Artist’s depiction of a pair of active black holes at the heart of two merging galaxies. They are both surrounded by an accretion disk of hot gas. Some of the material is ejected along the spin axis of each black hole. Confined by powerful magnetic fields, the jets blaze across space at nearly the speed of light as devastating beams of energy. Credit: NASA, ESA, Joseph Olmsted (STScI). Public Domain.
Astronomers have observed the closest confirmed pair of supermassive black holes in tight proximity.
The black holes 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 fuelled 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 several dozen ‘dual’ black holes have been found 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.
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.
Lead author Anna Trindade Falcão of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, 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.
A Hubble Space Telescope visible-light image of the galaxy MCG-03-34-064. Hubble’s sharp view reveals three distinct bright spots embedded in a white ellipse at the galaxy’s centre (expanded in an inset image at upper right). Two of these bright spots are the source of strong X-ray emission, a telltale sign that they are supermassive black holes. The black holes shine brightly because they are converting infalling matter into energy, and blaze across space as active galactic nuclei. Their separation is about 300 light-years. The third spot is a blob of bright gas. The blue streak pointing to the 5 o’clock position may be a jet fired from one of the black holes. The black hole pair is a result of a merger between two galaxies that will eventually collide. Credit: NASA, ESA, Anna Trindade Falcão (CfA); Image Processing: Joseph DePasquale (STScI). Public Domain.
Coincident
Falcão’s team then examined the same galaxy in X-rays light using the Chandra Observatory to drill into what’ s going on.
She 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 said: “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.”
Third source
The third source of bright light seen by Hubble 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.
Falcão advised: “We wouldn’t be able to see all of these intricacies without Hubble’s amazing resolution.”
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.
The study is published in The Astrophysical Journal.
