NASA Hubble discovers the first free-floating black hole

For the first time ever, astronomers using NASA’s Hubble Telescope have reported the sighting of a floating black hole completely independent of a stellar-mass companion.

So far, black holes have only been identified as supermassive objects at the center of massive galaxies like our own Milky Way, or gravitationally bound to a stellar companion. Although such floating black holes have long been predicted, this is the first time such an object has been detected.

Two separate observing teams – one led by the Space Telescope Science Institute in Maryland and the other led by the University of California, Berkeley – announced the results today and detail their observations in papers accepted in The Astrophysical Journal and Letters from the Astrophysical Journal.

After six years of meticulous observations, NASA’s Hubble Space Telescope has detected a wandering black hole located about 5,000 light-years away in our galaxy’s Carina-Sagittarius spiral arm, NASA has reported.

The teams used Hubble to capture the space-time warping object via gravitational microlensing. With microlensing, a foreground object can act as a gravitational lens to bend and amplify light from a distant background star. In this case, a black hole blasted a star about 19,000 light-years away into the galactic bulge of our galaxy, the Milky Way. One of the reasons the teams suspected that this lensed foreground object was actually a black hole was due to the duration of the lensing event which was amplified for 270 days.

Because two separate surveys captured the same object, this putative black hole has two names, known in part by the lensing events in which they were found: MOA-2011-BLG-191 and OGLE-2011-BLG-0462.

How do such black holes become solo objects?

The most likely scenario is that giant stars, at least 20 times the mass of the Sun, explode as supernovae. The resulting remaining stellar core is crushed by gravity into a black hole, NASA explains. Because the self-detonation is not perfectly symmetrical, the black hole can be kicked and blast through our galaxy like a blasted cannonball, NASA notes.

How big is this black hole?

Theoretical models suggest that the mass of the progenitor of such a black hole is 20 to 25 solar masses, which means that the progenitor must be an O-type star, Kailash Sahu, an astronomer at the Space Telescope Science Institute and lead author of The Astrophysical Journal article, told me.

O-type stars, which still burn hydrogen on what’s called the main sequence, are up to 90 times the mass of our Sun and can be a million times brighter than our own star.

Sahu says that if this solar-mass black hole is 7.1 times the mass of our Sun, its event horizon (or outer boundary) would be about 26 miles in diameter.

How fast does it travel across the galaxy?

Sahu’s team, NASA says, estimates the isolated black hole is moving through the galaxy at 100,000 miles per hour (fast enough to travel from Earth to the Moon in less than three hours).

Although the microlens is a point-in-time detection, Sahu says it might still be possible to make follow-up observations of this free-floating black hole.

“We estimate that the black hole is in a region of high density interstellar matter,” Sahu said. In this case, the black hole would accrete material from interstellar matter that would produce X-rays and radio waves, he says.

Thus, deep X-ray and radio observations can detect the black hole, which can be used to better characterize the object, Sahu says.

As for how many of these free floaters might be in our galaxy?

Estimates range from 100 to 200 million, with the closest being at least 80 light-years from Earth.

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