When you buy through links in our articles, Future and its syndication partners may earn commission.
For decades, astronomers have searched with little joy for signs of “missing link” black holes—black holes with masses between “stellar-mass black holes” and “supermassive black holes.” The former have been spotted scattered across the universe, and the latter are cosmic titans dominating the hearts of galaxies—but when it comes to intermediate-mass black holes, scientists have only seen evidence of about ten.
As reported earlier this month, scientists announced that they had used data from the Hubble Space Telescope to uncover the best evidence yet for an intermediate-mass black hole in Omega Centauri, the remnants of a galaxy torn apart by the Milky Way. And searching for intermediate-mass black holes is a lot like waiting for a bus in London—you wait a century for one, and then two of them pop up at the same time!
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*), which lies at the heart of the Milky Way, about 27,000 light-years from Earth.
The team led by Florian Peißker from the University of Cologne discovered this intermediate-mass black hole while evaluating the star cluster IRS 13, located approximately 0.1 light-years away from Sgr A*.
Relating to: Over the past 25 years, black hole physicists have uncovered the unimaginable
“IRS 13 appears to be a fundamental building block for the growth of our central black hole, Sgr A*,” said Peißker. “This fascinating star cluster has continued to amaze the scientific community since its discovery nearly two decades ago. At first, it was thought to be an unusually massive star. However, with high-resolution data, we can now confirm its building block composition with an intermediate-mass black hole at the center.”
Searching for the lost spaces of the universe
Stellar-mass black holes, with masses between five and 100 solar masses, are known to form from the collapse of stars at least eight times the mass of our star. But supermassive black holes must have a different origin, as no star can be massive enough to collapse and leave a remnant Millions or billions As big as the sun.
This knowledge has led scientists to theorize that supermassive black holes must be born through chains of mergers of larger and larger black holes. These cosmic titans are also thought to grow by voraciously eating the matter around them, including unfortunate stars that get too close and are torn apart and then stuffed into the black hole in a so-called “tidal disruption event” or “TDE.”
This means that there must be a population of black hole “seeds” in the universe, in that vast mass gap between stellar mass and supermassive black holes that haven’t yet reached “supermassive status” but are too large to form from a collapsing star. Still, these intermediate-mass black holes have been frustratingly hard to detect.
Like all black holes, intermediate-mass black holes are bounded by a light-trapping “surface” called an event horizon. Because of this boundary between the observable universe and everything inside the black hole, not only is it impossible for any signal to reach the wider cosmos from inside the black hole, but it is also impossible to “see” a black hole. That is, unless it is ripping apart stars or feasting on matter around it in bright TDEs, which would then heat up and shine brightly.
However, intermediate-mass black holes have not reached supermassive sizes because they are not surrounded by a wealth of matter to feed on, so they do not emit bright light around them, meaning they are almost completely dark. So scientists have to use clever techniques to detect intermediate-mass black holes that are not feeding. For example, they look at the behavior of visible matter around the black holes, such as stars, to see if they are affected by any gravitational effects. If so, these effects could be transmitted by a nearby black hole.
Looking at IRS 13, Peißker and his colleagues saw that the stars of this cluster at the heart of the Milky Way moved in a regular pattern. This was surprising because the team had expected the stars to line up randomly.
There are two possible explanations for this observation: either IRS 13 is interacting with Sgr A*, which is causing its stars to move in an orderly fashion, or there is a large gravitational pull on the cluster that is keeping it in order.
Using instruments such as the Very Large Telescope (VLT), the Atacama Large Millimeter/submillimeter Array (ALMA) and the Chandra X-ray space telescope, scientists were able to determine that the well-ordered, compact shape of IRS 13 could be due to the presence of an intermediate-mass black hole at the centre of IRS 13.
This finding is supported by the fact that astronomers saw X-rays coming from the star cluster, indicating ionizing gas spinning at hundreds of thousands of miles per hour. This hot ionized gas could be swirling around the mouth of the newly discovered intermediate-mass black hole.
Not only would this provide astronomers with another “missing” black hole link, but the findings could also explain an ongoing mystery surrounding IRS 13. The star cluster appeared to be much denser than other similar star clusters in our galaxy — but that’s to be expected if it hosts an intermediate-mass black hole.
RELATED STORIES:
— The supermassive black hole of the galaxy M87 is shooting out jets at speeds close to the speed of light
— Brightest quasar ever seen is powered by a black hole that ‘eats a sun a day’
— First human-imaged black hole has twisted magnetic fields, and scientists are excited
The team now plans to follow up this research by examining 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 July 18 in The Astrophysical Journal.