UCLA researchers have found that the black hole at the center of our galaxy may have a companion.
Many researchers currently believe that all stars in the Milky Way orbit a single supermassive black hole, known as Sagittarius A*. However, a recent UCLA-led study published in December discovered that a second black hole may exist alongside it.
Smadar Naoz, an associate professor of physics and astronomy at UCLA and the lead author of the paper, said most galaxies evolve by merging with other galaxies. As the majority of galaxies have a supermassive black hole at their centers, a galaxy could have multiple black holes at its center after merging with another galaxy, she added.
“It’s a cannibalism of galaxies, … and if there’s a supermassive black hole at the center of each galaxy, then just from gravity they will sink into the center,” Naoz said.
Since researchers know there’s a black hole in the center of the Milky Way, they expect that the black hole either had a younger sibling in the past or has one currently, Naoz said. She added that Sagittarius A* is unique because of its proximity to Earth.
“It’s a great lab to go and try to understand things that we have no chance of even detecting in other galaxies,” Naoz said.
Clifford Will, a distinguished professor of physics at the University of Florida and a co-author of the study, said if there were a companion to Sagittarius A*, then its gravity would affect the orbits of the stars revolving around it, which is how researchers would find if there is a second black hole.
“If there is a companion black hole to the big black hole that we know is there, its gravitational tug would perturb the orbits of the stars that we know are orbiting that black hole,” Will said. “Just the same way that the orbit of Jupiter perturbs the orbit of Uranus because Jupiter has its own gravitational attraction.”
Tuan Do, a research scientist and deputy director at the Galactic Center Group at UCLA and a co-author of the study, said understanding this process is interesting because it is one of the ways black holes can grow. He added it is also interesting because when two black holes merge, they produce a lot of gravitational waves.
Will added that finding a companion black hole, or finding an absence of one, will lead to interesting insights about the Milky Way’s past. If a second black hole is found, then the discovery could point out other processes that produce black holes in the center of existing galaxies. Additionally, the discovery could challenge current ideas of galaxy mergers, Will said.
Naoz said an absence of an additional black hole would mean that the Milky Way has not had any major mergers for about 10 million years. Sagittarius A* is only about 4 million solar masses, which means 4 million times the mass of the sun.
“Our supermassive black hole is not one of the larger ones,” Naoz said. “So it’s not a bonafide billion. … If we had any merger in the past, it was a minor merger. That means that smaller flimsy galaxies came to merge with us.”
Naoz said they used measurements of the brightest star orbiting Sagittarius A*, called SO-2, to predict how the star should move going forward if there were a second black hole within its orbit. She added the environment at the galaxy’s center is very dense and the second black hole’s gravity should affect the orbits of these stars if it exists.
“Between Earth (or) our solar system and the closest star, … there is nothing, there are no more stars,” Naoz said. “But in the same distance, … around the supermassive black hole, there are millions of stars. That’s how dense this place is.”
Do added that SO-2’s orbit around Sagittarius A* looks like an ellipse that slightly rotates every time the star completes one revolution. This motion is called precession, he said.
Naoz said precession resembles a flower pattern. If there was a second source of gravity affecting the orbits, she said, then this pattern would wobble slightly, creating a flower crown.
Using measurements of the orbit, Naoz said they could comfortably tell where the second black hole cannot be, as well as its range of mass.
“We have a part of the parameter of space and mass and the separation (between) the supermassive black hole and its friend, (within which) it’s fair game,” Naoz said. “It can be there.”