Sagittarius A*, the supermassive black hole in our Milky Way galaxy, may have company, scientists suggest
Sitting in the middle of our Milky Way galaxy, the supermassive black hole, Sgr A* or Sagittarius A* — which has the mass of about 4 million times that of our Sun — may not be alone. It may be sharing space with a smaller black hole, say scientists.
Smadar Naoz, Associate Professor of Physics & Astronomy, University of California, Los Angeles, and her colleagues suspect that there might be a second black hole lurking nearby which is about 1,00,000 times the mass of the Sun.
Pairs of supermassive black holes should be common in the universe, according to Naoz. These pairs can explain why supermassive black holes occupy the center of the galaxy. "Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the Sun," writes Naoz in The Conversation.
Their positioning in the center, scientists believe, can be explained by going back in time, to about 100 million years ago — the era of the very first galaxies. During this time, the death of the first few stars spurred the formation of the first few black holes. These black holes sank to the center of gravity, becoming the heart of their host galaxy, explains Naoz. As time progressed, galaxies merged and collided with one another, resulting in supermassive black hole pairs.
So, Noaz and team were interesting in figuring out whether Sgr A* has a companion in the Milky Way. To do so, they turned their attention to a star S0-2, which circles around SgrA* once every 16 years.
By monitoring the star, the team ruled out the existence of a companion supermassive hole with a mass greater than 1,00,000 times that of our Sun. "If there was such a companion, then I and my colleagues would have detected its effects on the orbit of SO-2," says Naoz.
"But that doesn’t mean that a smaller companion black hole cannot still hide there. Such an object may not alter the orbit of SO-2 in a way we can easily measure," she adds.
However, in the future, scientists will be able to search for the hidden second companion black hole. The search heavily relies on building instruments that can sense the gravitational waves released by black holes.
One such instrument is already in the works. “A planned space-based detector known as LISA may be able to detect these waves which will help astrophysicists figure out whether our galactic center black hole is alone or has a partner," says Naoz. LISA is set to be launched in 2034.