It seems we are finally finding at least some of the “missing” black holes in the universe.
A cluster of stars called IRS 13 near the center of the galaxy appears to harbor a black hole at its center. Moreover, the way these stars swarm and orbit suggest that a black hole is a rarity—one in the intermediate mass range, between those with similar masses to stars and supermassive monsters.
Such intermediate-mass black holes have been exceptionally rare, which in itself makes IRS 13 exciting… but the icing on the cake is its location.
Located just 0.1 light-year 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 core of the Milky Way, Sagittarius A* (Sgr A*).
This is a crucial clue that can help us understand how black holes grow and bridge the gap between stellar-range masses and supermassive black holes.
Black holes are the evolutionary endpoint in the lifetime of a massive star, but the observed masses of these objects are mysterious.
In the stellar mass range, we have black holes that form from the collapsed cores of massive stars (and mergers between them). For a black hole formed from a single star, the upper mass limit is approximately 80 times the mass of the Sun.
Supermassive black holes have an ill-defined mass range, but it is generally accepted that the mass is millions to billions of times that of the Sun. Anything between about 100 solar masses and 100,000 to a million solar masses is the middle mass range in which surprisingly few objects have been found.
The reason this is shocking is because it leaves an empty evolutionary path between small black holes and large ones.
There is a gap in the observational statistics between stellar mass and supermassive black holes that is not easily explained – meaning we have almost no evidence of growth from one point to another.
IRS 13 was discovered more than 25 years ago and has been puzzling astronomers ever since. It was initially thought to be a single massive star. Then a double star. Then a Wolf-Rayet star, a massive star on the verge of supernova.
It has since been identified as a small cluster, but still mysterious. It is so close to Sgr A* – a black hole with the mass of 4.3 million Suns – that such a cluster should not be able to maintain its close-in structure.
A team led by astrophysicist Florian Peißker of the University of Cologne in Germany wanted to solve this mystery, so the scientists looked at how the stars and clumps of gas in the cluster move. They expected the movement to be relatively random, but instead found it to be quite orderly.
There are two possible explanations for this. One of them is the influence of Sgr A*, which somehow modifies the orbits of the objects in IRS 13. But the team reasoned that there must be something inside the cluster that keeps it gravitationally intact.
They did observations and modeling to see if they could figure out what it was. By tracking the cluster’s movements, they identified where this dense object might be located.
At this location, they observed X-rays and a ring of ionized gas rotating at about 130 kilometers (81 miles) per second.
Then, using all these motions, they calculated the mass of the object at the center of the ring. Their object tipped the cosmic scales at about 30,000 solar masses. It can only be one thing: a medium-mass black hole.
Future observations with state-of-the-art instruments will give us more insight into this mysterious object, but for now, this cluster appears to represent an important step toward understanding the evolution of black holes.
“IRS 13 appears to be a fundamental building block for the growth of our central black hole SgrA*,” says Peißker.
“This fascinating star cluster has continued to surprise the scientific community since it was discovered about twenty years ago. It was initially thought to be an unusually heavy star. However, with high-resolution data, we can now confirm its building block composition with an intermediate-mass black hole at the center .”
The research was published in The Astrophysical Journal.