Simulations hint at a hidden population of companionless black holes
So far, almost all stellar-mass black holes detected by astronomers have been found in binary systems. However, astronomers have long suspected that these binary black holes are the exception and that, within the Milky Way, most exist on their own.
Through new simulations, researchers led by Wagg at the Flatiron Institute in New York have calculated that more than 90% of stellar-mass black holes in our galaxy could be isolated in this way, hinting at the possibility of a vast reservoir of as-yet unobserved bodies. Their results have been published on the arXiv preprint server.
Companionless and elusive
By definition, black holes cannot emit light at any wavelength. As a result, the only way to detect them is through their gravitational influence on other objects, such as a companion star being pulled apart or heated up. Since lone black holes have little surrounding material to disrupt, they are essentially invisible, making it extremely difficult for astronomers to study how black holes form and evolve over time.
Until observational techniques improve, there is no clear solution to this challenge—but there is still much astronomers can learn from simulations. In their study, Wagg's team developed a simulation of the Milky Way's history, modeling how stars are born, evolve and die across billions of years.
Their simulation also accounted for the supernova explosions that produce black holes and the recoil "kicks" that can catapult them across the galaxy. By running this model forward through cosmic time, the researchers could estimate how many black holes should exist today, where they are likely to be and how quickly they are moving.
Hidden population
The results suggest that around 91% of the Milky Way's stellar-mass black holes are currently isolated, with no companion star to give them away, and that roughly 3% have picked up enough speed from their birth kicks to escape the galaxy altogether. This would leave only a small fraction in the binary systems astronomers typically rely on for detection, implying that the population studied so far is a poor sample of what's really out there.
Through upcoming data from the Roman Space Telescope, Gaia's next data release and various spectroscopic surveys, many astronomers are hopeful that black holes could be revealed through more subtle methods, such as the way their gravity bends background starlight.
With their simulations, Wagg's team now offers a predicted map of where to look for the elusive bodies and what to look for. Using their predictions as a guide, astronomers may be better placed to test their models of stellar death and black hole formation against a much larger and more representative slice of the galaxy's hidden population.
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Publication details
Tom Wagg et al, Charting the Galactic Underworld I: Comprehensive simulations of the kinematics, rates, and demographics of Milky Way black holes, arXiv (2026). DOI: 10.48550/arxiv.2607.22814
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