The science of the WG 3
- Nowadays, it is generally accepted that so-called active galaxies contain a supermassive black hole at their center. Scientists reached this conclusion after a thorough analysis of the hints provided by astronomical observations. The strong, non-thermal radiation covering a wide range of the electromagnetic spectrum, the variability of the flux on timescales from hours to years and the very compact size of the emitting region are just some of the indications pointing towards the existence of an extremely small and powerful engine at the center of active galaxies. We may naturally ask ourselves ___ does the Milky Way host a supermassive black hole too? The reply is not obvious.
If we point a radio telescope towards the center of our galaxy (optical telescopes would be useless, because optical radiation is completely absorbed by the large amount of dust along the line-of-sight), looking for the typical fingerprints of an exceptional power generator as described above, we would see none. The Milky Way is not an active galaxy. What we would see, instead, is a strong radio source, known as Sagittarius A* (Sgr A*). Observations in the infrared bands, moreover, would reveal the existence of a star cluster around it.
Since many years, scientists are monitoring the inmost region of the Milky Way, taking frequent snapshots of the cluster for tracing the proper motion of several stars within. From the kinematics of these stars, scientists deduced that the stars orbit around a center of mass which, within the error bars, coincides with Sgr A*; the speed of the stars tends to increase as their distance towards the center decreases. These conclusions suggest the existence of a high concentration of mass close to the position of Sgr A* ___ according to the calculations, a mass of ~ 106M☉, confined within a maximum size of ~ 102 pc. By itself, such a high mass density does not necessary imply the presence of a black hole at the center of the Milky Way. The Schwarzschild radius of a black hole of 106M☉ is of the order of 106 km, about 5 orders of magnitude smaller than the estimated maximum size of the central mass. We could hypothesize that the strong attractor at the center of out Galaxy is a very dense star cluster. However, several arguments suggest that a considerable part of the mass is concentrated in one single source, Sgr A*, and that its size is significantly smaller than the upper limit obtained through kinematics arguments. There seems to be no reasonable alternative to the hypothesis that Sgr A* hides a supermassive black hole.
It is very likely that a supermassive black hole resides at the center of nearly every galaxy, whether active or not. The one hosted at the center of the Milky Way is by far the closest, and therefore it can be observed at incomparably high resolution, providing us with a unique opportunity to investigate how a supermassive black hole interacts with its environment. This is a key point to resolve some fundamental issues, as the connection between the formation of the black hole and the host galaxy, or the way they influence each other's evolution.
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