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A Hubble image of the black hole that's eating faster than we thought possible. Two lobes of hot gas, created by a pair of jets, are visible on either side of the central source (Image: W. P. Blair (JHU)/R. Soria (ICRAR-Curtin))

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A black hole in a nearby galaxy is blowing a mighty wind. The black hole is about 100 times the mass of the sun but is causing the emission of millions of times more energy, breaking a long-accepted rule about the way black holes feed. The discovery suggests that even small black holes may play a larger role in galaxy evolution than previously realised.

When black holes consume matter from their surroundings, the incoming gas and dust reach scorching temperatures just before falling in. The hot gas emits powerful "winds" of radiation, and theory has it that the energy in these winds cannot exceed a certain limit tied to the black hole's mass, called the Eddington limit. Winds more powerful than this limit would blow the incoming gas away and halt the black hole's growth – or so we thought.

Recently astronomers have been finding black holes blowing especially powerful jets, and they wondered if they could be breaking the Eddington limit. Roberto Soria of Curtin University in Western Australia and his colleagues have measured the mass of one of these apparent outlaws and found that it does in fact blow stronger winds than its mass should allow. That suggests the Eddington limit is more of a guideline than a rule, says Soria.

Faster feeding

The team found the black hole in a spiral galaxy called the Southern Pinwheel, one of the most-studied galaxies in our cosmic neighbourhood. They observed the bright black hole for over a year, measuring its output in visible light, X-rays and radio waves. The object weighs in at about 100 times the mass of the sun, making it relatively small, but the amount of energy it emits is a few times higher than its Eddington limit.

The black hole's brightness also means that it must have been eating at this extreme rate for at least 10,000 years, says team member Knox Long of the Space Telescope Science Institute in Baltimore, Maryland. "There was some question about whether one could exceed the Eddington luminosity on a very short timescale, but that wouldn't be as interesting," he says. "This shows that it's not a transient thing. It has to be pretty persistent."

Some black holes in the early universe grew hundreds of thousands of times more massive than the sun in a shorter time than theoretically possible, so astronomers had previously suggested they were breaking the Eddington limit. The researchers are not yet sure how the black hole in the Southern Pinwheel is breaking the limit, but figuring it out may be key to understanding how those early monstrous black holes grew so big so fast.

And if other bright black holes we see today are also breaking the limit, the objects might have had a bigger impact on the evolution of their host galaxies than previously thought. "We may have underestimated the effect that stellar-mass black holes have on the heating and ionisation of interstellar gas, especially in the early universe," says Soria.

Quelle: NewScientist