MOSCOW, April 10, Tatyana PichuginaThe first evidence that there are supermassive black holes in the middle of most galaxies was provided exactly five years ago. Now there are even more detailed images – from the Milky Way. What is known about active galactic centers – in the material .
An eye the size of a planet
Black holes were predicted by Albert Einstein's general theory of relativity. Their defining feature is the event horizon, which even quanta of light cannot overcome. The gravitational attraction of this region of space-time is so strong.
These objects are formed during the merger of large stars or the collapse of gas and dust clouds. There are small ones, with a mass of less than one hundred of our Suns, and there are supermassive ones – like millions and billions of Suns. According to one hypothesis, at the birth of the Universe, primordial black holes the size of a planet were formed. Perhaps these have survived to this day.
In 1975, Soviet astrophysicist Joseph Shklovsky suggested that at the center of our Galaxy there is a black hole with a mass of 30 thousand Suns. Its radiation is not thermal, but most likely synchrotron, which is emitted by charged particles accelerated in a magnetic field to sublight speed. It was discovered in accelerators in the 1940s.
Shklovsky's hypothesis was confirmed – he was only mistaken with the mass, underestimating the black hole by two orders of magnitude.
Until now, evidence of its existence has been convincing but indirect. In 2020, three scientists – theorist Roger Penrose, astronomers Reinhard Genzel and Andrea Ghez – were awarded the Nobel Prize for them.
Direct observation requires very powerful tools. In Russia, the Radioastron radio telescope was launched into orbit in 2011. Together with ground-based radio observatories, it formed the largest super-long-baseline interferometer at that time. This is another invention of Soviet scientists. An interferometer is a network of identical antennas located tens of kilometers from each other and even on different continents. Simultaneous observation of a specific, very distant, space object turns such a network into one large radio telescope.
There are several similar interferometers on Earth now. One of them, the Event Horizon Telescope (EHT), is the size of our planet. In 2019, scientists working on it reported reconstructing an image of a supermassive black hole in the elliptical galaxy M87* – 54 million light-years from Earth in the constellation Virgo. This galaxy is 1,500 times larger than the Milky Way and can be seen even with a regular telescope. And in the center is a black hole of 6.5 billion solar masses.
Observing Sagittarius A* (Sgr A*) in our Galaxy is much more difficult due to the clouds of interstellar gas covering it. It took several more years of data collection to present very average images of the center of the Milky Way in 2022.
Details cleared
The glowing orange rings in the TGS images, similar to the Eye of Sauron from The Lord of the Rings, are now known throughout the world. Their size and shape are predicted with good accuracy by the general theory of relativity. Previously, astronomers had only observed stars moving around an invisible compact object at the center of the Milky Way. Now they saw what he looked like. The dark spot in the center is the shadow of a black hole, from which no light comes. It is surrounded by a ring of bright radiation generated by gravitational forces.
A significant part of the collected radiation is polarized (this was noticed a long time ago). This will help to better understand the physics of galactic centers, in particular, to study the magnetic fields around black holes. In 2021, the same collaboration published images of the magnetosphere of M87, and recently of the Milky Way.
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“These images will help explain how magnetic fields pull matter towards themselves and eject some in the form of jets,” noted Andrew Chael from the Princeton Center for Theoretical Research at NASA.
Jets are jets of energy from the vicinity of a black hole. They have relativistic speeds. How exactly they arise is not entirely clear. What is clear is that some of the matter somehow escapes their gravity. The jets from the center of M87 are estimated to stretch out over five thousand light years, exceeding the size of the galaxy. The image of magnetic fields gives a clue. These fields are so strong that they are capable of throwing some of the matter away from the black hole and spinning the particles to the speed of light.
“Thanks to polarized light, we are learning much more about astrophysics, the properties of gas and the mechanism of “feeding” of the black hole,” says the co-author of the work. Angelo Ricarte from Harvard.
Images of the magnetosphere around M87* and Sagittarius A* are very similar, indicating similar processes that occur there, in particular the formation of jets. Perhaps the black hole in the center of the Milky Way also has them, but they have not yet been detected.
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