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How big can a black hole grow?
http://space.newscientist.com/article.ns?id=mg19926724.800&print=true
8.9.3
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. 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 30 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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Weblinks

Priya Natarajan, Yale University
http://www.astro.yale.edu/priya/