‘Missing’ black holes holding together a galaxy have been found deep in space

Astronomers have made a giant leap in the hunt for 10,000 missing black holes which have baffled scientists for decades.

The breakthrough came after researchers studied images from Nasa’s Hubble Space Telescope, along with new observations from the powerful James Webb Space Telescope.

By combining more than 20 years of records, they were able to identify a stellar-mass black hole in the vast Omega Centauri star cluster for the first time.

Omega Centauri is one of the largest globular clusters in the Milky Way, containing about 10million stars held together by gravity.

Scientists had long predicted that it should contain thousands of stellar-mass black holes left behind by the explosions of massive stars.

However, previous searches using methods such as measuring stellar motion through radial velocity, or detecting X-rays and radio signals from material falling into black holes, had found little evidence of this hidden population.

The new study used a different technique known as astrometry, which tracks tiny changes in the positions of stars over time.

By analysing decades of Hubble data and combining it with Webb’s infrared observations, researchers identified a star moving around an unseen object with enough mass to confirm it must be a black hole.

Named oMEGACat BH-2, the object is the first confirmed stellar-mass black hole found in Omega Centauri.

It also has unusual characteristics: its mass is lower than expected, and together with its visible companion star it forms the longest-period black hole binary system known to date.

‘With Hubble and Webb data, we were able to see the motion of the visible main sequence star that is part of this binary, which is about 18,000 light-years away in the dense environment of Omega Centauri,’ said Matthew Whitaker of the University of Utah, Salt Lake City, lead author of the paper.

‘The precision of these measurements is incredible, down to a fraction of a pixel on Hubble and Webb’s detectors. It would not have been possible to find this black hole without these two space telescopes.’

The discovery also overturns an earlier suggestion that the system contained a neutron star rather than a black hole. By extending previous Hubble research with additional astrometric measurements collected between 2002 and 2023, and incorporating Webb’s near-infrared data, the team was able to calculate the mass of the invisible object more accurately.

The visible companion star has a mass of about 0.78 times that of the Sun, while the black hole weighs around 4.46 solar masses – too large to be a neutron star.

‘While we already knew that the star was 0.78 solar masses, we can now calculate the black hole’s mass, which is 4.46 solar masses and therefore too heavy to be a neutron star.

‘However, its mass is much lower than would be expected in a metal-poor environment like Omega Centauri. This is surprising and exciting,’ said Anil Seth of the University of Utah, a coauthor of the study.

‘We now know that a metal-poor star is able to form a black hole like this, and we need to figure out how that happens. This detection is providing some data to those who do that kind of modelling.’

The detailed observations allowed researchers to trace the star’s orbit around its black hole companion for more than two decades. The star completes one orbit every 94 years, making oMEGACat BH-2 the longest-period black hole binary system discovered so far.

Scientists believe the pair may not have formed together but instead became linked through interactions within the crowded cluster.

Their calculations suggest the system is likely to survive for less than one billion years before being disrupted by encounters with nearby stars – a relatively short period compared with Omega Centauri’s estimated age of about 12 billion years.

‘It’s important to understand black hole populations in globular clusters because there’s uncertainty about their physics and formation,’ said Seth.

‘More specifically, understanding the process of forming black holes and then dynamically forming binaries is vital, because it affects our ability to interpret and understand gravitational wave events. Environments like Omega Centauri are the primary places where we think binaries are merging and creating these waves.’

Researchers say the discovery marks the beginning of a wider search for similar hidden black hole populations in globular clusters.

‘With Hubble and Webb, we can continue to look at Omega Centauri and expand our search for similar systems within other clusters,’ said Whitaker.

‘We’re also very excited for the launch of Nasa’s Nancy Grace Roman Space Telescope because it will image the crowded galactic bulge, including the galactic center, very regularly with Hubble-like resolution and with a much wider field of view.

‘We’re hoping we’ll be able to find black hole binary systems like this one because of the regular cadence of Roman’s observations.’

The findings were published on Monday in The Astrophysical Journal Letters.