Astronomers have simulated the Milky Way to uncover its hidden “Galactic Underworld,” estimating that roughly 170 million stellar-mass black holes are scattered throughout our galaxy, according to research led by Flatiron Institute astrophysicist Tom Wagg. Because invisible black holes rarely light up their surroundings with a feeding frenzy, finding them and taking a census remains a fundamental challenge in modern astronomy.
Simulating the Milky Way’s 13.6-Billion-Year History
To determine how many stellar-mass black holes exist today, researchers built virtual Milky Ways that followed billions of years of star formation, stellar deaths, and galactic evolution. According to the study, which has been submitted to AAS journals and is available on the preprint server arXiv, the Milky Way has ushered around 170 million stars into the afterlife of a black hole over its lifespan. Spread across the galaxy’s 13.6-billion-year history, that works out to an average of roughly one new stellar-mass black hole every 80 years. However, the team calculated that the galaxy’s black hole “baby boom” occurred much earlier during a period of profuse star formation.
Did you know? At the Sun’s distance from the galactic center, the distribution of black holes comes out to roughly one black hole per 6,250 cubic parsecs—a volume of space with sides measuring around 18.4 parsecs.
How Natal Kicks Shape Black Hole Distribution
Black holes frequently do not stay where they are born. According to the findings, many form in lopsided supernova explosions that impart a “natal kick,” punting the newly formed object across the galaxy. While some kicks reach escape velocity and leave the Milky Way entirely, most send black holes on long journeys inside the galaxy. This phenomenon alters their spatial distribution compared to normal stars. The team found that the Milky Way’s thin disk of stars has a height of around 306 parsecs, whereas the black hole disk reaches a height of around 790 parsecs due to these natal kicks.
Weighing Stellar Remnants and Supernova Models
The simulations also indicate that lighter black holes fly farthest from the galactic plane because they receive the strongest kicks at birth. According to the research team, supernova explosions that create lighter objects eject more star material, increasing the recoil that sends the newborn black hole flying. Heavier black holes form with more material falling back onto the remnant, which damps the kick. Consequently, future surveys measuring the masses and motions of these objects could help astronomers distinguish between competing models of supernova explosions and black hole formation.
Pro Tip: Upcoming sky surveys won’t just count black holes; their present-day locations and motions preserve an archive of the Milky Way’s past evolution. Read more about recent galactic discoveries in our astronomy news archive.
Frequently Asked Questions
How many stellar-mass black holes are in the Milky Way?
Simulations led by Flatiron Institute astrophysicist Tom Wagg estimate that around 170 million stellar-mass black holes exist in our galaxy.
Why are stellar-mass black holes hard to find?
Unless they actively feed on surrounding material and emit light, stellar-mass black holes remain invisible and difficult to detect.
What is a natal kick?
A natal kick is an energetic impulse imparted to a newborn black hole during a lopsided supernova explosion, sending the object traveling across the galaxy.
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