The center of our galaxy, the Milky Way, is an interesting place in many ways. First, it is home to the supermassive black hole Sagittarius A* (Sgr A*). Second, it is home to so many different objects — from dust and gas to stars and black holes — that scientists sometimes get lost in the diversity. And although all of this is hidden from us by a veil of interstellar matter that is difficult to penetrate, models and statistics help make amazing discoveries.
Image Credit: Mark Garlick/Science Photo Library
Studying the center of the Milky Way in the infrared and radio ranges allows us to find stars there even behind dense clouds of dust. It is much more difficult to search for stellar-mass black holes in this “shroud”. According to star formation models, the region closest to the supermassive black hole Sagittarius A* may contain about 300 stellar-mass black holes. As is known, when large enough stars die, their cores collapse and turn the remains of the star into a black hole. This can be taken into account by statistics, which allows us to roughly estimate the number of black holes near the center of the galaxy.
The new work goes further and claims that there are many more stellar-mass black holes near the center of the Milky Way — not hundreds, but hundreds of millions and even billions. Scientists call the center of our galaxy a veritable “meat grinder of stars” and a “swarm of stellar-mass black holes.”
The basic idea of this new model is that the central region near Sagittarius A* is extremely rich in gas and dust compared to the rest of the galaxy. This means that massive O- and B-type stars can easily form there. Such stars have very short lives and die as supernovae. Their cores collapse into black holes, and the remaining matter dissipates and can be used to create new stars. Over time, as stars are born and die in this region, black holes will inevitably accumulate.
Eventually, so many black holes will accumulate in the region that collisions between them and stars will become common. The black holes will gradually tear the stars apart, mixing the matter in the region and accelerating the formation of new stars and black holes. The authors of the study called this model a “star crusher.”
If this hypothesis is correct, then the center of our galaxy could contain millions or even billions of black holes with a stellar mass per cubic parsec (a parsec is 3.26 light years). Any star that falls into this region would be at risk. To confirm their concept, the scientists turned to statistical analysis.
Given the density of black holes in a region, the average time it takes for a star to collide with a black hole can be calculated. The collision time depends on the number of black holes and the size of the star: the more black holes, the shorter this period, and the more massive the star, the higher the probability of a collision.
Having carried out calculations and compared them with observations, scientists found out that in the central region of the galaxy there are the fewest O-type stars and more B-type stars. Both of these types are massive but short-lived stars. They are clearly visible due to their hot shells, which makes their statistical analysis possible. As a result, the calculations showed that in the specified region there are about 100 million black holes of stellar mass per cubic parsec. This is an incredibly high density, which radically changes our understanding of the processes in the center of the galaxy.
These calculations are indirectly confirmed by observations of more than a dozen runaway stars that are escaping from the center of the galaxy at speeds exceeding normal intragalactic values. The stars could only reach such colossal speeds during close interactions with black holes, accelerating in their gravitational wells to values that allow them to leave the Milky Way. The number of such stars is too large, indicating a high density of black holes in this region.