A supermassive black hole in the Large Magellanic Cloud (LMC) may be the source of nine stars zooming through our galaxy – a surprising hint that dwarf galaxies can host large black holes.
“This is the first compelling evidence for a supermassive black hole in [a dwarf] galaxy,” says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. He estimates the mass of the black hole inside the LMC would be about 600,000 times that of the sun. For comparison, the one at the centre of the Milky Way is about 4 million times the mass of the sun.
Han and his colleagues found evidence for the LMC’s supermassive black hole by analysing so-called hypervelocity stars in our galaxy. This is the name for stellar objects deemed to be travelling more than 500 kilometres per second, which is about twice the speed our sun orbits the centre of the Milky Way. However, some hypervelocity stars can move as fast as 2000 kilometres per second – around 0.6 per cent of the speed of light.
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The only known way to accelerate a star to such speeds is by gravitationally slingshotting it around a supermassive black hole, says at the Max Planck Institute for Astronomy in Germany. “It’s really impossible to explain those high speeds with supernova ejections” or other methods, she says. As such, the assumption has been that the nearly two-dozen known hypervelocity stars in the Milky Way were accelerated by the supermassive black hole at the centre of our galaxy.
Han and his colleagues have now suggested an alternative. In research yet to be peer-reviewed, they say that about half of the known hypervelocity stars in the Milky Way may actually be explained by a supermassive black hole in the centre of the LMC, a galaxy about a tenth the size of the Milky Way. The LMC orbits our galaxy at a distance of about 163,000 light-years.
In total, the team identified nine stars that could have come from the LMC. These are clustered in a region of the northern sky called the Leo Overdensity. “We can rewind the paths of these stars to see where they come from, and they track directly back to the LMC,” says Han.
The idea is “entirely plausible and deserves further investigation”, says at the University of Surrey in the UK. We might be able to confirm the idea by looking for more hypervelocity stars in the sky tracing a path back to the LMC. “That would be a very nice confirmation,” he says.
There have been previous hints of supermassive black holes inside dwarf galaxies, but confirmation has so far proved impossible. “They’re too far away to image,” says Han. The LMC, however, is close enough that we might be able to spot stars orbiting its black hole. “I think I know where to point, but I’m not going to give away the coordinates just yet,” he says.
However, this object would probably be too small to be directly imaged even by a powerful black hole observatory such as the Event Horizon Telescope, says Han.
Its discovery could be useful for telling us how black holes grow. “One of the outstanding questions in astronomy is how did supermassive black holes begin,” says Han. “The dwarf galaxy supermassive black hole population is a good tracer of the initial seed mechanisms. And this would be the first [direct] detection of a supermassive black hole in a dwarf galaxy.”
arXiv
