Fattest Black Holes Feasted On Two Buffets


One of the biggest conundrums facing astrophysicists is how the most massive black holes became so, well, massive. Now it seems we have a potential answer — these greedy behemoths may have gobbled down two courses of matter rather than just the one.

Supermassive black holes are thought to lurk in the centers of most galaxies. The supermassive black hole living in the core of the Milky Way — known to live in the radio source Sagittarius A*, or Sgr A* — is around four million times the mass of our sun. But that’s not the biggest, not by a long shot.


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The uber-supermassive black holes recently discovered in the galaxies NGC 3842 and NGC 4889 weigh in at 9 billion times the mass of our sun — or 2,000 times the mass of Sgr A*.

This cosmic game of “my supermassive black hole is bigger than yours” may be fun, but there’s an underlying problem. Current black hole formation theories dictate that these monsters shouldn’t exist; there is simply not enough time in the Universe for supermassive black holes to grow so big. Yes, even 13.75 billion years (the time from the Big Bang to now) is not enough time for these black holes to fatten to such proportions.

Now, astrophysicists in England and Australia think they have an answer.

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Somehow, these supermassive black holes needed to gain weight quickly, and most of their growth must have occurred in the early Universe. “We know they grew very quickly after the Big Bang,” said Andrew King of the Department of Physics and Astronomy at the University of Leicester. “These hugely massive black holes were already full-grown when the universe was very young, less than a tenth of its present age.”

Current formation theories account for black holes that feast on material from an “accretion disk” that forms as a black hole sucks in matter. This super-heated disk is composed of gas, dust, blended stars and smashed-up planets that strayed too close to the black hole’s gravitational field. Forming a thin disk, the matter will supply mass to the black hole, fattening it up.

But there’s a problem — a single accretion disk cannot provide mass fast enough to explain the supermassive black holes we see in our Universe today.

“We needed a faster mechanism,” said Chris Nixon, also from the University of Leicester, “so we wondered what would happen if gas came in from different directions.”

With the help of astrophysicist Daniel Price, of Monash University in Australia, the trio simulated the formation of two accretion disks around a black hole to see what would happen.

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The computer model creates an inner accretion disk (closest to the black hole’s event horizon) and an outer accretion disk tilted at an angle. When the simulation starts, the two disks start to spread out and collide. The collision causes the disk material to rapidly slow down — it then succumbs to the black hole’s immense gravity, falling into the voracious singularity. (One of the 3D simulations are shown below.)

By modeling the interaction of two accretion disks, black hole growth is dramatically increased over a very short period of time. The researchers have found that black hole growth can be increased 1,000-fold.

“We don’t know exactly how gas flows inside galaxies in the early universe,” King concluded, “but I think it is very promising that if the flows are chaotic it is very easy for the black hole to feed.”

So, as this research alludes to, the gas swirling inside galactic cores may have been a lot more chaotic than we observe today. And with a large supply of mass in the “black hole buffet,” young black holes weren’t restricted to feasting on one course; the black holes could devour matter from two accretion disks, causing them to grow more rapidly than current theories of black hole formation allow.

This research is due for publication in the Monthly Notices of the Royal Astronomical Society.

To see some of the awesome black hole simulations, take a look at the videos on Chris Nixon’s research pages.

theta = 150 (3D) from Chris Nixon on Vimeo.

Source: University of Leicester

Image: A snapshot of the two accretion disks swirling around the black hole. Credit: University of Leicester

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