Scientists waited ages to find a 'missing link' black hole — then stumbled upon 2

For decades, astronomers have searched without much joy for signs of "missing link" black holes, aka black holes with masses between those of "stellar-mass black holes" and "supermassive black holes." The former have been seen scattered across the universe and the latter are cosmic titans that dominate the hearts of galaxies — but when it comes to intermediate-mass black holes, scientists have seen evidence of only about ten.

As reported earlier this month , scientists announced that using data from the Hubble Space Telescope revealed the best evidence yet of an intermediate-mass black hole in Omega Centauri, the remains of a galaxy cannibalized by the Milky Way . And, well, it looks like searching for intermediate-mass black holes is a lot like waiting for a bus in London; you wait an age for one, and then two turn up at once!

A separate team of researchers has discovered evidence of another medium-sized black hole, this time lurking near the supermassive black hole Sagittarius A* (Sgr A*) at the heart of the Milky Way, some 27,000 light-years from Earth.

The team, led by Florian Peißker of the University of Cologne, discovered this intermediate-mass black hole while assessing the star cluster IRS 13, located around 0.1 light-years from Sgr A*.

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"IRS 13 appears to be an essential building block for the growth of our central black hole Sgr A*," Peißker said. "This fascinating star cluster has continued to surprise the scientific community ever since it was discovered around twenty years ago. At first, it was thought to be an unusually heavy star. With the high-resolution data, however, we can now confirm the building-block composition with an intermediate-mass black hole at the center."

Searching for the universe's missing voids

Stellar-mass black holes with between five and 100 solar masses are known to form via the collapse of stars with at least eight times the mass of our star. Supermassive black holes, however, must have a different origin, as no star could be massive enough to collapse and leave a remnant millions or even billions of times as massive as the sun.

This information has led scientists to theorize that supermassive black holes must be born through merger chains of progressively larger and larger black holes. These cosmic titans are also thought to grow by hungrily feasting on the matter around them, including the any unfortunate star that wanders too close and is shredded then stuffed into the black hole in a so-called "tidal disruption event," or "TDE."

This means there should be a population of black hole "seeds" in the cosmos that exist in that vast mass gulf between stellar-mass and supermassive black holes that haven't yet achieved "supermassive status" but are still too massive to have formed from a collapsing star. Yet, these intermediate-mass black holes have been frustratingly difficult to spot.

Like all black holes, intermediate-mass black holes are bounded by a light-trapping "surface" called the event horizon . Thanks to this boundary between the observable universe and whatever's inside a black hole, not only is it impossible for any signal to travel from the interior of a black hole to the wider cosmos, but it is also effectively impossible to "see" a black hole. That is, unless it's ripping apart stars in bright TDEs or feasting on the matter around it, which would heat and glow brightly.

However, intermediate-mass black holes have failed to reach supermassive sizes because they aren't surrounded by a wealth of material to feed on, so they don't exhibit bright emissions of light in their surroundings, which means they are pretty much completely dark. Thus, scientists have to use clever techniques to detect non-feeding intermediate-mass black holes. For instance, they look at the behavior of visible matter around black holes, like stars, to see if they're impacted by any gravitational effects. If so, those effects could be imparted by a nearby black hole.

While looking at IRS 13, Peißker and colleagues saw the stars of this cluster at the heart of the Milky Way moving in an orderly pattern. This was surprising because the team had expected the stars to be randomly ordered.

There could be two explanations for this observation. Either IRS 13 is interacting with Sgr A* , and this is making its stars move in an orderly way, or there is a large gravitational influence in this cluster that is keeping it well-ordered.

Using instruments such as the Very Large Telescope (VLT) , the Atacama Large Millimeter/submillimeter Array (ALMA) and the Chandra X-ray space telescope , the scientists were able to determine the well-ordered compact shape of IRS 13 could indeed be due to the presence of an intermediate-mass black hole located at the center of IRS 13.

This finding is supported by astronomers' seeing X-rays from the star cluster, which indicate ionizing gas rotating at speeds of many hundreds of thousands of miles per hour. That hot ionized gas could be swirling around the maw of this newly discovered intermediate-mass black hole.

This would not only provide astronomers with another "missing" black hole link, but the findings also may explain a lingering mystery surrounding IRS 13. The star cluster had seemed to be much more dense than other similar star clusters in our galaxy — but that would be expected if it hosts an intermediate-mass black hole.

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The team now intends to follow up on this research by investigating IRS 13 with the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT), currently under construction atop Cerro Armazones in the Atacama Desert of northern Chile.

The team's research was published on July 18 in The Astrophysical Journal.