A wandering supermassive black hole moving far from the center of its galaxy may have revealed its presence by tearing apart a star that came too close.
In June 2023, telescopes spotted a new light in a distant galaxy. The flare, named TDE 2023mfm, grew brighter and then faded over the following weeks. Astronomers continued observing it with several instruments, including the Hubble Space Telescope, to understand what had caused this sudden appearance.
Six illustrated stages of a star’s destruction by a black hole: passing near the black hole, stretching, fragmentation, accumulation of matter, and emission of light. This drawing represents the AT2024tvd event, distinct from TDE 2023mfm.
Illustration: NASA, ESA, STScI, Ralf Crawford.
The most likely scenario is that a star passed too close to a black hole. Its gravity would have pulled much more strongly on the part of the star nearest to it, eventually tearing it apart. Some of the stripped material would then have orbited the black hole while heating up, producing intense light.
But the location from which this light came surprised the researchers. The most massive black holes are generally found at the centers of galaxies. Yet this phenomenon occurred about 3,500 light-years from the center of its galaxy, based on its apparent position in the sky. More precise observations confirmed this offset.
Astronomers infer the likely presence of a black hole weighing several million times the mass of the Sun. It would therefore be far more massive than a black hole born from the collapse of a star, without being located at the center of its galaxy.
Where could it have come from? One possibility is that it once belonged to a small galaxy that was absorbed by a larger one. Most of its stars would have been dispersed during this encounter, but its black hole may have remained away from the center, perhaps accompanied by a few stars.
Images taken by Hubble provide a clue: faint light remains visible at the site of the event, long after its initial flare. It could come from a cluster of stars surrounding the black hole, or even from the remnants of its former galaxy. The observations do not yet make it possible to decide.
The researchers also detected radio waves that appeared at least a year after the first light signal. They could have been produced by material ejected outward after the star’s destruction. Here again, their exact origin remains to be verified.
This black hole is therefore not necessarily leaving its galaxy: “wandering” here means that it lies far from its center. The study, currently published as a preprint, still needs to be examined by other scientists. Above all, it shows how the disappearance of a star can reveal an otherwise nearly invisible object.