Astronomers Find Milky Way’s First Rouge Black Hole

The dark object weighs in at seven times the mass of the sun, making it one of the best candidates for a free-floating black hole of stellar mass to date

Photo by Aman Pal on Unsplash

Black hole hunters are enjoying a boom time. In most galaxies, black holes weighing billions of suns can be found at their cores, and they have even been imaged. In the meantime, scientists routinely detect gravitational waves from merging black holes. Our own galaxy’s supermassive black hole and its more diminutive cousins can produce dramatic celestial fireworks when they feed on gas clouds or even entire stars. It is the first time we have ever witnessed an unavoidable phenomenon: a black hole flung through space by a collapsing star.

A free-floating black hole discovered some 5,000 light-years away from Earth has been identified for the first time. It represents more than a decade’s worth of search efforts when it was published January 31 on the arXiv preprint server but hasn’t yet been peer-reviewed. Marina Rejkuba, co-author of the paper from the European Southern Observatory in Germany, says that the finding is “super exciting.” “We are actually proving the existence of isolated black holes.” Future surveys and missions should uncover dozens or even hundreds more of the dark, lonely travelers. Kareem El-Badry, an astronomer at Harvard Smithsonian Center for Astrophysics, says it’s just the tip of the iceberg.

A famous experiment was performed by Arthur Stanley Eddington in 1919. Gravitational lensing occurs when massive objects bend rays of light in close proximity due to the force of gravity. According to Einstein’s theories of special and general relativity, massive objects bend light in spacetime. A total solar eclipse allowed Eddington to observe stars in the heavens adjacent to the sun when the sun’s glare was minimized. He measured the stars’ locations with the help of astrometry, showing a subtle change in their position due to the gravitational pull of our star. Despite the absence of a paper by Feryal *zel, the apparent positions of the stars shifted slightly.

A new application of this technique was discovered in the following decades. Black holes form when a star has a mass greater than 20 times that of our sun and its core collapses under its own weight after its thermonuclear fuel is exhausted. Black holes of stellar mass-spheres with mass dozens of times our sun-are often accompanied by supernova explosions from the energy released when their core collapses. Black holes can be flung out of their wombs by forces so great that they sometimes travel through interstellar space forever. Black holes should be hard to see because of their small size and inherent darkness, combined with their cosmic wanderlust. Eddington’s work suggests that these outcasts can be found by observing their lensing effects, which are usually transient brightenings of stars in our field of view when the black holes flit across them. A singular black hole would have slim chances of causing such an event, but millions of black holes of stellar mass are predicted to be drifting through our galaxy, so some may turn up in sufficiently broad and deep surveys.

Scientists in Japan and New Zealand have searched for microlensing events with the Microlensing Observations in Astrophysics (MOA) survey and the Optical Gravitational Lensing Experiment (OGLE). This resulted in a star suddenly brightening 20,000 light-years away from a densely packed bulge in the Milky Way’s center; this was seen in June 2011. Has a rogue black hole caused this microlensing event? Researchers investigated.

A lead author on the arXiv preprint detailing the object’s discovery was Kailash Sahu of the Space Telescope Science Institute in Baltimore. In just a few weeks after the star became bright, he and his colleagues focused on it with the Hubble Space Telescope, then revisited it repeatedly over the next six years. A lensing object had been detected, which indicated that the light had been magnified, but this wasn’t the most important finding. An apparent shift of a minuscule distance had happened in the star’s position in space. Sahu says that the effect measured by Hubble was “1,000 times smaller than Eddington’s,” and was near the limits of Hubble’s capability. There was something amplifying and distorting the star’s light. But who should it be? Its mass is 7.1 times that of the sun, making it an invisible black hole.

According to Sahu, there was no other possibility than a black hole. For that to be confirmed, there were two things to consider. As a first step, none of the light from the lens had to come from the lens, he says, to rule out more practical objects such as a failed star called a brown dwarf. Given a black hole’s gravitational sphere of influence’s expansive size, the magnification effect should last a long time. The June 2011 event did just that, lasting about 300 days. El-Badry says the analysis is thorough and detailed. “The team has worked hard.”

Sahu and his collaborators were able to calculate the mass of the suspected black hole by lensing and deflecting light from the star. As a result, it lies “just about in the middle” of what one might expect for stellar-mass black holes, he says. Researchers were able to calculate its speed as well. It moves at a speed of 45 kilometers per second, says Sahu. If an ejecting massive star had kicked the black hole into action, this would be relatively fast compared with nearby stars. The exact date of the event is not known, but Sahu believes it may have happened close to 100 million years ago. “We don’t know where it originated, so we can’t really tell from where it came.”

There are, however, several other candidates that predate this one that yield microlensing evidence from stellar-mass black holes. Instead of simply amplification, now we are able to measure the gravitational deflection of the lens and determine its mass-and therefore its true nature. It is not the first time black hole candidates have been detected, but the previous detections did not include these astrometric measurements, says David Bennett of NASA’s Goddard Space Flight Center, co-author with Sahu and others on the discovery paper. “This technique is best for detecting black holes with stellar masses isolated from one another. We are trying it for the first time. Since black holes are not isolated, all those that have been discovered before have been found.”

Black holes like this one can rise from the ashes of particularly massive stellar progenitors, proving astrophysicists’ models of their formation are correct. It is, however, possible that black holes can also form within binary systems and then become nomads in the void. With this particular object, it is impossible to say which origin story occurred with certainty. Research into and refinement of the models of these black holes will be greatly enhanced by finding more isolated black holes. The study of black holes by themselves has never been possible, Özel says. It is certainly exciting to be able to find them and determine their mass this way. Does their formation differ from one to the other? Do they have different masses?””

Quite soon, we should know the answers to these questions. Gaia, the astronomical instrument of the European Space Agency, is currently mapping the positions of billions of stars in our Milky Way galaxy. The lensing data from the project will be released in 2025, and it is expected to reveal the presence of many more massive stars bolting about our galaxy. This latest discovery paper was co-authored by *ukasz Wyrzykowski from Warsaw University, who is also searching for rogue black holes with Gaia. “The data from Gaia will be similar to or even better than Hubble’s data,” he says. Several dozen additional candidates are expected to be included in the coming lensing data, he estimates.

Several other observatories, including the Vera C. Rubin Observatory in Chile, which is planning to begin a survey of the night sky next year, are expected to find black holes rogues in the night sky, as well as NASA’s Nancy Grace Roman Space Telescope, which is due to launch in 2027. The wide fields of view of Rubin and Roman allow them to capture stellar vistas in which a great number of free-floating black holes may lurk. El-Badry says he expects this data to be available. In the hope of determining this astrometric shift for many stars, Rubin and Roman hope to measure it.

Now that this dark discovery has been made, the future of the hunt seems bright. Black holes with stellar masses have long been predicted but were only observed recently. There is a possibility that their populations may be common enough to be studied demographically. We may uncover new gaps in understanding stellar evolution if we can confirm their true abundance, mass and other properties. The discovery has been awaiting us for a long time,” Wyrzykowski says. “It proves its effectiveness.” The only way to locate black holes is with gravitational microlensing.

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