Astronomers found counter-rotating discs in a supermassive black hole

Astronomers have found counter-rotating discs in a supermassive black hole
Image from The Astrophysical Journal Letters, 2019

Scientists have discovered in the galaxy NGC 1068 two gas-dust tori around a central supermassive black hole, and found that they rotate in opposite directions. This is the first direct evidence of such a configuration near black holes. The interaction of such components can lead to a sharp increase in the rate of accretion on the central body and effective mass gain by a black hole.

Today, it is believed that in the center of all large galaxies is a supermassive black hole. Moreover, if the center of the galaxy is noticeably active, then this is also associated with a black hole, namely, the fall of matter on it with the transformation of gravitational energy into other species.

Most observational data on active galactic nuclei (AGNs) can be explained within the framework of a theoretical model that involves several components. In the center is a supermassive black hole, the gravity of which controls the movement of all surrounding bodies, the black hole is surrounded by a cloud of hot gas, and behind it is a gas and dust torus with a noticeable concentration of molecules.

Depending on the orientation of the system relative to the line of sight, we can directly see the hot gas in the form of lines of radiation extended due to high chaotic velocities, or just through the torus radiation that is partially absorbed in it. If the central part is visible, then they talk about the AGN of the first type, and otherwise, of the second.

Astronomers from Chile, the USA, Canada, Germany, Great Britain, and Italy, led by Violette Impellizzeri from the U.S. National Radio Astronomy Observatory, analyze new observations of the galactic nucleus of NGC 1068 by the millimeter-wave telescope ALMA. This object is one of the closest AGNs of the second type, and detailed studies of this object formed the basis of a single model.

Previous studies of the NGC 1068 core showed complex gas kinematics on a scale of about 10 parsecs, as well as the presence of emissions of matter from the center at a speed of up to 450 kilometers per second. New observations with a spatial resolution of 1.4 parsecs made it possible to understand the gas movements in the immediate vicinity of the black hole – it turned out that two disks rotating in opposite directions were located around it.

The inner disk is located at a distance of 0.5 to 1.2 parsecs from the black hole, while the outer one extends up to 7 parsecs. The gas motion of the inner disk corresponds to the Kepler velocities of revolution around the black hole and coincides with the kinematics of water megamasers — extended cosmic sources of coherent radiation — already observed in this system. The external disk also spins according to Kepler’s laws, but the particle speeds in it are directed in the opposite direction.

The law of conservation of angular momentum prevents the occurrence of counter-rotating discs due to a single act of loss of matter. This situation is also not expected as a result of the evolution and curvature of the disk. Scientists conclude that the external disk was supposed to form from a gas that fell into this region, regardless of the substance, closer to the black hole and, most likely, after it. Scientists also note that the direction of rotation of the outer disk is also opposite to the movement of the torus on a scale of 100 parsecs and in general the movement of the galaxy on a scale of kiloparsecs.

Counter-rotation as a whole is not an exceptional phenomenon in space. However, a similar phenomenon was previously observed in merging galaxies, in which case the spatial scale of the flows is orders of magnitude larger. According to the authors, when the NGC 1068 disks start touching each other, the external one will collapse in a time of the order of one orbital period, that is, 30,000 years, which is very fast for such processes. This will lead to a sharp increase in accretion to the black hole and an increase in its mass – such a chaotic accretion is considered one of the possible mechanisms for the rapid growth of supermassive black holes that have gained mass of about a billion solar cells in the first billion years of the existence of the Universe.