“Combined with the lack of detected light from the lens at late HST epochs, the BH nature of the lens is conclusively verified,” the scientists wrote in their landmark paper in The Astrophysical Journal. The confirmation is a milestone in black hole astronomy as it proves the existence of the first lone black hole a lone animal without a companion star wandering the Milky Way.
The black hole, OGLE-2011-BLG-0462, lies approximately 5,000 light-years away towards Sagittarius. While our home galaxy’s central supermassive black hole is a solitary resident, i.e., it isn’t traveling around another star, this stellar-mass black hole is a solitary wanderer, traveling at a record velocity of 51 kilometers per second compared to the stars it encounters. Its passage through the universe probably began with a supernova explosion so large that it may have given it the “kick” that launched it to travel at high velocities through space.
Part of what makes this find so terrifying is not that the black hole was alone, but the manner in which it was discovered. Black holes, by definition, do not emit any light whatsoever, and are thus notoriously hard to detect unless they are impacting material nearby. In this instance, the black hole was rendered visible through the effect of gravitational microlensing. As it passed in front of a background star far away, its massive gravity distorted the fabric of space-time, bending and compressing the starlight. This single observation gave astronomers a brief but precious insight into the black hole’s existence.
The object was first seen in July 2011, and the Hubble Space Telescope sighting of the object took place between 2011 and 2017. Yet many years of cautious observation, and additional observations in 2021 to 2022 based on observations by the Gaia spacecraft were required to ascertain that the object indeed existed. The group was led by a Baltimore Space Telescope Science Institute astronomer, Kailash Sahu. “It’s the only one so far,” Sahu described how wonderful the finding is.
The mass of the black hole was estimated to be seven solar masses with the error margin of 0.8 solar masses. This number effectively excludes the object from being a neutron star, whose limit is much lower. Luckily, a second team of scientists who initially rejected the theory of black holes re-examined data in 2023 and arrived at the same conclusion, although with marginally different mass measurements.
The find has numerous implications beyond the discovery of one black hole. The Milky Way galaxy contains anywhere from 100 million black holes, astronomers estimate, but only the largest are observable because they are reticent. OGLE-2011-BLG-0462’s discovery is a proof of principle for discovering other lone black holes with gravitational microlensing. Future flights, including the 2027 launch of the Nancy Grace Roman Space Telescope, will be certain to transform this area by opening up the possibility for many microlensing events to be observed.
This one black hole also brings some interesting questions about its fate. Though at present it is producing no electromagnetic radiation or light, in the future it may “turn on” and produce X-rays or radio waves if it does start interacting with nearby matter. It may even not remain alone for good. The newly constructed Vera C. Rubin Observatory will also be able to detect more objects of the same kind, and friends or interactions could be found by it.
Something that one of the black hole discoveries shows is as much about the power of will and ability of astronomers. They spent more than a decade searching, chaos, and technological advancement to be certain that it was there. But that is only the starting point. As Sahu and his team embark on this expedition, the universe very well might reveal more of its hidden travelers, rewriting our understanding of black holes and their role in galactic society.
