Though an estimated 100 million black holes are predicted to roam among the stars in our Milky Way galaxy, astronomers have never conclusively identified an isolated black hole — until now. Previously, black holes with the mass of the newly identified one had only been detected in binary systems in which the black hole is gravitationally bound to a normal, visible star. Now, following six years of meticulous observations, NASA’s Hubble Space Telescope has provided, for the first time ever, strong evidence for a lone black hole drifting through interstellar space.
A paper describing the discovery, by a team that includes Penn State astronomer Howard Bond, has been accepted for publication in the Astrophysical Journal.
“Black holes have been in the news recently as a spectacular image of the black hole at the center of our Milky Way galaxy was released a few weeks ago,” said Bond, professor of practice in astronomy and astrophysics at Penn State. “This black hole has a mass of 4 million times that of our sun; it’s what’s called a supermassive black hole. Our object, by contrast, has a mass of only 7.1 times that of the sun. It’s a ‘stellar-mass’ black hole. There are very likely many of these objects in the Milky Way, but they are extremely difficult to detect since they emit no light. Ours is the first one to be identified based on the black hole deflecting the image of a background star that it passed closely in front of.”
The black hole that was detected lies about 5,000 light-years away, in the Carina-Sagittarius spiral arm of our galaxy. For context, the nearest star to our solar system, Proxima Centauri, is a little over 4 light-years away.
Black holes roaming our galaxy are born from rare, monstrous stars (less than one-thousandth of the galaxy’s stellar population) that are at least 20 times more massive than our sun. These stars explode as supernovae, and the remnant core is crushed by gravity into a black hole. Because the self-detonation is not perfectly symmetrical, the black hole may get a kick, and go careening through our galaxy like a blasted cannonball.
Telescopes can’t photograph a wayward black hole because it doesn’t emit any light. However, a black hole warps space, which then deflects and amplifies starlight from anything that momentarily lines up exactly behind it.
Ground-based telescopes, which monitor the brightness of millions of stars in the rich star fields toward the central bulge of our Milky Way, look for such a tell-tale sudden brightening of one of them when a massive object passes between us and the star. Then Hubble follows up on the most interesting such events.
Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, along with his team, made the discovery in a survey designed to find just such isolated black holes. The warping of space due to the gravity of a foreground object passing in front of a star located far behind it will briefly bend and amplify the light of the background star as it passes in front of it. The phenomenon, called gravitational microlensing, is used to study stars and exoplanets in the approximately 20,000 events seen so far inside our galaxy.
The signature of a foreground black hole stands out as unique among other microlensing events. The very intense gravity of the black hole will stretch out the duration of the lensing event for over 200 days.
Next, Hubble was used to measure the amount of deflection of the background star’s image by the black hole. Hubble is capable of the extraordinary precision needed for such measurements. The star’s image was offset from where it normally would be by two milliarcseconds. That’s equivalent to measuring the diameter of a 25-cent coin in Los Angeles as seen from New York City.
This astrometric microlensing technique provided information on the mass, distance, and velocity of the black hole. The amount of deflection by the black hole’s intense warping of space allowed the team to estimate it weighs about seven solar masses. The isolated black hole is traveling across the galaxy at 90,000 miles per hour (fast enough to travel from Earth to the moon in less than three hours). That’s faster than most of the other neighboring stars in that region of our galaxy.
“Astrometric microlensing is conceptually simple but observationally very tough,” said Sahu. “It is the only technique for identifying isolated black holes.”
The existence of stellar-mass black holes has been known since the early 1970’s, but all of them — until now — are found in binary star systems.
“Detections of isolated black holes will provide new insights into the population of these objects in our Milky Way,” said Sahu.
NASA’s upcoming Nancy Grace Roman Space Telescope will discover several thousand microlensing events out of which many are expected to be black holes, and the deflections will be measured with very high accuracy.