• Black holes only 300 light-years apart – closer than any other observed pair of supermassive black holes – have been found at the centers of two merging galaxies.
• The two black holes are still drawing closer and closer together, scientists say.
• They say this unusual black hole duo will merge, creating a single, new, giant black hole.

NASA published this article on September 9, 2024. Edits by EarthSky.

Black hole duo will eventually merge

For all practical purposes, stars traveling across space never smash into each other. The ratio between a sunlike star’s diameter and its distance to the next neighboring star is about 1:10 million. Galaxies do collide, though. By comparison, the separation between our Milky Way galaxy and neighboring Andromeda galaxy is a staggering 2.2 million light-years. That’s a ratio of only 1:20. And eventually, there will be a collision and merger between the two giants.

When two galaxies collide, their central supermassive black holes merge into a single giant black hole. In fact, nearly all galaxies contain supermassive black holes in their centers. Now the Hubble Space Telescope and the Chandra X-ray Observatory have looked into the heart of a pair of colliding galaxies and uncovered twin supermassive black holes waltzing around each other. The black hole duo, engorged with infalling gas, blazes brightly as active galactic nuclei. They are approximately 300 light-years apart, the closest such pair seen in visible-light and X-ray wavelengths.

And they’re getting closer.

Cores of two galaxies on a collision course

Like two sumo wrestlers squaring off, the closest confirmed pair of supermassive black holes have been observed in tight proximity. These are located approximately 300 light-years apart and were detected using NASA’s Hubble Space Telescope and the Chandra X-ray Observatory. These black holes, buried deep within a pair of colliding galaxies, are fueled by infalling gas and dust, causing them to shine brightly as active galactic nuclei (AGN).

This AGN pair is the closest one detected in the local universe using multiwavelength (visible and X-ray light) observations. While astronomers have found several dozen “dual” black holes before, their separations are typically much greater than what was discovered in the gas-rich galaxy MCG-03-34-64. Astronomers using radio telescopes have observed one pair of binary black holes in even closer proximity than in MCG-03-34-64, but without confirmation in other wavelengths.

AGN binaries like this were likely more common in the early universe when galaxy mergers were more frequent. This discovery provides a unique close-up look at a nearby example, located about 800 million light-years away.

How Hubble spotted the black hole duo

The discovery was serendipitous. Hubble’s high-resolution imaging revealed three optical diffraction spikes nested inside the host galaxy, indicating a large concentration of glowing oxygen gas within a very small area.

Anna Trindade Falcão of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, lead author of the peer-reviewed paper published September 9, 2024, in The Astrophysical Journal, said:

We were not expecting to see something like this. This view is not a common occurrence in the nearby universe, and told us there’s something else going on inside the galaxy.

Diffraction spikes are imaging artifacts caused when light from a very small region in space bends around the mirror inside telescopes.

An X-ray view into a cosmic collision

Falcão’s team then examined the same galaxy in X-ray light using the Chandra observatory to drill into what’s going on. Falcão said:

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. We put these pieces together and concluded that we were likely looking at two closely spaced supermassive black holes.

To support their interpretation, the researchers used archival radio data from the Karl G. Jansky Very Large Array near Socorro, New Mexico. The energetic black hole duo also emits powerful radio waves. Falcão stated:

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 AGN duo.

Unexplained light?

The third source of bright light that Hubble saw is of unknown origin, and more data is needed to understand it. That might be gas that is shocked by energy from a jet of ultra high-speed plasma fired from one of the black holes, like a stream of water from a garden hose blasting into a pile of sand.

The two supermassive black holes were once at the core of their respective host galaxies. A merger between the galaxies brought the black holes into close proximity. They will continue to spiral closer together until they eventually merge – in perhaps 100 million years – rattling the fabric of space and time as gravitational waves.

Bottom line: The Hubble Space Telescope and the Chandra X-ray Observatory spotted a pair of black holes at the cores of two merging galaxies. These black holes will spiral together and eventually collide.

Source: Resolving a Candidate Dual Active Galactic Nucleus with ?100 pc Separation in MCG-03-34-64

Via NASA