How massive can a supermassive black hole get?

There’s a limit to how big a supermassive black hole can become before it stunts it own growth, calculations suggest.

Black hole
In this artist’s illustration, a supermassive black hole billions of times the mass of our sun accretes matter in the heart of a galaxy. A new study calculates if it has a maximum mass.
NASA / JPL-Caltech

Kohei Inayoshi and Zoltan Haiman from Columbia University in New York, US, modelled the evolution of a supermassive black hole over the lifetime of the universe – 13.8 billion years – and found once they get to around 10 billion times the mass of the sun, they top out.

At this point, the majority of the gas flowing to and feeding a supermassive black hole instead gets caught in the surrounding disc, triggering star formation light-years away and more than far enough to remain safe from the black hole.

Starved of fuel, the black hole slows growth. The work, published in The Astrophysical Journal, explains why we don’t detect supermassive black holes more than ten billion solar masses today.

Most large galaxies, it’s thought, harbour a supermassive black hole in their centre. We have one in the centre of our galaxy, the Milky Way – it’s thought to be around 4.5 million times more massive than the sun.

But it’s tiny to some of the monsters we’ve detected. Some are in the tens of billions of solar masses. But that’s as massive as they seem to get. Why is this?

Inayoshi and Haiman thought the process of rapid black hole growth could eventually be the very thing that slows them. They decided to model it and see what happened.

For a supermassive black hole to grow larger than the largest we’ve seen, it would need to guzzle around 1,000 solar masses each year.

That fuel has to come from somewhere and as time wears on, they must use gas from further afield. But once the gas is pulled from the outer reaches of the galaxy, it gets stuck tens of hundreds of light-years from the black hole. 

Food slows to a trickle. This, in turn, changes the physics of the black hole’s disc.

The inner portion of the disc puffs up and blasts strong jets, further suppressing feeding and thus, growth.

The pair’s upper limit size calculations support another study published in the Monthly Notices of the Royal Astronomical Society by the UK’s University of Leicester’s Andrew King in December last year, which attributed slowed growth to fragmentation of the disc.

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