New ‘missing link’ black hole spotted in galactic center

It seems that we may finally be able to locate at least some of the universe’s “missing” black holes.

A star cluster called IRS 13, located near the center of the galaxy, appears to harbor a black hole within it. What’s more, the way these stars swarm and orbit each other suggests that the black hole is a rarity—a medium-mass hole, somewhere between star-like ones and supermassive monsters.

Such intermediate-mass black holes have been discovered extremely rarely, which makes IRS 13 exciting in itself… but the cherry on top is its location.

Located just 0.1 light-years from the galactic center, the black hole appears to be one of the building blocks fueling the continued growth and evolution of the supermassive black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*).

This is a key clue that can help us understand how black holes grow, bridging the gap between stellar-range masses and supermassive black holes.

Black holes are the endpoint of a massive star’s evolution, but the observed masses of these objects are puzzling.

In the stellar mass range, we have black holes that form from the collapse of the cores of massive stars (and their merger). For a black hole formed from a single star, the upper limit on mass is about 80 times the mass of the Sun.

Supermassive black holes have a poorly defined mass range, but are generally considered to be millions to billions of times the mass of the Sun. The intermediate mass range is between 100 solar masses and 100,000 to a million solar masses, in which surprisingly few objects have been discovered.

The reason this is shocking is that it leaves open an evolutionary pathway between small black holes and large ones.

There is a gap in observational statistics between stellar-mass black holes and supermassive black holes that cannot be easily explained – meaning we have little to no evidence of growth from one point to the other.

IRS 13 was discovered just over 25 years ago and has fascinated astronomers ever since. At first, it was thought to be a single, massive star. Then a binary star. Then a Wolf-Rayet star, a massive star on the verge of a supernova.

It has since been identified as a small, but nonetheless mysterious, cluster. It is so close to Sgr A* – a black hole with a mass of 4.3 million Suns – that such a cluster would not be able to maintain its compact structure.

A team led by astrophysicist Florian Peißker of the University of Cologne in Germany wanted to solve this mystery. So the researchers studied how stars and clumps of gas move within the cluster. They expected the motion to be relatively random, but they found that it was actually orderly.

There are two possible explanations for this. The first is that Sgr A* could have somehow altered the orbits of the IRS 13 objects. But the team concluded that there must be something inside the cluster that is keeping it intact through gravity.

They conducted observations and modeling to see if they could determine what it was. By tracking the movements of the cluster, they identified the location of this dense object.

At this location, they observed X-rays and a ring of ionized gas, rotating at a speed of about 130 kilometers per second.

They then calculated the mass of the object at the center of the ring, using all these motions. Their object tipped the cosmic scales at about 30,000 solar masses. It could only be an intermediate-mass black hole.

Future observations with advanced instruments will give us more information about this enigmatic object, but for now, this cluster appears to represent an important step towards understanding the evolution of black holes.

“IRS 13 appears to be a critical component in the growth of our central black hole SgrA*,” Peißker says.

“Since its discovery some twenty years ago, this fascinating star cluster has continued to surprise the scientific community. It was initially thought to be an unusually heavy star. Thanks to high-resolution data, we can now confirm its elemental composition, with an intermediate-mass black hole at its center.”

The research was published in The Journal of Astrophysics.