Almost every large galaxy has a supermassive black hole churning away at its core. In most cases, these black holes spin in concert with their galaxy, like the central hub of a cosmic wagon wheel. But on December 18, 2024, NASA researchers announced they had discovered a galaxy whose black hole appears to have been turned on its side, spinning out of alignment with its host galaxy.
The galaxy, NGC 5084, was discovered centuries ago by German astronomer William Herschel, but it took new techniques, recently developed at NASA’s Ames Research Center, to reveal the unusual properties of the black hole.
The new method is called SAUNAS (Selective Amplification of Ultra Noisy Astronomical Signal). It enables astronomers to tease out low-brightness X-ray emissions that were previously drowned out by other radiation sources.
When the team put their new technique to the test by combing through old archival data from the Chandra X-ray observatory – a space telescope that acts as the X-ray counterpart to Hubble’s visible-light observations – they found their first clue that something unusual was going on in NGC 5084.
Four large X-ray plumes, made visible by the new technique, appeared in the data. These streams of plasma extend out from the centre of the galaxy, two in line with the galactic plane, and two extending above and below.
While plumes of hot, charged gas are not unusual above or below the plane of large galaxies, it is unusual to find four of them, rather than just one or two, and even more unusual to find them in line with the galactic plane.
To make sure that they weren’t just seeing some error or artifact in the Chandra data, they started looking more closely at other images of the galaxy, including both the Hubble space telescope and the Atacama Large Millimeter Array (ALMA).
These observations revealed a dusty inner disk spinning in the centre of the galaxy at a 90-degree angle to the rest of NGC 5084.
The team also looked at the galaxy in radio wavelengths using the NRAO’s Expanded Very Large Array. All together, these observations painted a picture of a very strange galactic core.
“It was like seeing a crime scene with multiple types of light,” said Ames research scientist Alejandro Serrano Borlaff, lead author of the paper published this week in The Astrophysical Journal. “Putting all the pictures together revealed that NGC 5084 has changed a lot in its recent past.”
Borlaff’s coauthor and astrophysicist at Ames, Pamela Marcum, added that “detecting two pairs of X-ray plumes in one galaxy is exceptional. The combination of their unusual, cross-shaped structure and the ‘tipped-over,’ dusty disk gives us unique insights into this galaxy’s history.”
The plumes of plasma suggest that the galaxy has been disturbed in some way during its lifetime. It might be explained, for example, by a collision with another galaxy, which caused the black hole to tip on its side.
With this discovery, SAUNAS has demonstrated that it can bring new life to old data, uncovering new surprises in familiar galaxies. This surprise twist on a galaxy we’ve known about since 1785 offers tantalizing hope that there might be other weird and wonderful discoveries to come, even in places we thought we’d seen everything.
Learn more:
“NASA Finds ‘Sideways’ Black Hole Using Legacy Data, New Techniques.” NASA.
Alejandro S. Borlaff et al. “SAUNAS. II. Discovery of Cross-shaped X-Ray Emission and a Rotating Circumnuclear Disk in the Supermassive S0 Galaxy NGC 5084.” The Astrophysical Journal.