- Astrophysicists discover a model that explains the mysterious behavior of eccentric stellar orbits surrounding suppressive black holes.
- A supermassive black hole swallows one star every year in a post-galactic merger period.
- This is 10,000 times more often than other estimations.
Researchers at University of Colorado at Boulder have discovered a technique that explains a longstanding astronomical mystery of eccentric stellar orbits. It states that during post galactic merger (when two or more galaxies collide), orbiting stars could fling into a supermassive black hole at the rate of one per year.
The study tries to explain the behavior of eccentric stellar orbits surrounding giant black holes in some galaxies, and why the unsteady dynamic survives long term.
Stellar Orbits
The enormous gravity of supermassive black hole forms a nuclear star cluster surrounding it. We simply assume that these nuclear star clusters are spherically symmetric, but many galaxies, including the nearest major galaxy to the Milky Way, Andromeda (M31), have been observed to be highly asymmetric and coherent torques between stellar orbits might dominate over two body interactions.
Due to the merger between two gas-rich galaxies, the clusters take form of eccentric disks. All stars follow an elliptical orbit, revolving around the supermassive black hole, within this disk. Most of the time, the orbits of different stars closely overlap and interact with each other.
The super-strong force builds up in stellar orbits, which alters their formation. Ultimately, stars reach its nearest approach to the supermassive black hole and get shredded.
According to the research, a supermassive black hole will swallow one star every year in a post-galactic merger period. This is 10,000 times more often than other estimations.
The Model Makes Number of Predictions
- The model predicts the presence of less massive outer disk in the Andromeda galaxy, which precesses in the opposite direction to the stable eccentric nuclear disk. The inner and outer disk could reproduce Andromeda’s double nucleus structure.
- Along with negative eccentricity gradient in Andromeda, the model also predicts a negative inclination gradient.
- The model predicts a small population of counter-orbiting stars at low semi-major axes because of inclination flips.
- The eccentric disks might be frequent in neighbor galaxies, especially in post-merger galaxies. These disks will only manifest as double nuclei over a small range of parameter space. However, it may appear as a central ‘hole’ or offset nuclei in the stellar distribution.
Reference: Astrophysical Journal | doi:10.3847/1538-4357/aaa714 | University of Colorado at Boulder
What’s Next?
A few galaxies with supermassive black holes at the center have higher rates of stellar mortality as compared to others. The findings indicate that eccentric nuclear disks may be more usual than previously anticipated. Further studies could give us more insights about evolution of the universe and galactic mergers.
The next step is to study the shape of eccentric nuclear disks as a function of initial disk parameters like surface density and eccentricity. The team will also explore how disks respond to background stellar potentials and losing stellar mass through tidal disruption events. They will measure the time-dependent tidal disruption events due to evolving eccentric nuclear disks, taking all major factors into account, such as general relativity, stellar mass segregation and background gravitational potentials.
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The Andromeda galaxy has past the peak of this process – it has already undergone a merger long ago. However, data with higher resolution will let researchers uncover younger eccentric disk in far-away galactic nuclei.
