Astronomers say they have witnessed the final moments of several pairs of galaxies merging together for the first time.

In a study published in Nature, researchers led by Michael Koss from the University of Maryland used images stretching back 20 years from NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii to observe hundreds of nearby galaxies.

And they found several instances of galaxy mergers taking place – something thought to be frequent in the early universe but less so now. When these galaxies merge, so do the supermassive black holes at their cores, producing noticeable bursts of energy in the form of gravitational waves, which we detected for the first time in 2016.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” Koss said in a statement. “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.”

In total they looked at nearly 500 galaxies at an average of 330 million light-years away, finding that 17 percent had a pair of black holes at their centers. This suggested these were galaxies in the final stages of merging before the black holes themselves merged together to produce a single black hole.

It’s thought black holes do not spend much time in this phase, so finding so many in this late stage was somewhat surprising. To check they were right, they looked at more sedate galaxies that lacked actively growing central black holes, finding only 1 percent with late-stage merging black holes.

“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,” Laura Blecha from the University of Florida, a co-author of the study, said in the statement.

“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.”

The research was in part made possible using X-ray data from NASA’s Burst Alert Telescope (BAT), which could peer through the dust of these galaxies. Hubble and Keck were then used to track down the images spotted in the X-ray data.

While the finding was somewhat surprising, it could be useful in helping us learn more about how galaxies evolve, and how gravitational waves are formed. And future telescopes, like the James Webb Space Telescope (JWST), could get an even better look at what’s happening inside these galaxies.