For the first time ever, astronomers using NASA’s Hubble Telescope have reported the observation of a free-floating black hole completely independent of a stellar-mass companion.
So far, black holes have only been identified as supermassive objects at the centers of massive galaxies like our own Milky Way, or as gravitationally bound to a stellar companion. Although the existence of such free-floating black holes has long been predicted, this is the first time such an object has been discovered.
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 are describing their observations in accepted papers The Astrophysical Journal and The Letters of the Astrophysical Journal.
After six years of meticulous observations, NASA’s Hubble Space Telescope has discovered a wandering black hole about 5,000 light-years away in our galaxy’s Carina Sagittarius spiral arm, NASA reported.
The teams used Hubble to capture the object that distorts spacetime through gravitational microlensing. With microlensing, a foreground object can act as a gravitational lens to bend and enhance light from a distant background star. In this case, a black hole struck a star estimated to be 19,000 light-years away in the galactic bulge of our Milky Way. One reason the teams suspected this foreground lensed object was actually a black hole was the duration of the lensing event, which amplified to 270 days.
Because two separate surveys captured the same object, this putative black hole has two names, known in part from the lensing events where 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 remnant star core is being crushed by gravity into a black hole, NASA says. Because the self-detonation isn’t perfectly symmetric, the black hole can get a kick out of it and hurtle through our galaxy like a blasted cannonball, NASA notes.
What is the real size of this black hole?
Theoretical models suggest that the mass of the progenitor of such a black hole is 20 to 25 solar masses, meaning the progenitor must be an O-type star, Kailash Sahu, astronomer at the Space Telescope Science Institute and lead author of The Astrophysical Journal paper , told me.
O-type stars, still burning hydrogen on the so-called main sequence, reach up to 90 times the mass of our Sun and can be millions of times as bright as our own star.
Sahu says that if this solar-mass black hole had 7.1 times the mass of our Sun, its event horizon (or outer limit) would be about 26 miles across.
How fast is it traveling through the galaxy?
According to NASA, Sahu’s team estimates that the isolated black hole is traveling through the galaxy at 100,000 miles per hour (fast enough to travel from Earth to the Moon in under three hours).
Although microlensing is a once-in-a-lifetime discovery, Sahu says it may still be possible to make follow-up observations of this free-floating black hole.
“We estimate that the black hole is located in a region with a high density of interstellar matter,” Sahu said. In that case, the black hole would accumulate material from interstellar matter that would produce X-rays and radio waves, he says.
Therefore, deep X-ray and radio observations could detect the black hole, which can be used to better characterize the object, Sahu says.
How many of these free floaters could be in our galaxy?
Estimates range from 100 to 200 million, with the nearest expected to be at least 80 light-years from Earth.