How big can a black hole grow?

10:45 03 September 2008
NewScientist.com news service
David Shiga

Just how big can a black hole grow? Two astronomers reckon they have worked out the answer: colossal black holes with a mass of up to 50 billion suns could be lurking out there – but that's the limit.

Giant black holes sit at the cores of virtually all galaxies, and are thought to have grown from smaller seed black holes that swallowed lots of matter. The biggest well-measured one resides in the galaxy Messier 87 and has the mass of 3 billion suns, a measurement based on the speed of the gas swirling around it.

Even bigger black holes are waiting to be found, say Priya Natarajan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Ezequiel Treister of the European Southern Observatory in Santiago, Chile.

In a study to appear in Monthly Notices of the Royal Astronomical Society, the pair examined the "feeding habits" and growth of black holes. They used data from surveys carried out by other teams that observed the X-rays and visible light emitted by matter as it is devoured by black holes. The properties of this radiation can be used to estimate a black hole’s mass and how quickly it is gobbling up its surroundings.

The team analysed how many galactic black holes of various masses were present at each stage in the universe's history. The distribution of masses they found today and in the past can only be explained if there is a limit on how fast black holes can grow, the researchers say.

Ultramassive black hole

Previous studies have also suggested this, and it may be due to the way radiation from infalling matter blasts the black hole's neighbourhood free of additional sustenance. "They self-regulate," says Natarajan. "They never grow beyond a certain mass in any epoch."

Knowing this growth rate allowed them to work out the modern-day size of the biggest known black holes that existed in the early universe. Back then, they are estimated to have had the mass of about a billion suns. According to Natarajan and Treister, a few black holes of this size may have bloated to "ultramassive" size by now, with between 5 and 50 billion times the sun's mass, at the most. Even a black hole at the lower end of this range would be gargantuan – more than 3 times as wide as our solar system.

One ultramassive black hole may already have been spotted 3.5 billion light years away in the galaxy OJ 287, which is thought to harbour a pair of giant black holes circling each other at its centre. The larger of the two has been estimated to be 18 billion solar masses, based on the properties of radiation outbursts from the system, but astronomers disagree on how accurate this is.

Scott Tremaine at Princeton University says that examining the growth history of black holes is important because it appears to be closely tied to the growth of galaxies, including our own. But he cautions that estimating black hole masses from the amount of radiation they give off – as Natarajan and Treister have done – is fraught with uncertainty because a black hole's brightness can vary depending on how much material it eats.

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