"groundbreaking": First image of the Milky Way’s central black hole

The Event Horizon Telescope has captured the first image of the supermassive black hole Sagittarius A* at the center of the Milky Way. The organizations involved in the radio telescope network presented the image to the public on Thursday. It is the long-awaited first glimpse of the supermassive object at the center of our galaxy and outstanding visual evidence that the object is in fact the black hole that has so far been primarily described theoretically. The recording provides “valuable information on the functioning of such giants,” explains the European Southern Observatory. They are thought to be at the center of most galaxies, but prior to the image of Sagittarius A* (Sgr A*), only the historic image of the black hole of M87 was successful.

Size comparison of the two supermassive black holes pictured now.

Although Sagittarius A* is much closer to us than galaxy M87, capturing it was a lot more difficult, explains Steward Observatory’s Chi-kwan Chan. This is partly due to the fact that Sgr A* is significantly smaller. While the gas orbiting M87’s black hole takes days and weeks to orbit even at speeds close to the speed of light, Sgr A* takes just minutes. Brightness and appearance therefore change much faster: It’s a little “like trying to take a sharp picture of a puppy that’s constantly wagging its tail in front of the camera”. The photo presented now is an average of several recordings. To create it, “sophisticated new methods” had to be devised.

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Zoom on Sagittarius A*

The Max Planck Institute for Radio Astronomy (MPIfR ) now calls the work “groundbreaking” and explains that it is based on the ingenuity of more than 300 researchers from 80 institutes around the world. The evaluation of the data collected in 2017 would have required five years of hard work and supercomputers. Unlike the image of M87, the image now presented is also much better suited for testing theories on the nature of gravity. Because for Sgr A* you have reliable data about the mass. The size of the shadow that can be seen can therefore be compared with the theories: “And it fits very well!” says Michael Kramer from the MPIfR.

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Grouping and averaging the images of Sgr A*

Sgr A* is about 27,000 light-years from us and about the size in the sky of a lunar donut. While the black hole itself cannot be seen naturally, the immensely accelerated and heated gas in its immediate vicinity shows the telltale trail already known from M87. “We were amazed at how well the size of the ring matched the predictions of Einstein’s general theory of relativity,” says EHT project scientist Geoffrey Bower of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei. The results are published in a special issue of the scientific journal The Astrophysical Journal Letters.

The Event Horizon Telescope consists of a global network of telescopes that are interconnected to simulate a gigantic radio telescope almost the size of the Earth – one participating observatory is on Greenland, for example, one on the South Pole. The maximum angular resolution of the world-spanning radio telescope is up to 15 micro arc seconds. That’s smaller than a golf ball on the moon’s surface. The individual telescopes were connected for the first time in 2017, and thanks to the good weather, several objects were photographed.

The Atacama Large Millimeter/submillimeter Array (ALMA) of the European Southern Observatory in Chile

(Image: ESO/C.Malin)

First, the picture of the event horizon of the black hole in the center of the galaxy M87, 55 million light-years away, was calculated with unexpected sharpness. The recordings were also analyzed further because follow-up observations had to be canceled due to the corona pandemic. It was discovered that a significant part of the light is polarized.

An enormous technical effort is made for the recordings. Radio telescopes on several continents targeted the black holes at exactly the same times and are recording as much data as possible. At some locations, memories were used that can be written at 16 gigabits per second. At the European Southern Observatory’s ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope array alone, hard drives with a capacity of more than 1 petabyte were installed.

The collected data was then transported by ship or plane to be finally merged at two high-performance computers in Germany (at the Max Planck Institute for Radio Astronomy in Bonn) and the USA. Extremely precise clocks at the various locations were essential for this, in order to be able to precisely synchronize the collected data with their time stamps at the end. Each of these clocks would advance or lose just one second in a million year period.

Astrophysicists from Germany and the USA had shown that a black hole can be imaged from Earth at all only calculated at the turn of the millennium. Using simulated images, they had shown that the dark shadow appears five times larger than previously expected. This was the only way that the black holes now being targeted had advanced into the area in which they were to be imaged on Earth. The astrophysicist responsible for this, Heino Falcke, was surprised at how close the mapping was already and was confident that it would soon succeed. Once again it has now been confirmed.


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