Black holes have limits

No matter how hungry, they eventually stop growing

Hungry black holes chew stars apart with their immense gravitational fields, like pure appetite at the gates of hell. But a growing cadre of astrophysicists believe that it’s time to rethink a black hole’s ability to consume matter without limit.

A mass of a few tens of billion times that of the sun — admittedly enormous — may be the practical limit of ultramassive black holes in the universe.

“It’s not just that we’ve only had so much time since the universe formed for black holes to accrete matter and grow as large as the biggest ones are now,” says Priyamvada Natarajan, an associate professor of astronomy at Yale University currently on fellowship at Harvard’s Radcliffe Institute for Advanced Study. “It is a tighter limit than that, and it controls how much matter can funnel down to it.” Maybe, her work and that of others in the field suggests, the black hole itself says, “No more.”

Natarajan and Chilean astronomer Ezequiel Treister of the European Southern Observatory report, in a paper to appear in the Monthly Notices of the Royal Astronomical Society, on multiple strands of evidence from many researchers that black holes near the size limit unplug their own feeding tubes. They do so, via means not fully understood, by heating their surroundings to a scorching temperature. This stops stars from forming anywhere nearby and blows most gas and dust far out of reach.

Ultramassive black holes presumably form from mergers of galaxies that feed plenty of fresh gas and dust to the big new baby. The biggest galaxies, those with ultramassive black holes, may have consumed 100 smaller galaxies. But in principle, there should be more of these most massive of the black holes, and some should be larger, than direct evidence — including the movements of stars near the cores — suggests.

Signs that such monsters have not been feeding much for the last few billion years has been coming together for the last 10 years or so as astrophysicists examined large, mature clusters of galaxies. “The big surprise was that the gas in the clusters reaches temperatures of tens of millions of degrees,” says Avi Loeb of the Harvard-SmithsonianCenter for Astrophysics and HarvardUniversity.

Somehow, apparently, the inevitable trickle of matter that does reach the black hole is whipped into such a frenzy that some escapes, emitting radio jets or wispy, supersonic wind outflows. Thus the black hole’s near-starvation diet creates such a fuss as it goes down that it blows most of the rest of the food off the table.

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