Astronomers have discovered compelling evidence of the closest known supermassive black hole outside the Milky Way. This enormous black hole, with a mass about 600,000 times that of the Sun, resides in the Large Magellanic Cloud (LMC), one of our galaxy’s nearest neighboring dwarf galaxies.
The discovery was made by tracking the motion of 21 hypervelocity stars—stars moving so fast they will eventually escape the gravitational pull of the Milky Way. By analyzing their trajectories, researchers determined that while some were flung out by the Milky Way’s supermassive black hole, others had been propelled by a previously undetected black hole in the LMC.
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Jesse Han of the Center for Astrophysics | Harvard & Smithsonian (CfA), who led the study, highlighted the significance of this finding: “It is astounding to realize that we have another supermassive black hole just down the block, cosmically speaking. Black holes are so stealthy that this one has been practically under our noses this whole time.”
Unveiling the Black Hole’s Existence
The discovery was made possible through data collected by the European Space Agency’s Gaia mission, a satellite that has been tracking over a billion stars in the Milky Way with unprecedented accuracy. By combining Gaia’s data with recent models of the LMC’s orbit, scientists were able to pinpoint the hidden black hole’s location. “We knew that these hypervelocity stars had existed for a while, but Gaia has given us the data we need to figure out where they actually come from,” said co-author Kareem El-Badry of Caltech.
Hypervelocity stars are created when a binary star system ventures too close to a supermassive black hole. The black hole’s immense gravitational pull captures one star while slingshotting the other into space at speeds of millions of miles per hour. The research team predicted that if a supermassive black hole existed in the LMC, it would create a cluster of hypervelocity stars in a specific region of the Milky Way. The data confirmed this pattern, providing strong evidence for the LMC’s hidden black hole.
Ruling Out Other Explanations
To ensure the accuracy of their findings, scientists considered alternative explanations for the movement of these hypervelocity stars. Some stars can be ejected by supernova explosions or gravitational interactions with other stars, but the observed pattern did not match these scenarios. “The only explanation we can come up with for this data is the existence of a monster black hole in our galaxy next door,” said Scott Lucchini, a co-author of the study.
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By studying the speeds and number of stars ejected by both the LMC and Milky Way’s supermassive black holes, researchers estimated that the LMC’s black hole is significantly smaller than the Milky Way’s, which has a mass of about 4 million Suns. However, in the grand scheme of the universe, black holes with billions of solar masses dwarf both.