It seems we are finally finding at least some of the universe’s 'missing’ black holes.
A cluster of stars called IRS 13 near the center of the galaxy appears to have a black hole at its center. Also, the way these stars cluster and orbit suggests that a black hole is a rarity—one in the middle mass range, between star-like mass and supermassive monsters.
Such intermediate mass black holes are rarely detected, which excites IRS 13… but the icing on the cake is its location.
Just 0.1 light-years from the galactic center, the black hole appears to be one of the building blocks feeding the ongoing growth and evolution of the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*).
This is a key clue to understanding how black holes grow, narrowing the gap between stellar-limit masses and supermassive black holes.
Black holes are the evolutionary endpoint of a massive star’s lifetime, but the observed masses of these objects are confusing.
In the interstellar mass range, black holes form from the collapsed cores of massive stars (and mergers between them). For a black hole formed from a star, the upper mass limit is around 80 times the mass of the Sun.
Supermassive black holes have a well-defined mass range, but it is generally accepted to be between a million and a billion times the mass of the Sun. The mass range is between about 100 solar masses and 100,000 to one million solar masses, in which surprisingly few objects have been discovered.
This is shocking because it leaves an evolutionary path between small black holes and large black holes empty.
There is a gap in the observational data between interstellar-mass and supermassive black holes that cannot be easily explained – meaning we have no evidence of growth from one point to the other.
IRS 13 found 25 years ago, and has been puzzling astronomers ever since. At first, it was thought to be a massive star. Then A A binary star. After A Wolf-Rayet starA massive star on the verge of supernova.
Then it is identified A small cluster, but still a puzzle. It is so close to Sgr A* — a black hole with 4.3 million suns — that such a cluster could not maintain its close-knit structure.
A team led by astrophysicist Florian Beisker of the University of Cologne in Germany wanted to solve this mystery, so the researchers studied the way clusters of stars and clumps of gas move. They expected the motion to be relatively random, but instead found it to be highly ordered.
There are two possible explanations for this. One is that the influence of Sgr A* somehow modifies the orbits of objects in IRS 13. But the team reasoned, there must be something inside the cluster to keep gravity intact.
They made observations and modeling to see if they could figure out what it was. By tracing the cluster’s motions, they identified the location of that dense material.
There, they observed X-rays and a ring of ionized gas spinning at 130 kilometers (81 miles) per second.
They then used all these motions to calculate the mass of the object at the center of the ring. Their material tipped the cosmic scales at about 30,000 solar masses. There can only be one thing: an intermediate mass black hole.
Future observations with more sophisticated instruments will give us more insight into this enigmatic object, but for now, this cluster represents a significant step toward understanding black hole evolution.
„IRS 13 appears to be an essential building block for the development of our central black hole SgrA*” Biskar says.
„This spectacular star cluster has puzzled the scientific community since its discovery twenty years ago. It was originally thought to be an unusually massive star. However, with high-resolution data, we can now confirm the structure—a block composition with an intermediate-mass black hole at the center.”
Published in the thesis The Astrophysical Journal.