New Class of Black Hole Found?

Victoria Jaggard
National Geographic News
July 1, 2009

An extraordinarily bright object in a galaxy 290 million light-years away could be a new type of black hole—one that Goldilocks would approve of.

Scientists have been trying to confirm whether intermediate-mass black holes, those that are not too light and not too heavy, really exist. The newfound object, dubbed HLX-1, is "the best candidate yet" for this proposed black hole class, according to a new study in tomorrow's issue of the journal Nature.

Right now astronomers are confident that two types of black holes are out there.

Smaller, so-called stellar-mass black holes are created by dying stars and only reach 20 to 30 times the mass of our sun. Supermassive black holes thought to sit at the centers of most galaxies, meanwhile, clock in at millions to billions of solar masses.

"There's a big gap in between where we just don't know if nature makes black holes that weigh that much," said Philip Kaaret, an astrophysicist at the University of Iowa, who was not part of the study team.

New Black Hole Is "Just Right?"

All black holes grow by feeding off nearby matter. They're visible because the matter falling in gets hotter and brighter as it's compacted.

But a black hole will cut off its own food supply when it reaches a point where the "push" of radiation from in-falling matter equals the "pull" of a black hole's gravity.

This means there's a cap on how bright a black hole can be.

Theoretically, a middleweight black hole could form inside dense groupings of older stars known as globular clusters, noted study co-author Natalie Webb, an astronomer with the Centre d'Etude Spatiale des Rayonnements in France.

In these clusters, a stellar-mass black hole might swell to intermediate size as it collides and merges with other objects while pulling in surrounding gases and dust.

(Related: "Space-Time 'Ripples' Created by Merging Black Holes?")

The University of Iowa's Kaaret also thinks that middleweight black holes might have formed in the gas-rich depths of the very early universe, when stars could get much more massive than the modern variety.

Previous observations had found objects that are much brighter than known stellar-mass black holes. That brightness—combined with other properties—suggests to some astronomers that the objects might be the missing "in between" black holes.

Tough to Prove

The recently studied HLX-1 is one such candidate, glowing so brightly in x-rays that astronomers think it could be a black hole more than 500 times the mass of the sun.

"Where our object stands out is that it is about ten times brighter than the previous record holder" among middleweight candidates, study leader Sean Farrell, also of the French astronomy center, said via email.

But confirming any of the known candidates presents significant challenges. To make a "gold plated" case, Kaaret said, you'd have to see the companion star, which would be incredibly hard given the black hole's glare and distance from Earth.

You could also look iron emissions that appear distorted by the odd gravitational effects of matter falling into a black hole, said co-author Olivier Godet, of the University of Leicester in the U.K. But current instruments aren't sensitive enough to detect such emissions.

Next-generation telescopes might be able to help find the distorted iron emissions or detect ultraviolet emissions from the candidate objects that would aid modeling efforts, study leader Farrell said.

"This is the next step for us, and so far it's looking promising."

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