A massive black hole sits at the center of our home galaxy. For the first time, researchers manage to take a picture of it. It is made possible by a global network of radio telescopes that are combined to form a super telescope.

It is an astronomical world premiere: Researchers are able to take the first picture of the black hole in the center of our home galaxy, the Milky Way. The European Southern Observatory (ESO) and the international telescope project “Event Horizon Telescope” (EHT) presented the first image of the object Sagittarius A* at a press conference in Garching, Washington, Santiago de Chile and other cities worldwide. The Max Planck Institute for Radio Astronomy (MPIfR) in Bonn played an important role in creating and interpreting the observation results.

“We have achieved something very new, very exciting,” said Xavier Barcons, Director General of ESO, presenting the image. “We were often so close.” The picture was taken in the radio wave range. It was made possible by a network consisting of eight radio observatories around the world, combined into a single Earth-sized virtual telescope, the Event Horizon Telescope. The EHT is named after the “event horizon,” the limit of the black hole beyond which no light can escape.

Because black holes don’t even let light escape due to their extreme mass, they are practically invisible. Still, glowing gas orbiting the black hole reveals a telltale signature: a dark central region (dubbed the “shadow”) surrounded by a bright annular structure. The new image captures light being diffracted by the black hole’s strong gravity.

Just over three years ago, the EHT took the first ever image of a black hole. At that time, the image of the black hole in the galaxy Messier 87 (M87) caused a sensation around the world. The researchers are now repeating this success with Sagittarius A* (pronounced Sagittarius A star), the black hole in the center of the Milky Way – although it took longer than expected.

One reason: The evaluation of the data from Sagittarius A* was much more difficult than with M87 – Sagittarius A* is only 27,000 light years away, but M87 55 million light years. But there is a huge difference in size: Sagittarius A* is gigantic at about 3.7 million solar masses and has a diameter of about 20 million kilometers (our sun would fit into it about 15 times side by side). But M87 is much larger with 6.5 billion solar masses.

What makes the difference is the rotation of the gas around the black hole, explains EHT scientist Chi-kwan (“CK”) Chan: “The gas near the black holes moves at the same speed – almost as fast as light – around Sagittarius A* and M87. But while the gas takes days to weeks to orbit the larger object M87, the much smaller Sagittarius A* completes its orbit in just a few minutes.” This means that the brightness and appearance of the gas around Sagittarius A* changed rapidly during the observation – “a bit like trying to get a sharp picture of a puppy constantly wagging its tail in front of the camera. “

The scientists are particularly pleased that they finally have images of two very different sized black holes, allowing them to study how the two objects are similar and how they differ. They have also started using the new data to test theories and models about how gas behaves around supermassive black holes. This process is not yet fully understood, but it is believed to play a key role in the formation and evolution of galaxies.

(This article was first published on Thursday, May 12, 2022.)

6