A massive black hole exists at the center of our galaxy. Sagittarius A* has a mass of 4.2 million Suns and is only 27,000 light-years away from Earth. Sag A* is the nearest supermassive black hole, and one of only two that we have directly detected. We can see stars tightly orbiting it since it is so close. Some of those stars have been seen for more than 20 years, so we have an excellent understanding of their orbits. We’ve used those orbits to calculate Sag A*’s mass, but a recent study investigates a different question: does our galaxy’s black hole have a companion?
Most galaxies have a supermassive black hole, and some have two. This is created by galactic mergers, in which one galaxy’s black hole is captured by another. Observations show that our galaxy does not have two supermassive black holes. There may be star mass black holes orbiting Sag A, but current observations are insufficient to find them. Another theory is that an intermediary black hole orbits Sag A, the subject of this study.
A black hole with an intermediate mass (IMBH) has a mass between hundreds and thousands of Suns. They are not the gravitational seeds of galaxies, nor are they generated by the collapse of a huge star. They were very recently found via gravitational waves from black hole mergers, thus they are not well understood. We have no idea how common they are. However, if an IMBH orbits Sag A*, the gravitational force of the IMBH will impact the orbits of neighboring stars that are also orbiting our supermassive black hole.
The researchers investigated the orbit of a star known as S0-2, or S2. With an orbital period of only 16 years, it has one of the closest orbits to Sag A*. It orbits the black hole so closely that general relativity must be used to calculate its orbit, and we have over two decades of observational data on it. If an IMBH is orbiting nearby, S0-2 should be affected.
To the best of their ability, the scientists discovered no indication of gravitational perturbations on S0-2’s orbit. This does not rule out the possibility of an intermediate-mass black hole in the vicinity, but it does place some upper boundaries on its mass if one exists. Based on the data, a hypothetical IMBH orbiting outside the orbit of S0-2, say with an orbital radius of 1,000 to 4,000 AU, may have a mass of 1,000 to 10,000 Suns. If an IMBH orbits Sag A* closer than S0-2, its mass cannot be more than 400 Suns.
These aren’t strict limitations, but they do confirm that Sagittarius A* does not contain a massive intermediate black hole. If it has a smaller IMBH companion, we are currently unable to detect it.
Source: Will, Clifford M., et al. “Constraining a companion of the galactic center black hole, Sgr A*”