A team of astronomers led by University of Colorado Boulder scientists has discovered a mechanism that explains the persistence of asymmetrical star clusters surrounding supermassive black holes in post-merger galaxies and suggests that during post-merger periods, stars could be flung into the black hole and destroyed at a rate of one per year.
This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. When a star passes within a certain distance of a black hole — close enough to be swallowed up — the stellar material gets stretched and compressed as it is pulled in. A black hole’s destruction of a star, called ‘stellar tidal disruption,’ releases an enormous amount of energy, brightening the surroundings in an event called a flare. Image credit: NASA / JPL-Caltech.
A supermassive black hole’s gravity creates a nuclear star cluster surrounding it, which gravitational physics would expect to be spherically symmetric.
However, several galaxies — including the nearby Andromeda Galaxy — have been observed with an asymmetrical star cluster that takes the form of a disk instead.
Eccentric disks are suspected to be formed in the wake of a recent merger between two gas-rich galaxies.
Within the disk, each star follows an elliptical orbit that revolves around the supermassive black hole over time.
The stars’ orbits nearly overlap and interact with each other frequently. Eventually, gravitational disruptions to one star’s orbit will bring it too close to the black hole.
“The force builds up in these stellar orbits and changes their shape. Eventually, a star reaches its nearest approach to the black hole and it gets shredded,” said lead author Dr. Ann-Marie Madigan, from the Department of Astrophysical and Planetary Sciences & JILA at the University of Colorado Boulder.
“We predict that in a post-galactic merger period, a supermassive black hole will swallow one star per year. That’s 10,000 times more often than other rate predictions,” said co-author Heather Wernke, a graduate student at the University of Colorado Boulder.
The finding bolsters observational evidence that some galaxies with supermassive black holes at their center have higher stellar mortality rates than others and suggests that eccentric nuclear disks may be more common than initially expected.
Further studies could help researchers better understand galactic mergers and the evolution of the Universe.
“Andromeda Galaxy is likely past the peak of this process, having undergone a merger long ago,” Dr. Madigan said.
“But with higher resolution data, we may be able to find younger eccentric disks in more distant galactic nuclei.”
The research appears in the Astrophysical Journal.
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Ann-Marie Madigan et al. 2018. Dynamical Properties of Eccentric Nuclear Disks: Stability, Longevity, and Implications for Tidal Disruption Rates in Post-merger Galaxies. ApJ 853, 141; doi: 10.3847/1538-4357/aaa714
