Astronomers have detected enormous black holes as multiple pairs of galaxies collide and merge into larger galaxies.
Using powerful telescopes to see through the galaxies' thick walls of gas and dust surrounding their cores, the academics have managed – for the first time – to observe supermassive black holes fall into each other and coalesce into an even more giant black hole.
Black holes are areas of space-time which are so dense that nothing – whether matter or energy – can escape them.
Astrophysicists currently believe there are about 10,000 black holes at the centre of our own galaxy, the Milky Way, all of which surround a supermassive black hole at its core.
A team led by research scientist Michael Koss surveyed hundreds of nearby galaxies using existing imagery and published their findings in the journal Nature.
"Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing," said Mr Koss said.
"In our study, we see two galaxy nuclei right when the images were taken. You can't argue with it; it's a very 'clean' result, which doesn't rely on interpretation."
The high-resolution images have provided the scientists with a close-up of galactic collisions, something which is believed to have been more frequent in the early universe.
When the black holes at the centre of galaxies collide they will unleash incredibly powerful energy in the form of gravitational waves – ripples in the very fabric of space-time which have only recently been detected.
The team said it was inspired to conduct the survey by an image captured by the Hubble telescope of two interacting galaxies which are collectively called NGC 6240.
They initially scanned for active black holes that were hidden from normal visual wavelengths of light by analysing a decade's worth of X-ray data from the Burst Alert Telescope (BAT) aboard NASA's Neil Gehrels Swift Observatory.
"The advantage to using Swift's BAT is that it observes high-energy, 'hard' X-rays," said the study's co-author, Richard Mushotzky, a professor of astronomy.
"These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths."
Mr Koss explained: "People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust.
"We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.
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"Computer simulations of galaxy smashups show us that black holes grow fastest during the final stages of mergers, near the time when the black holes interact, and that's what we have found in our survey," added Laura Blecha.
Ms Blecha, who is an assistant professor of physics at the University of Florida and a co-author of the study, added: "The fact that black holes grow faster and faster as mergers progress tells us galaxy encounters are really important for our understanding of how these objects got to be so monstrously big."