Galaxies smash into each other on a pretty regular basis, but the collisions don't destroy them – rather, the two galaxies just merge together. Their stars jostle each other around before settling into new orbits, and the supermassive black holes at their centers meld into one even larger object. Using data and observations from Hubble and the Keck Observatory, astronomers have now imaged the late stages of this incredibly slow process for the first time.

According to the team, the new images mark the closest pass of two black holes ever seen – a cosmic hair's breadth, at just 3,000 light-years apart. That might still sound like a fair hike, but in the previous closest images the black holes have been separated by about 10 times that distance.

The observations came out of a census of the cores of galaxies, taken in near-infrared light. The galaxies in question lie about 330 million light-years from Earth on average, and were identified from 10 years' worth of X-ray data from the Burst Alert Telescope, and 20 years of Hubble observations.

"Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing," says Michael Koss, lead researcher on the team. "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 slowest dance

Few things happen in slower motion than galactic mergers, which play out on the scale of billions of years. Given that dizzying amount of time, we've obviously never been able to see the whole show play out, but we can piece things together by observing pairs of galaxies at various stages of this process.

Astronomers have found evidence of these kinds of cosmic collisions all through space and time. Our own Milky Way swallowed up a smaller sausage-shaped galaxy about 10 billion years ago, and in around 4 billion years our home galaxy will smash into the neighboring Andromeda galaxy. Further abroad, some galaxies are just beginning to nibble away at the outer edges of their neighbors, while others have been spotted with two supermassive black holes locked in a slow dance as the result of an ancient meeting.

It's these final stages, taking place over the last 10 or 20 million years of the collision, that astronomers are the most interested in, but unfortunately this is also the hardest part to see. That's because the huge clouds of dust and gas kicked up by the cataclysms blot out the action in the center, as the two supermassive black holes get closer and closer and finally merge into one.

But although that cloud blocks visible light, astronomers can peer through the veil using X-rays and infrared. To do so, the team first consulted a decade's worth of X-ray data from the Burst Alert Telescope instrument, which highlighted the glow of supermassive black holes as they guzzle the gas surrounding them.

"Gas falling onto the black holes emits X-rays, and the brightness of the X-rays tells you how quickly the black hole is growing," says Koss. "I didn't know if we would find hidden mergers, but we suspected, based on computer simulations, that they would be in heavily shrouded galaxies. Therefore we tried to peer through the dust with the sharpest images possible, in hopes of finding coalescing black holes."

Once they did identify some candidates, the researchers then located those galaxies in archival Hubble data, then used Keck to image even more galaxies in near-infrared light. All up, they analyzed 96 galaxies from Keck and 385 from Hubble, which they say make it an accurate sample of the cosmos. These survey galaxies were also compared to 176 other galaxies, to confirm that the glowing signatures are indeed a mark of impending collisions.

The next steps for the team are to follow up the observations with the OSIRIS instrument at Keck. That will allow the researchers to measure the rotation rate and mass of the black holes, which could then be used to figure out how long it might take before the objects finally merge, as well as how big the resulting gravitational waves would be.

The research was published in the journal Nature.

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