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Astronomers Finally Discover the Elusive Mid-Sized Black Hole

Photo credit: NAOJ
Photo credit: NAOJ

From Popular Mechanics


Astronomers have discovered a black hole 50,000 times the mass of our sun in a faraway star cluster. But it's not just any black hole.

The new discovery is an elusive intermediate-mass black hole—a celestial "missing link" that astronomers believe may reveal information about how supermassive black holes like Sagittarius A* form.

There are four different types of black holes. Supermassive black holes lie at the center of most galaxies and slurp up nearby stars. (Last year, astronomers photographed a black hole for the first time, a supermassive black hole at the center of M87 galaxy.)

When stars die, they either form neutron stars or incredibly dense stellar-mass black holes. Extremely low mass, miniature black holes, which astronomers believe came into existence around the time the universe formed, are largely theoretical.

And then there are intermediate-mass black holes. Astronomers have long been hot on the trail of this curious cosmic oddball, but so far, the evidence has been thin.

In 2006, researchers using NASA’s Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mirror Mission spotted an peculiar X-ray flare they eventually dubbed 3XMM J215022.4−055108. The strange X-ray signature, which they believed to be a black hole ripping a star to shreds, wasn't emanating from the center of a galaxy. After ruling out one theory—that a neutron star in our own galaxy was giving off the flares—the race was on to find the location of the flare.

Dacheng Lin of the University of New Hampshire and his colleagues trained the Hubble Space Telescope on the tiny patch of sky where astronomers first observed 3XMM J215022.4−055108 to see if they could get a closer look. The images revealed that the flare originated in a dense star cluster at the edge of a distant galaxy—just the type of place astronomers might find an intermediate-mass black hole.

“Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate,” Lin said in a statement. “That is what Hubble has allowed us to do for our candidate.” The scientists published their findings this week in the Astrophysical Journal Letters.

Because intermediate-mass black holes aren't as powerful as supermassive black holes, they're more difficult to find. But they're an incredibly important piece of the cosmic puzzle. Astronomers suspect that supermassive black holes, which are powerful enough to slurp up entire star clusters, may actually get their start as intermediate-mass black holes.

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