The little dots in this image are not stars or galaxies. These are black holes
The image above may look like a fairly normal image of the night sky, but what you are looking at is much more special than just twinkling stars. Each of these white dots is an active supermassive black hole.
And each of those black holes is devouring matter inside a galaxy millions of light years away – that’s how they could be located.
Totaling 25,000 of those points, astronomers created the most detailed map to date of low-frequency radio black holes in early 2021, an achievement that took years and a European-sized radio telescope to compile.
“This is the result of many years of working on incredibly difficult data”, explained the astronomer Francesco de Gasperin from the University of Hamburg in Germany. “We had to invent new methods of converting radio signals into images of the sky.”
When they’re just lying around, black holes don’t give off any detectable radiation, making them much harder to find. When a black hole actively accumulates matter – coiling it from a disk of dust and gas that surrounds it like water surrounds a drain – the intense forces involved generate radiation at multiple wavelengths. that we can detect across the vastness 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 around 20,000 radio antennas, spread over 52 sites across Europe.
Currently, LOFAR is the only radio telescope array capable of deep, high resolution imaging at frequencies below 100 megahertz, providing a view of the sky like no other. This release of data, covering four percent of the northern sky, was the first in the network’s ambitious plan to image the entire northern sky in ultra-low frequencies, the LOFAR LBA Sky Survey (LoLSS).
Because it is Earth-based, LOFAR has an important obstacle to overcome that does not afflict space telescopes: the ionosphere. It is particularly problematic for ultra-low frequency radio waves, which can be reflected in space. At frequencies below 5 megahertz, the ionosphere is opaque for this reason.
The frequencies that enter the ionosphere can vary depending on atmospheric conditions. To overcome this problem, the team used supercomputers running algorithms to correct for ionospheric interference every four seconds. In the 256 hours that LOFAR looked at the sky, that made a lot of corrections.
This is what gave us such a clear view of the ultra-low frequency sky.
“After many years of software development, it’s so wonderful to see that it has now really worked” said astronomer Huub Röttgering of the Leiden Observatory in the Netherlands.
Another benefit of having to correct for the ionosphere is that it will allow astronomers to use the LoLSS data to study the ionosphere itself. Traveling ionospheric waves, twinkle, and the relationship of the ionosphere to solar cycles could be characterized in much more detail with LoLSS. This will allow scientists to better constrain ionospheric models.
And the investigation will provide new data on all kinds of astronomical objects and phenomena, as well as possibly undiscovered or unexplored objects in the region below 50 megahertz.
“The final publication of the survey will facilitate progress in a range of areas of astronomical research”, the researchers wrote in their article.
“[This] will enable the study of over a million low-frequency radio spectra, providing unique information on physical models of galaxies, active nuclei, galaxy clusters and other areas of research. This experiment represents a unique attempt to explore the ultra-low frequency sky at high angular resolution and depth. “
The results were published in Astronomy & Astrophysics.
A version of this article was first published in February 2021.