Astronomers using NASA’s Hubble Space Telescope and Chandra X-ray Observatory have discovered a possible 'missing link' black hole, offering a rare glimpse of a cosmic heavyweight in the middle of a celestial meal.
The newly identified object, named NGC 6099 HLX-1, sits 40,000 lightyears from the centre of a giant elliptical galaxy located 450 million lightyears away in the constellation Hercules.
This is a possible intermediate-mass black hole (IMBH), a long-sought class of black hole that seems to evade detection by even our most powerful telescopes.
What makes this black hole so special
Black holes usually come in two types:
- Stellar black holes, which form from dying stars and weigh less than 100 times the mass of our Sun
- Supermassive black holes, millions or billions of times more massive, lurking in the centres of galaxies
Intermediate-mass black holes have remained tricky to spot.
They’re too big to form from single stars and too small to glow with the brightness of supermassive black holes.
Scientists believe they play a key role in the growth of galaxies, but finding them remains a challenge.
"X-ray sources with such extreme luminosity are rare outside galaxy nuclei and can serve as a key probe for identifying elusive IMBHs." says study said lead author Yi-Chi Chang of the National Tsing Hua University, Hsinchu, Taiwan.
"They represent a crucial missing link in black hole evolution between stellar mass and supermassive black holes."
Caught in the act of devouring a star
Astronomers first spotted something strange in 2009: a bright source of X-rays picked up by the Chandra X-ray Observatory.
Over the next decade, it flared up dramatically in 2012 – becoming 100 times brighter – before slowly dimming again by 2023.
That kind of behaviour points to a dramatic event: a black hole tearing apart a star, an explosive occurrence known as a tidal disruption event.
The shredded star forms a hot, glowing disk of gas spiralling into the black hole, and that's when telescopes can finally catch it.
X-rays from NGC 6099 HLX-1 indicate a temperature of 3 million degrees, which is consistent with a tidal disruption event.
And the Hubble Space Telescope found evidence for a small cluster of stars around the black hole.
This seems to fit the picture, as the star cluster would give the black hole plenty to feast on. The stars are so close together they're just a few lightmonths apart (about 500 billion miles).
"If the IMBH is eating a star, how long does it take to swallow its gas? In 2009, HLX-1 was fairly bright. Then in 2012, it was about 100 times brighter. And then it went down again," says study co-author Roberto Soria of the Italian National Institute of Astrophysics.
"So now we need to wait and see if it’s flaring multiple times, or there was a beginning, there was peak, and now it's just going to go down all the way until it disappears."
The building blocks of giant black holes
Discoveries like this help scientists understand how black holes – and the galaxies around them – grow and evolve over time.
One leading theory suggests that small IMBHs slowly merge together, eventually forming the supermassive black holes seen at the hearts of most galaxies.
Another theory suggests they form directly from collapsing gas clouds in the early Universe.
And this discovery hints that galaxies could be hiding satellite black holes in their outer regions, orbiting far from the centres where supermassive black holes usually live.
"If we are lucky, we're going to find more free-floating black holes suddenly becoming X-ray bright because of a tidal disruption event," says Soria.
"If we can do a statistical study, this will tell us how many of these IMBHs there are, how often they disrupt a star, how bigger galaxies have grown by assembling smaller galaxies."
