Heavyweight and lightweight black holes abound in the universe, but nobody has detected a middleweight--and some scientists argue they don't exist. Now, astronomers say they have found the first conclusive evidence for one of these elusive objects at the fringe of a distant galaxy. Estimated to be at least 500 times more massive than the sun, the discovery could plug a large gap in the cosmic menagerie, though it leaves unanswered questions about this type of black hole's origins.
Small black holes--also known as stellar-mass black holes--form as the relics of giant exploding stars. They grow by gobbling up nearby gas, dust, and even unfortunate planets and stars that wander too close. But they have a built-in feeding-control mechanism. Called the Eddington limit, it kicks in when the massive amount of radiation spawned by the black hole matches the inflow of material. That effect limits the amount of x-rays the black hole emits, and astronomers can calculate the object's mass based on the strength of the radiation. Astronomers can also use the Eddington limit to measure supermassive black holes, which can weigh more than billions of stars.
The newly discovered object, called HLX-1, is the first to show an intermediate weight between stellar-mass and supermassive black holes. Astronomers led by Sean Farrell, formerly at the Centre d'Etude Spatiale des Rayonnements in France, detected HLX-1, hanging just outside a galaxy about 290 million light-years away, during a search for unusual white dwarfs and neutron stars. As the researchers report tomorrow in Nature, they found that HLX-1 is spewing 10 times as much x-radiation as stellar-mass black holes normally do, suggesting that the object is much heavier. Based on its radiation intensity, the team conservatively estimates its mass at 500 times that of the sun, says Farrell, now at the University of Leicester in the United Kingdom.
The find still leaves open an important question about intermediate black holes. "If they exist, it's not at all clear where they came from," says Christopher Reynolds, an astronomer at the University of Maryland, College Park. It's possible that small black holes that formed within a densely packed stellar cluster could merge into a midsize variety. Or, intermediate-mass black holes could be leftovers from the very earliest stars in the universe, which scientists think were much larger than today's versions. The former scenario may be more likely, notes Farrell, as observations place HLX-1 far away from its parent galaxy's center and apparently outside of the main disk--which is exactly where a tightly packed globular cluster of stars would be located.
"We must now take the existence of intermediate-mass black holes very seriously," says Reynolds. And that means getting "on with the business of figuring out how nature makes them."