Observations of gravitational waves tell about the formation of black holes

Physicists described how the observations of gravitational waves limit possible scenarios that explain the formation of black holes outside our Galaxy; These black holes either rotate more slowly than the black holes of the Milky Way, or they rotate swiftly, but “lie on their sides”, that is, their spins are oriented with respect to the planes of their orbits randomly.

This new work is based on data obtained by observing gravitational waves with the help of the LIGO Observatory in 2015 and then again in 2017.

In our Galaxy, we can “see” black holes by observing the radiation generated in their neighborhoods if the companion stars are moving around black holes. These observations show that the black holes of the Milky Way rotate at a relatively high speed.

Gravitational waves carry information about the origin of such black holes, which can not be observed by other methods. Physicists came to the conclusion that the gravitational waves detected in September 2015 were formed as a result of the fusion of two black holes of stellar masses. The new paper states that the black holes discovered at that time differ in properties from the black holes of our Galaxy, which were studied earlier.

This difference can be explained in accordance with one of two possible scenarios, the authors indicate. The first scenario assumes a slower rotation of extragalactic black holes, compared to the black holes of the Milky Way. The second possibility is that the extragalactic black holes rotate as fast as the black holes of the Milky Way, but their formation occurs in a denser stellar environment (for example, in a dense star cluster), and interaction with this environment results in black Holes “turn on their side”, as a result of which their spins are directed arbitrarily relative to the planes of their orbits.