Two black holes just 300 lightyears apart have been observed buried deep within two colliding galaxies.
The black hole pair was detected using the Hubble Space Telescope and Chandra X-ray Observatory.
While binary black holes have been observed before, these two are the closest pair ever detected.
The black holes are known as active galactic nuclei because they glow brightly due to matter falling inwards and heating up as it does so.
Colliding galaxies and their black holes
Astronomers now know that many galaxies contain supermassive black holes at their core, and that supermassive black holes likely play a key role in the evolution of galaxies.
These two supermassive black holes once sat at the at the core of their host galaxies, but the merger of the two galaxies has brought them close together.
They're expected to get closer and closer and, likely in 100 million years, they will merge, producing ripples in spacetime known as gravitational waves.
And because active galactic nuclei mergers occurred frequently in the chaotic early Universe, when galaxy collisions were more common, this current black hole pair is giving astronomers a rare insight into the phenomenon.
How the MCG-03-34-64 black hole collision was discovered
The galaxy merger is known as MCG-03-34-64 and is located 800 million lightyears away, which is close-by in astronomical terms.
The discovery was made when the Hubble Space Telescope captured three so-called diffraction spikes in the host galaxy, which astronomers were able to determine as a large concentration of glowing oxygen gas in a small area.
"We were not expecting to see something like this," says Anna Trindade Falcão of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, USA and lead author of the paper published in The Astrophysical Journal.
"This view is not a common occurrence in the nearby Universe, and told us there's something else going on inside the galaxy."
Chandra's x-ray vision was then used to get a closer look.
"When we looked at MCG-03-34-64 in the x-ray band, we saw two separated, powerful sources of high-energy emission coincident with the bright optical points of light seen with Hubble," says Falcão.
"We put these pieces together and concluded that we were likely looking at two closely spaced supermassive black holes."
And, the team were also able to use archival radio data from the Karl G. Jansky Very Large Array near Socorro, New Mexico.
"When you see bright light in optical, x-rays and radio wavelengths, a lot of things can be ruled out, leaving the conclusion these can only be explained as close black holes. When you put all the pieces together it gives you the picture of the active galactic nucleus duo," says Falcão.
There is a third source of bright light seen by Hubble, however, and the team still don't yet know exactly what this is.
It could be gas shocked by a jet of plasma fired from one of the black holes, but more investigation is needed.
What is clear, however, is that using the power of three incredible observatories, astronomers have been given a view of the tightest black hole binary ever observed, at the centre of a catastrophic galactic collision.
Read the full paper at iopscience.iop.org/article/10.3847/1538-4357/ad6b91
