There could be a second black hole gravitating around the main one at the center of the galaxy even though we haven’t found any evidence yet. That’s what a new article in The Conversation written by astrophysicist Smadar Naoz, Professor of Physics and Astronomy at the University of California at Los Angeles, who has made a new study that has appeared on arXiv for the time being.
According to Naoz and colleagues the black hole that lies at the center of our galaxy (its presence is now accepted by the vast majority of astronomers and astrophysicists), called Sagittarius A* and having a mass of about 4 million times that of our Sun, may in fact have a companion, another black hole, smaller, that orbits around it.
Naoz and colleagues have found no direct evidence of the presence of a second black hole, but they are confident that it may actually exist because they rest on the idea that galaxies evolve mostly by merging with each other, a phenomenon that we are also increasingly finding through the observations of other galaxies and that is becoming increasingly common in our eyes.
And since it is believed that each galaxy has a black hole at its centre that guides its entire gravitational process, this means that pairs of black holes must exist at the centre of many galaxies (in those cases where they have not merged). And among these galaxies there could also be the Milky Way since at the same time we have not even found real evidence that a second black hole cannot exist.
If at the center of a galaxy there are two black holes and not just one, the gravitational attraction that the two bodies generate is different because the two supermassive black holes orbit around each other and at the same time exert an attraction on the stars and matter around them.
To understand this complex gravitational movement, the researchers first analyzed S0-2, the one that can be considered as the most studied star that is located in the whirling central region of our galaxy and that completes an orbit around the supermassive central black hole every 16 years.
Examining the orbit of this star, the researchers found no evidence that there could be another supermassive black hole with at least 100,000 times the mass of the Sun (100,000 solar masses is usually considered the minimum threshold for a black hole to be considered “supermassive”). However, there could be one with a smaller mass, so small that it does not alter the orbit of SO-2 in a way that is detectable with the tools we have today.
The astronomers themselves are waiting for the next minimum distance approach that the star S0-2 will have to Sagittarius A* in 16 years’ time. At that time it will be possible to test several of Einstein’s predictions on general relativity and among these is the one about the change in the orientation of the elongated orbit of the stars. If there really is a second black hole, it could alter this result.
Also, the gravitational waves of the two black holes orbiting each other could be detected. Currently, it is not possible to detect them with the LIGO and Virgo detectors because they are waves with too low a frequency, however it might be possible to detect them with the new space detector called LISA of the European Space Agency that should be launched around the mid 2030s.