometric network consists of about 20,000 radio antennas, distributed over 52 locations throughout Europe.
Currently, LOFAR is the only network of radio telescopes capable of obtaining deep, high-resolution images at frequencies below 100 megahertz, offering a view of the sky like no other. This data publication, which covers four percent of the northern sky, is the first of the network’s ambitious plan to imagine the entire northern sky at ultra-low frequencies, the LOFAR LBA Sky Survey (LoLSS).
Because it is based on Earth, LOFAR has a major hurdle to overcome that does not affect space-based telescopes: the ionosphere. This is particularly problematic for low-frequency radio waves, which can be reflected back into space. At frequencies below 5 megahertz, the ionosphere is opaque for this reason.
The frequencies that penetrate the ionosphere can vary depending on atmospheric conditions. To overcome this problem, the team used supercomputers that ran algorithms to correct ionospheric interference every four seconds. Throughout the 256 hours LOFAR looked at the sky, there are many corrections.
This is what has given us such a clear view of the ultra low frequency sky.
“After many years of software development, it’s so wonderful to see that this has already worked,” said astronomer Huub Röttgering of the Leiden Observatory in the Netherlands.
Having to correct the ionosphere also has another advantage: it will allow astronomers to use LoLSS data to study the ionosphere itself. Ionospheric traveling waves, flashes, and the relationship of the ionosphere to solar cycles could be characterized in much more detail by LoLSS. This will allow scientists to better restrict ionospheric models.
And the survey will provide new data on all sorts of astronomical objects and phenomena, as well as possible undiscovered or unexplored objects in the region below 50 megahertz.
“The final launch of the survey will facilitate advances in various areas of astronomical research,” the researchers wrote in their article.
“[This] will allow the study of more than one million low-frequency radio spectra, providing unique information on physical models of 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 resolution and angular depth. “
The results will be published in Astronomy and astrophysics.