Setting Limits to Black Hole Gluttony

Black holes may have a limit to how much they can eat in the public eye.

Black Hole

Artist's rendering of a supermassive black hole. The black hole itself is dark, but these beasts can be seen from across the observable universe by the light emitted from the accretion disks that feed them.
NASA / JPL-Caltech

Even the most gluttonous black hole reaches a point when it pushes itself away from the public buffet line, preferring instead to sneak its treats on the sly..

The gluttony limit of a black hole is around 50 billion times the mass of the Sun, according to calculations by Andrew King (University of Leicester, UK, and University of Amsterdam, The Netherlands). By some deceptively simple reasoning published in the February 11th Monthly Notices Letters of the Royal Astronomical Society, King shows that once a black hole reaches this mass, the disk of gas that acted as the black hole’s dinner buffet begins to crumble apart, collapsing under its own weight into stars.

The gaseous disk that feeds growing black holes is what enables us to see these dark objects, even from a distant universe less than 1 billion years old. Take away the gas and you take away the visible and ultraviolet light that signals a black hole’s gorging.

“If the black hole is very massive, then the gas disc would have to be correspondingly large and massive,” explains Zoltan Haiman (Columbia University). “The main idea in King’s paper is that above a certain mass, the gas in such a disk would be gravitationally unstable — i.e., it would collapse into clumps under its own weight, before the gas can funnel inward into the black hole.”

In other words, even the immense gravitational pull of a 50 billion solar-mass black hole can’t overcome the self-gravity that clumps up the surrounding matter.

“I find this idea very compelling,” Haiman says.

But that’s not to say the black hole stops growing altogether. It just has to gobble down mass in secret, without emitting any light. A star might happen to fall straight into it, swallowed whole, or it could merge with another black hole.

Astronomers have found black holes with masses of around 10 billion Suns, near King’s theoretical limit, but they’ve found them by looking for the accretion disk’s beacon of light. “The mass limit means that this procedure should not turn up any masses much bigger than those we know, because there would not be a luminous disk,” King said in a press release.

Yet it’s possible that even bigger behemoths might sit silently in nearby galactic centers. To find them, astronomers will have to turn to more indirect means of detection, such as gravitational lensing.

3 thoughts on “Setting Limits to Black Hole Gluttony

    1. Peter WilsonPeter Wilson

      The phenomenon is related to the Roche limit. Tidal forces near large bodies tend to disrupt smaller bodies. Tidal forces broke apart comet Shoemaker-Levy 9, because it flew within Jupiter’s Roche limit. Saturn’s Roche limit lies just beyond its rings, which is why its rings do not gravitate together to form a single moon.
      The Roche limit depends on the ratio of the density of the two bodies. What happens is a black-hole’s event horizon grows linearly with mass, so its density decreases as it grows. The density of a black hole 50 billion times the mass of the sun is only 0.2 kg/m^3, or six times thinner than air. When the density of the gas in the accretion disk exceeds the density of the BH, then the Roche limit retreats to inside the event horizon, and gas in the accretion disk can collapse to form stars. Once the gas collapses to form stars, friction is eliminated, and the orbits are stable, like planets circling the sun. Hence, the feeding stops.

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