Formation of an eccentric nuclear disk after the impact of a gravitational recoil of a supermassive black hole. Credit: Tatsuya Akiba and Ann-Marie Madigan, UCB.
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We observe galaxies quite easily. But there is often a problem with their central areas, where there is not much to see due to dust and gas. When astronomers observed the iconic Galaxy in Andromeda (M31), they first thought that its supermassive black hole would be surrounded by a more or less symmetrical cluster of stars, similar to ours in the Milky Way.
Ann-Marie Madigan. Kredit: University of Colorado at Boulder.
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But when the Hubble Space Telescope thoroughly explored the Andromeda Galaxy in the 1990s, it revealed a remarkable thing. This galaxy is as pretty spiral as the Milky Way, but its central part doesn’t look like we imagined. The stars orbit there in strange orbits, creating a strange oval shape. Experts call it the “eccentric nuclear disk.”
Ann-Marie Madigan of the University of Colorado, Boulder, and her colleague grabbed this mystery by the throat. They believe that the strange shape of the center of the Andromeda Galaxy is the result of a gigantic gravity kick, a “kickback” that occurred during an ancient collision with another galaxy, apparently several billion years ago. The kick was so overwhelming that it derailed millions of stars and sent them into orbits, which are raising eyebrows for astronomers today.
Madiganova et al. they modeled galaxy collisions and watched what would happen to their galactic centers when their supermassive black holes merged into one. Their simulations showed that the gravitational forces torn from the chain by the fusion of supermassive black holes would easily be able to form the center of the Andromeda Galaxy, which we are now observing there.
Logo. Kredit: University of Colorado at Boulder.
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When black holes collide, the surrounding universe waves with gravitational waves. But they do not directly affect the stars. On the contrary, it has a fundamental effect when the recoil moves the newly created supermassive black hole, which was created by merging the original pair. As Madigan points out, the recoil can be so intense that it moves a supermassive black hole at speeds of up to thousands of miles per second. It could even fly out of its galaxy.
However, if a supermassive black hole does not fly out of the galaxy after such a gravitational kick, it can easily move many stars in orbit during its dramatic movement. The results of the simulations look very similar to what we observe at the center of the Andromeda Galaxy.
Video: Black Hole Sun | Ann-Marie Madigan | TEDxBoulder
Literature
University of Colorado at Boulder 2. 11. 2021.
Astrophysical Journal Letters 921: L12.
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