Hidden black hole could explain mystery at the heart of our galaxy

The centre of our galaxy is a strange and chaotic place, but we may finally have an explanation for the unusual stars that orbit there. Our supermassive black hole, Sagittarius A*, is surrounded by three populations of stars, all strikingly different from one another but with similar ages, and researchers have come up with a relatively simple model that can explain all of them at once.

The closest objects to Sagittarius A* are called S-stars: a spherical swarm of stars, many of which are on elongated orbits that take them dangerously close to the black hole. Their distribution also has a strange, unexplained gap called a zone of avoidance. The next layer contains clockwise disc stars, which are massive stars that sit in a relatively orderly disc outside the orbits of the S-stars. Finally, there are the off-disc stars, which are on more scattered orbits, including some that appear to circle in the opposite direction from the rest.

There have been many explanations proposed for each of these populations, but thus far, none has been able to consistently explain all of them. Xiaochen Zheng at the Beijing Planetarium in China and her colleagues may have a solution.

They constructed a model with one main extra component – an intermediate-mass object, most likely a black hole several hundred to a thousand times the mass of the sun. In their model, all of these stars were born together in the same disc of gas and dust, all orbiting neatly within that circular disc.

But if this intermediate-mass object was also near the centre of the galaxy, orbiting on a steep tilt relative to the disc, it could create a series of complex interactions among the stars of the disc. It would have the strongest effect on the outermost stars, stretching and tilting their orbits so much that some could even appear to orbit in the wrong direction, as some of the off-disc stars do.

For the centre layer, the clockwise disc stars, the dominant effect would be what’s called a resonance, where the gravity of the intermediate-mass object and Sagittarius A* balance out to stretch the orbit just a little bit without perturbing it too much. And the motion of the S-stars would remain largely unaffected by the extra object, instead thrown into chaos by interactions among the stars themselves – interactions which also rip binary stars from one another, naturally creating the zone of avoidance.

“Through three distinct gravitational dances, this cosmic companion pulled the family apart,” says Zheng. This is the simplest way to explain all three of the populations near the galactic centre, she says: “In doing so, it avoids the far greater complexity of postulating multiple, independent formation events with no obvious reason to coincide in space and time.”

However, not all of the details are worked out, most importantly, the cosmic companion itself. “They still have to find that perturber, and finding these intermediate mass black holes is not easy,” says Albert Zijlstra at the University of Manchester in the UK. “All the potential ones that have been found so far in this mass range have fallen through, mostly due to lack of evidence.”

The researchers do have a candidate: a cluster of stars called IRS-13E that orbits near the galactic centre and may have a black hole at its centre. However, we aren’t even sure that IRS-13E is a genuine cluster and not just a temporary coincidence of stars – it will take more precise measurements over an extended period of time to confirm whether it can truly explain the mysteries at the centre of the galaxy.