. Scientific Frontline: Astronomers Identified the Nature of Instability in the Accretion Disk of the Galaxy NGC 4258

Tuesday, July 5, 2022

Astronomers Identified the Nature of Instability in the Accretion Disk of the Galaxy NGC 4258

The galaxy NGC 4258 is 22.8 million light-years from Earth.
Photo Credit: NASA

An international group of researchers, including Andrey Sobolev, a leading researcher at the Kourovka Astronomical Observatory of the Ural Federal University, for the first time examined the details of the distribution of maser emissions in the accretion It was found that in this disk acts magneto-rotational instability. Scientists reported the discovery in the journal Nature Astronomy.

"The discovery of a disk around this galaxy was reported by Miyoshi and Greenhill back in 1995 in articles in Nature and in The Astrophysical Journal. That was the first time we knew there was a disk. But now, with the RadioAstron's ultra-high angular resolution, we have been able to ascertain for the first time the details of the distribution of the maser emission spots. The regularity in their maser locations is explained by the fact that there is a magneto-rotational instability in the accretion disk," says Andrey Sobolev.

The instabilities determine the evolution of disks. We can use them to know whether the disk is stationary or whether everything in it is changing rather rapidly. In other words, the instabilities help determine the physical status or physical state of the disk: how it is formed, what happens in it, and predict whether it will change over time. Therefore, to understand the processes that occur in the accretion disk, scientists need to understand what instabilities are operating there, and the detection of the magneto-rotational instability is extremely important. At the same time, scientists are not going to put an end to the research of the unique object around the supermassive black hole. According to Sobolev, now it is the turn of theorists to explain the unique data obtained at the cosmic interferometer - the largest device created by mankind. This interferometer was created as part of the RadioAstron project, in which Russian scientists play a leading role.

"We have many terabytes of data to study and analyze. To understand - it took a team of professionals four years to process the array of information obtained with the interferometer in configurations as small as 19.6 Earth diameters during the three observations in 2014 and 2016. However, we still have data obtained at interferometer sizes of 26.5 Earth diameters. And they have not yet been analyzed, it is very difficult work. Research will certainly continue, but it will take time," concludes Andrey Sobolev.

The galaxy NGC 4258 is far from Earth - at a distance of 7.6 megaparsecs (the light would travel from Earth to this galaxy 22.8 million light years away). To see the accretion disk of this galaxy and what is going on there, a very high angular resolution is required. The highest angular resolution for water masers was achieved in the RadioAstron project.

The first observed evidence for the existence of black holes was obtained from observations of the galaxy NGC 4258. Scientists have shown that there is such a large mass of matter inside this volume that it can only be a black hole.

The study involved researchers from the Netherlands Institute of Radio Astronomy ASTRON (the Netherlands), the Astro Space Center of the Physical Institute of the Russian Academy of Sciences (Russia), the Ural Federal University (Russia), the Chinese Academy of Sciences (China), the Max Planck Society Institute for Radio Astronomy (Germany) and Kagoshima University (Japan).


Accretion disks occur around stars, galaxies, and black holes. For example, under the action of gravity, matter from the surrounding space falls on a black hole. This is stars, gas and dust from the interstellar medium, as well as matter trapped from the surface of neighboring stars. In the black hole's gravitational field, the matter is accelerated to enormous speeds comparable to the speed of light. The matter does not fall vertically onto the black hole, but twists, forming a dense and hot accretion disk around the black hole. Layers of gas in the disk move around the center in the same direction, but with different velocities - the closer to the center, the higher the velocities. Therefore, friction occurs between the layers of gas, and it turns the kinetic energy of the gas into heat. As a result, the disk heats up to such a high temperature that it glows in the radio, infrared and optical ranges, in X-rays and gamma rays.

An accretion disk is involved in the formation of any planet: planets arise when the accretion disk turns into a protoplanetary disk. This is the same accretion disk, but from it the matter stops falling on the star, and starts to collect and form a planet.

RadioAstron is an international space project with leading Russian participation to conduct fundamental astrophysical research in the radio range of the electromagnetic spectrum using the space radio telescope.

Source/Credit: Ural Federal University


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