¿Por que no los dos? A star has been destroyed by a wandering supermassive black hole Second supermassive black hole is a long way from the galaxy's core. John Timmer – May 9, 2025 9:37 am | 0 It's easy to see that the bright spot of AT2024tvd is not at the center of its host galaxy. Credit: NASA, ESA, STScI, Yuhan Yao (UC Berkeley, Joseph DePasquale It's easy to see that the bright spot of AT2024tvd is not at the center of its host galaxy. Credit: NASA, ESA, STScI, Yuhan Yao (UC Berkeley, Joseph DePasquale Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more Back in 2024, a system set up to identify objects that suddenly brighten found something unusual. Unfortunately, the automated system that was supposed to identify it couldn't figure out what it was looking at. Now, about a year later, we know it's the first tidal disruption event—meaning a star being ripped apart by a supermassive black hole—identified at visual wavelengths. It's also a rather unusual one, in that the supermassive black hole in question does not reside at the center of its galaxy. Instead, there's an even more massive object there, which is feeding on matter at the same time. A mystery object The object, now called AT2024tvd, was identified by the Zwicky Transient Facility, which is set up to scan the entire northern sky over a period of just two days, after which it repeats the process. Combined with software that scans the data for changes, these repeated exposures allow the system to identify objects that suddenly brighten (or, potentially, anything that suddenly goes dark). Among the events it can identify are tidal disruption events, where a star gets spaghettified by the enormous gravity of a supermassive black hole. Normally, supermassive black holes live at the center of galaxies. So, the software that does the scanning will only flag something as a potential tidal disruption event if it coincides with the presence of a previous light source at the same location. And that wasn't the case with AT2024tvd, which appeared to be over 2,500 light years from the center of the galaxy. As a result, the software didn't flag it as a potential tidal disruption event; people didn't figure out what it was until they looked more closely at it. Fortunately, researchers were able to arrange for follow up observations at wavelengths ranging from x-rays to radio waves. At least two of the observatories—the Hubble Space Telescope and the Very Large Array radiotelescopes—could resolve both the object that brightened suddenly (meaning AT2024tvd) and a bright spot at the center of the galaxy, which appears to correspond to the supermassive black hole at the galaxy's center. The fact that it's bright indicates that it's presently feeding on matter. All of the observations indicated that AT2024tvd is a tidal disruption event. For example, it maintained a high temperature throughout the observations, unlike a supernova, which tends to cool down over time. There were also fewer high-energy X-rays than one would expect from a supernova. The UV spectrum also looked like previously identified tidal disruption events, with the signature of elements like carbon and nitrogen that don't require a supernova to be produced. That makes this the fourth tidal disruption event we've identified that's the product of a supermassive black hole not located at the center of the galaxy. It's also the first that was initially identified at visible wavelengths. Wandering black holes This raises two questions: why are there two supermassive black holes here, and why is one of them not in the center of the galaxy? The first is relatively easy to answer. It appears that large galaxies are the product of galaxy mergers, essentially built by combining multiple smaller galaxies. Each of these small galaxies would come into the merger with its own black hole. In most cases, these newly arrived supermassive black holes would end up at the center of the galaxy, and eventually merge with the central supermassive black hole. But note the phrasing there: "in most cases" and "eventually." Even in the cases where a merger takes place, the process is slow, potentially taking millions or even billions of years. As a result, a large galaxy might have as many as 100 extremely large black holes wandering about, with about 10 of them having masses of over 106 times that of the Sun. And the galaxy that AT2024tvd resides in is very large. One consequence of all these black holes wandering about is that not all of them will end up merging. If two of them approach the central black hole at the same time, then it's possible for gravitational interactions to eject the smallest of them at nearly the velocity needed to escape the galaxy entirely. As a result, for millions of years afterwards, these supermassive black holes may be found at quite a distance from the galaxy's core. At the moment, it's not possible to tell which of these explanations account for AT2024tvd's location. The galaxy it's in doesn't seem to have undergone a recent merger, but there is the potential for this to be a straggler from a far-earlier merger. It's notable that all of the galaxies where we've seen an off-center tidal disruption event are very large. The paper that describes AT2024tvd suggests this is no accident: larger galaxies mean more mergers in the past, and thus more supermassive black holes floating around the interior. They also suggest that off-center events will be the only ones we see in large galaxies. That's because larger galaxies will have larger supermassive black holes at their center. And, once a supermassive black hole gets big enough, its event horizon is so far out that stars can pass through it before they get disrupted, and all the energetic release would take place inside the black hole. Presumably, if you were close enough to see this happen, the star would just fade out of existence. The arXiv. Abstract number: 2502.17661 (About the arXiv). To be published in The Astrophysical Journal Letters. John Timmer Senior Science Editor John Timmer Senior Science Editor John is Ars Technica's science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots. 0 Comments