Supermassive Black Holes (SMBHs) reside at the center of large galaxies, where they dominate their surroundings and sometimes eat stars. When they gobble up a star, they emit a distinctive light flare. This makes it easier for astronomers to pinpoint their location. Astronomers have detected one of these flares offset from a galactic center. Is the black hole shifting its location?
When an SMBH consumes a star, it’s called a Tidal Disruption Event (TDE). During a TDE, the star is stretched out or ‘spaghettiefied’ by the black hole’s powerful gravity. The remnants of the star are drawn into a circular orbit around the black hole, which generates outflows and shocks that flare with radiation.
TDEs are scientifically valuable because of what they show scientists about black holes. They can reveal the black hole’s mass and spin, details of mass accretion under extreme physical conditions, show how jets form, and even act as probes for the stellar populations in galactic nuclei.
In new research to appear in The Astrophysical Journal Letters, an international team of researchers used the Hubble Space Telescope, the Chandra Observatory, and the VLA to pinpoint an MBH in a galaxy about 600 million light-years away. The research is titled “A Massive Black Hole 0.8 kpc from the Host Nucleus Revealed by the Offset Tidal Disruption Event AT2024tvd.” It’s available at arxiv.org, and the lead author is Yuhan Yao of the University of California at Berkeley.
The research reveals two surprises. The first is that the star-eating culprit is offset from the galactic nucleus by about 2,600 light-years. The second surprise is that there’s another SMBH right where it should be: in the galactic centre.
These images show how the Hubble, Chandra, and the VLA worked together to understand the pair of black holes in the galaxy. The red x marks the location of the TDE, while the red plus marks the galactic center, where the larger SMBH resides. Image Credit: Yao et al. 2025, The Astrophysical Journal Letters.
“AT2024tvd is the first offset TDE captured by optical sky surveys, and it opens up the entire possibility of uncovering this elusive population of wandering black holes with future sky surveys,” said lead author Yao. “Right now, theorists haven’t given much attention to offset TDEs. I think this discovery will motivate scientists to look for more examples of this type of event.”
The SMBH at the galaxy’s center contains about 100 million solar masses, while the massive black hole (MBH) responsible for the TDE has only about one million solar masses. The SMBH is an Active Galactic Nucleus (AGN), which means it’s actively accreting material and emitting powerful radiation visible to telescopes. About 2,600 light-years separate the two. So, even though they share the same galaxy, they’re not bound to one another gravitationally.
“Tidal disruption events hold great promise for illuminating the presence of massive black holes that we would otherwise not be able to detect,” said co-author Ryan Chornock, associate adjunct professor at UC Berkeley. “Theorists have predicted that a population of massive black holes located away from the centers of galaxies must exist, but now we can use TDEs to find them.”
This Hubble Space Telescope image shows a distant galaxy that hosts the telltale signature of a roaming black hole. Credit: NASA, ESA, STScI, Yuhan Yao (UC Berkeley), Joseph DePasquale (STScI)
The press release accompanying the research says that the black hole responsible for the TDE is “prowling inside the bulge of the massive galaxy,” pointing out that it’s only visible every few tens of thousands of years when it emits a flare while devouring a star. In the paper, the researchers write that the discovery is “consistent with cosmological simulations that massive halos host more wandering black holes.” It’s almost as if the black hole is chasing its food, to put it colourfully.
Although scientists are interested in TDEs for various reasons concerning how black holes behave, this finding has something more to it. By finding galaxies with both central SMBHs and separate, offset black holes, they may be able to shed light on how SMBHs grow so large through mergers.
The researchers point to two primary explanations for offset TDEs.
The first one says that the TDE originates from one black hole in a dual or inspiraling pre-merger MBH system. The second says it’s from a gravitational slingshot where an MBH is ejected from a triple BH system. “If the black hole went through a triple interaction with two other black holes in the galaxy’s core, it can still remain bound to the galaxy, orbiting around the central region,” said Yao.
An alternative scenario is that the offset black hole results from a galaxy merger within the last one billion years. If so, it will eventually merge with the larger SMBH in the galaxy’s center. Co-author Erica Hammerstein, a post-doctoral researcher at UC Berkeley, examined the evidence and found no evidence to support such a merger.
However, that didn’t mean there wasn’t one.
“There is already good evidence that galaxy mergers enhance TDE rates, but the presence of a second black hole in AT2024tvd’s host galaxy means that at some point in this galaxy’s past, a merger must have happened.”
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