Turns out, the nearest supermassive black hole to be found outside our own Milky Way Galaxy lies shockingly close, only 158,000 light years away in the Large Magellanic Cloud. The LMC, a dwarf galaxy which straddles the southern constellations of Dorado and Mensa, is clearly visible to the naked eye from the southern hemisphere.
But what’s been long hidden is its supermassive black hole that likely lies at its center, although its exact location is still not precisely known.
The detection of this supermassive black hole, some six hundred thousand times the mass of our own sun was enabled by data from the European Space Agency’s Gaia mission as well as a years-long study of hypervelocity stars.
Hypervelocity stars are created when a double-star system ventures too close to a supermassive black hole, notes the Harvard Smithsonian center. The intense gravitational pull from the black hole rips the two stars apart, capturing one star into a close orbit around it, notes the center. The other star is jettisoned away at extraordinarily high speeds, says the Harvard Smithsonian center.
The authors of a paper accepted for publication in The Astrophysical Journal have determined that LMC*, as this newfound massive black hole is now dubbed, was the only solution to the mystery of so-called unbound, hypervelocity stars spotted in our own Milky Way Galaxy’s halo. That is, hypervelocity stars that are on trajectories that will soon send them out of the galaxy and into the void beyond.
Despite the LMC being one of the best studied galaxies, this is the first evidence that the LMC has a supermassive black hole in its center, Jesse Han, the paper’s lead author and an astronomer at the Harvard Smithsonian Center For Astrophysics, told me via email.
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The lines of evidence led to the Leo Overdensity, a pronounced overdensity of hypervelocity stars on the sky.
We showed that the hypervelocity stars that make up this overdensity all trace back to the LMC, says Han. And if the LMC were to indeed harbor a SMBH that produces hypervelocity stars, it would produce an overdensity on the sky at precisely this observed location, he says.
Out of 21 hypervelocity stars that were traced, Han notes that he and colleagues can confidently classify nine as being from the LMC. Seven can be traced from our Milky Way Galaxy’s own supermassive black hole, Sag A*.
We realized that half the stars cannot come from Sag A* based on their direction of orbit. Instead, their direction pointed directly towards the LMC, says Han.
So, Han and colleagues concluded that this stellar overdensity must be due to the LMC's production of hypervelocity stars. This means that the LMC must harbor a supermassive black hole.
But why did it take so long to detect this supermassive black hole in the LMC?
The LMC is quite large on the sky, and due to ongoing deformations of the galaxy, its "center" is not well defined, says Han. So, we haven't found an obvious source of photons that could be directly linked to the black hole, he says. But in this study, we've found strong evidence for the presence of the SMBH by its production of hypervelocity stars, says Han.
The Bottom Line?
Han says it’s still debated whether dwarf galaxies elsewhere in the cosmos might harbor such massive black holes. At present, Han says there’s no evidence for such a massive black hole in the Small Magellanic Cloud, the LMC’s companion galaxy, located some 200,000 light years away.
But it’s long been known that large spiral galaxies like our own Milky Way harbor supermassive black holes. At some 4 million times the mass of our own sun, the Milky Way’s own Sag A* dwarfs the newfound LMC*.
What's Next?
Pinpointing the location of LMC* by finding an optical, radio or x-ray counterpart to the black hole, says Han. I’d like to know where exactly the SMBH is, and what its immediate surroundings look like, he says.