Employees of the GAISH MSU named after M. V. Lomonosov investigated changes in matter radiation near the supermassive black hole in the center of the galaxy, known to astronomers as NGC 2617. These changes were discovered several years ago: the object became much brighter, and gas clouds near the supermassive black hole, which were not previously visible, began to be observed. Such a dramatic change can provide valuable information for understanding what is the environment of a giant black hole, how and what happens near it. The research results were published in the Monthly Notices of the Royal Astronomical Society (MNRAS).
Most galaxies, including the Milky Way, have giant black holes in their cores. They have masses from a million to a billion times the mass of the Sun. The nucleus of our Galaxy is not active, but in some galaxies the black hole, "feeding on" the gas that falls into it in a spiral, emits a huge amount of radiation. The nuclei of such galaxies are called active galactic nuclei (AGN). The energy output from the environment of black holes in AGN can exceed the luminosity of hundreds of billions of stars in the rest of the galaxy. How supermassive black holes form in these galaxies is a mystery. The nuclei of galaxies, where supermassive black holes vigorously capture gas, are divided into two types: those where we directly observe matter spiraling into a black hole (at a speed thousands of times faster than the speed of sound), and thosewhere the interior areas are covered with dust. In the nuclei of the second type galaxies, only the more slowly moving gas is visible, which is much farther from the black hole than in the nuclei of the first type.
For decades, astronomers have wondered why we see interior regions in some active galactic nuclei but not in others. The most common explanation for the two types of active galactic nuclei is that they are actually the same objects, different for us only because we view them from different points of view. If we see them flat on, then we can observe hot gas spiraling into the black hole directly. If the active galactic nucleus is tilted to the line of sight, then the dust around it blocks the central region from our view, and we see only more slowly moving gas clouds at a distance of a light year or more from the black hole.
The initiator of the project, on which the research was carried out by an international group of scientists, Viktor Oknyanskiy, senior researcher at the State Aviation Institute of the Moscow State University named after M. V. Lomonosov, says: “Cases of transition of an object from one type to another are a certain problem for this popular model. We know dozens of active galactic nuclei that have changed their type. In a recent study, we focused on one of the striking examples of such objects - NGC 2617. In 2013, a team of researchers in the United States discovered that NGC 2617 changed its type of active galaxy, and the inner regions that were hidden from us became visible. We do not know how long the object will remain in this new state. This can continue for a short period of time or for tens of years."
According to the authors, there is no generally accepted explanation for the type changes when the previously hidden inner regions of the active galactic nucleus become visible. Viktor Oknyansky comments: “It is absolutely clear that this phenomenon is not very rare, on the contrary, we believe that it is quite typical for AGN. There are various possible explanations for these phenomena. One of them is that perhaps some star has come too close to the black hole and was torn apart by its gravity. Nevertheless, the destruction of a star by a black hole occurs very rarely, and we do not think that such events can explain the observed frequency of changes in the type of active galactic nuclei. Instead, we consider a model in which the black hole's “feeding regime” changes dramatically. As matter spirals in the direction of the black hole, it emits powerful radiation. We assume that this intense radiation destroyed some of the dust surrounding the core, allowing us to see the interior regions around the black hole."