The picture above may look like a fairly normal picture of the night sky, but what you are looking at is much more special than just twinkling stars. Each of those white dots is an active supermassive black hole.
And each of the black holes devours material in the heart of a galaxy millions of light-years away – that’s how it could be fixed.
Astronomers have made a total of 25,000 such dots to date with the most detailed map to date of black holes at low radio frequencies, an achievement that took years, and a Europe-sized radio telescope to put together.
“This is the result of many years of work on incredibly difficult data,” explains astronomer Francesco de Gasperin of the University of Hamburg in Germany. “We had to invent new methods to convert the radio signals into the aerial images.”
(LOFAR / LOL recording)
If they just do not do much, black holes give off no detectable radiation, which makes it much harder to find. When a black hole collects active material – which flushes out of a dust and gas disk that circles it like water in a drain – the forces in question generate radiation over various wavelengths that we can observe in large space.
What makes the above image so special is that it covers the ultra-low radio wavelengths, as detected by the LOw Frequency ARray (LOFAR) in Europe. This interferometric network consists of about 20,000 radio antennas, spread over 52 locations in Europe.
Currently, LOFAR is the only radio telescope network that can display deep, high resolution images at frequencies below 100 megahertz, offering an aerial view like no other. This release of data, which accounts for four percent of the Northern Sky, is the first for the network’s ambitious plan to image the entire Northern Sky in ultra-low frequencies, the LOFAR LBA Sky Survey (LoLSS).
Because it is based on Earth, LOFAR does have an important obstacle to overcome that does not affect space-based telescopes: the ionosphere. This is especially problematic for ultra-low frequency radio waves, which in turn can be reflected in space. At frequencies below 5 megahertz, the ionosphere is opaque for this reason.
The frequencies entering the ionosphere can vary according to atmospheric conditions. To overcome this problem, the team used supercomputers that use algorithms to correct ionospheric interference every four seconds. Over the 256 hours that LOFAR stared into the air, these are many corrections.
This is what gave us such a clear picture of the ultra-low frequency air.
“After many years of software development, it’s so wonderful to see that it’s really worked out now,” said astronomer Huub Röttgering of the Leiden Observatory in the Netherlands.
Correcting for the ionosphere also has another advantage: it will allow astronomers to use LoLSS data to study the ionosphere itself. Ionospheric moving waves, glitter and the relationship of the ionosphere with solar cycles can be characterized with the LoLSS in much greater detail. This will enable scientists to better limit ionospheric models.
And the survey will provide new data on all sorts of astronomical objects and phenomena, as well as possibly undiscovered or unexplored objects in the area below 50 megahertz.
“The final launch of the survey will enable progress in a variety of astronomical research areas,” the researchers wrote in their paper.
“[This] will enable the study of more than 1 million low-frequency radio spectra, providing unique insights into physical models for galaxies, active nuclei, galaxies, and other research fields. This experiment represents a unique attempt to explore the ultra-low frequency air with high angular resolution and depth. ‘
The results must be published in 2008 Astronomy & Astrophysics.