Black Hole Caught Shredding a Star 3,500 Light-Years From Its Galaxy's Center
TDE 2023mfm is a rare off-nuclear tidal disruption, hinting that a wandering black hole of 1 to 10 million solar masses roams the host galaxy.
Astronomers have found a black hole tearing apart a star far from the middle of its galaxy, a sign that a massive black hole may be wandering through its host rather than sitting at the center. The event, catalogued as TDE 2023mfm, was described in a preprint posted to the arXiv server on September 28 by Itai Sfaradi and colleagues, and has not yet been peer reviewed.
A tidal disruption event, or TDE, happens when a star strays too close to a black hole and is pulled apart by gravity, producing a flare of light that fades over months. Nearly all known examples occur in the nuclei of galaxies, where the biggest black holes live. This one took place about 1.08 kiloparsecs, roughly 3,500 light-years, from the center of its host galaxy, which makes it an off-nuclear event.
The team followed the flare with a long list of instruments: the Zwicky Transient Facility, the Hubble Space Telescope, Swift's ultraviolet telescope, the Keck and Lick observatories, the Chandra and XMM-Newton X-ray telescopes and the Very Large Array in radio. Optical brightness peaked about 30 days after discovery and declined slowly over roughly 100 days. The source held a temperature near 20,000 kelvin for more than a month after the peak. A clear broad hydrogen line in its spectrum led the researchers to classify it as a TDE-H, a type whose spectra show only hydrogen emission.
Three independent methods put the black hole's mass at about 1 million to 10 million times the Sun's, similar to many known TDEs. No X-rays came from the disruption itself, though a faint X-ray source probably tied to the galaxy's nucleus was seen. About 40% of optically discovered TDEs eventually become X-ray bright, so the team calls the lack of X-rays not unprecedented. Radio emission appeared only about 991 days, or 2.7 years, after the event, then evolved slowly. That contrasts with a close look-alike, TDE 2024tvd, which matches it in peak brightness, duration and host galaxy size. Why the two differ in radio is unclear; the authors list a missing early outflow, a different gas environment or viewing angle as possibilities.
The more intriguing question is how a million-solar-mass black hole ended up so far from the center. The authors consider three explanations. The black hole could be the leftover core of a small galaxy that merged with the host, which fits a clump of 10 million to 100 million solar masses of stars at the site, though the clump is too light for the black hole, so the small galaxy must have lost many of its stars. It could have been flung out of a triple black hole system by a gravitational-wave recoil, which the researchers estimate could explain about 1% of TDEs. Or two black holes could have merged and the recoil pushed the remnant outward.
The team thinks the last two are less likely because the host galaxy's center has its own black hole, revealed by a faint active nucleus. Two black holes in one galaxy favors the minor-merger story. Future radio observations of the off-nuclear event could confirm the wandering black hole and measure how far apart the two black holes are.
