These are not stars, these are black holes

An international team of astronomers has just published a remarkable map of the night sky without a single star, but which is instead filled with 25,000 supermassive black holes.

We know what you are thinking. How is that possible if black holes don’t radiate light, you fool geese? This is only half true, however. The black hole itself is more or less invisible, but if it nibbles at a star or other object, the tidal forces of its gravity tear it apart and create a flattened accretion disc around it.

The disc material rotates around the black hole’s event horizon at significant fractions of the speed of light and sometimes this material is ejected by the black hole’s intense magnetic field, creating jets of particles at the hole’s magnetic poles. noir.

These relativistic jets, as they are called, emit a parcel of radio waves. This is what astronomers mapped in the northern skies using an array of 52 radio telescopes across Europe known as the Low Frequency Array (LOFAR).

Sky map showing 25,000 supermassive black holes.  Each white point is a supermassive black hole in its own galaxy.

Each white point on the map is a supermassive black hole in a distant galaxy. (Image credit: LOFAR / LOL survey)

“This is the result of many years of working on incredibly difficult data”, said lead researcher Francesco de Gasperin, formerly of the University of Leiden but now at the Universität Hamburg, Germany. “We had to invent new methods to convert radio signals into images of the sky.”

The incredible map shows low-frequency radio signals from 25,000 supermassive black holes at the center of distant galaxies and only covers about 4% of the night sky in the northern hemisphere. The researchers eventually hope to create a complete sky atlas of supermassive black holes visible in the northern hemisphere.

The card was included as part of a to study which has been accepted for publication in a future edition of the journal Astronomy & Astrophysics.


Analysis: Computer Algorithms Help Us See the Universe

One of the problems with seeing radio waves from supermassive black holes is that they often produce low-frequency radio waves. Normally this would not be a problem, but for terrestrial radio telescopes, the earth’s ionosphere completely reflects radio signals below 3 MHz and distorts signals up to 30 MHz.

“It’s like when you try to see the world while submerged in a swimming pool,” said study co-author Reinout van Weeren, of the Leiden Observatory. “When you look up, the waves on the pool water deflect the light rays and distort the view.”

In order to counter this, the researchers developed a supercomputer algorithm that corrects the distortion effect of the ionosphere every four seconds of the 256 hours of observation.

“After many years of software development, it’s so wonderful to see that it has now really worked,” said study co-author Huub Röttgering, scientific director of the Leiden Observatory.

This is not the first time that a computer algorithm has helped astronomers map the skies. The most famous, in 2019, Katie bouman designed the algorithm that put together the various data streams of the Event Horizon Telescope to give us the first image of a black hole’s event horizon.

Computer algorithms have long played a pivotal role in astronomy, and as our computers become even more powerful, the power of these algorithms to open our eyes to the universe will only increase.

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