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Smallest Supermassive Black Hole Ever Found Could Reveal How Galaxies Grow


Jonathan O'Callaghan

Senior Staff Writer

1735 Smallest Supermassive Black Hole Ever Found Could Reveal How Galaxies Grow
Shown is an optical image of the black hole, and inset the view in X-ray. V.F. Baldassare et al./NASA/CXC/University of Michigan/SDSS.

If you think about it, it’s a bit odd. Why does almost every galaxy we see have a supermassive black hole at its core? Finding an answer has proved difficult, but astronomers may now be a step closer.

Research led by the University of Michigan has found the smallest supermassive black hole ever seen at the center of a galaxy, more than twice as small as the nearest such black hole in size. The relatively small black hole in question was found in the dwarf galaxy RGG 118, located about 340 million light-years from Earth. The discovery will be published in The Astrophysical Journal Letters, and a preprint is available on arXiv.


Found by astronomers using NASA’s Chandra X-ray Observatory and the 6.5-meter Clay Telescope in Chile, the black hole contains about 50,000 times the mass of the Sun. This makes it 100 times less massive than the Milky Way’s central black hole, and 200,000 times less massive than the biggest supermassive black holes we know of. 

Finding such a small black hole is useful, as it could help to answer questions about how galaxies grew in the early universe 13 billion years ago to have supermassive black holes. Two theories prevail: Either they are the result of a large cloud of gas 10,000 to 100,000 times the mass of the Sun collapsing, or they may result from the collapse of a 100-solar-mass star.

RGG 118’s supermassive black hole could therefore serve as a “proxy for those in the very early universe and ultimately may help us decide which of the two [theories] is right,” said study coauthor Elena Gallo of the University of Michigan in a statement.

Using data from Chandra, the scientists found that this black hole behaves much like its bigger cousins. Specifically, its outward push of radiation pressure of hot gas sourced from its surroundings is about 1% of its gravitational pull, the same as other galaxies. This suggests that black holes of all sizes grow in a similar way.


“It might sound contradictory, but finding such a small, large black hole is very important,” added study lead author Vivienne Baldassare of the University of Michigan in the statement. “We can use observations of the lightest supermassive black holes to better understand how black holes of different sizes grow.”

She told IFLScience that this black hole mass is 1/50,000th that of its host galaxy, which is smaller than is typically seen for massive galaxies. “However, there is also a strong relation between supermassive black hole mass and the range of speeds of stars in the host galaxy, and this system follows that relation,” she added. “This could suggest that small galaxies and their supermassive black holes coevolve similarly to how more massive galaxies do with their bigger supermassive black holes.”

Astronomers will now search for similar or even smaller objects to get to the bottom of how supermassive black holes formed, and why most galaxies seem to have one.


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