The largest galactic core ever seen may be the remnant of a battle for black hole supremacy.
Omar Lopez-Cruz of Mexico's National Institute of Astrophysics, Optics and Electronics and his team measured the core of galaxy Holm 15A, 650 million light years from Earth. They found it was a record-breaking 15,000 light years across – about one-sixth the diameter of the entire Milky Way.
When two galaxies merge, their central black holes start to orbit each other as a binary system. Nearby stars get slung out of their orbits away from the core. This steals energy from the black hole binary and extends the size of the core, making it more diffuse. Eventually, the black holes merge.
Holm 15A's core's size suggests the black hole it hosts could weigh 100 billion times the mass of our sun – nearly as much as the Milky Way. If so, it probably formed as two or three separate black holes jostled for position before merging into one, puffing up Holm 15A's core in the process.
Journal reference: Astrophysical Journal Letters , DOI: 10.1088/2041-8205/795/2/L31
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