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Galaxies in the early universe
Rocking the cradle
Aug 13th 2009 | RIO DE JANEIRO
From The Economist print edition
The discovery of some massive infants forces a rethink of galactic evolution
IN THE crazy world of Dr Seuss, an American children’s author, a bird called a Pelf lays eggs that are three
times as big as herself. At this week’s meeting of the International Astronomical Union, astronomers were
asked to entertain equally odd thoughts when they were presented with the latest evidence that some early
galaxies, although smaller than their more recent counterparts, contain much more mass. It is like being
handed a baby that weighs three times as much as its mother.
The objects in question are “red compact” galaxies, in particular a well-studied one called 1255-0 that formed
just 3 billion years after the Big Bang. As the universe is thought to be 13.7 billion years old, the light reaching
Earth from this galaxy shows what it was like some 10.7 billion years ago, when it was the equivalent of a
newborn. Images suggest that the galaxy is 3,000 light years across—just a fifth of the size of the Earth’s
home galaxy, the Milky Way—but about four times as massive.
To check how compact 1255-0 really was, a group of astronomers led by Pieter van Dokkum of Yale University
analysed the speed at which its stars were moving. The researchers reasoned that if it really was as small and
as massive as it appeared, those stars would be whizzing round its centre at high velocity.
They were able to calculate the speed of the stars in question using the Doppler effect—the change in
frequency, and thus in colour, that a light wave undergoes according to whether the thing that emitted it is
moving towards or away from the observer. (The changing pitch of an ambulance siren is the equivalent effect
for sound.) The Doppler shifts of the light from different parts of 1255-0 showed Dr van Dokkum and his
colleagues that the average orbital velocity of this galaxy’s stars was 510 kilometres a second, the highest
value ever recorded. This suggests that previous estimates of its size and mass are right.
Growing and shrinking
Red compact galaxies are thought to evolve by growing into blob-shaped galaxies called ellipticals, which are
common today, and there are also several of these near 1255-0. Although ellipticals are bigger than red
compacts, they are usually less massive. That makes the process of transition from one to the other
problematic. Mergers are ruled out. Though they would make the babies bigger, they would also make them
heavier, rather than lighter. The shock involved would create new, younger stars as well, and these do not
appear to be there.
One possibility discussed at the meeting is whether the distribution in space of dark matter might help explain
what happens. It may be that the red compacts are heavy not because they contain more stars but because
they contain more of this substance, whose nature remains unknown, but which can be detected by its
gravitational effects, and which seems to be about six times as abundant as the familiar matter of atoms and
molecules. Yet this explanation is not terribly satisfactory either. Existing observations suggest that dark
matter was smoothly distributed around elliptical galaxies by some 8 billion or 9 billion years ago. If extra dark
matter is what is making earlier galaxies particularly massive, then something must have happened to
redistribute the stuff between 10 billion years ago (when it clumped together in baby galaxies) and 9 billion
years ago (by which time it was no longer lumpy).
That something would have to have been strange indeed, though—possibly a shift in the value of a
fundamental constant such as the speed of light—to have caused compact galaxies to inflate while losing
weight. Dr Seuss’s hero never did find out how the Pelf learned to pull off her egg-laying trick. Perhaps
astronomers will have better luck.
Copyright © 2009 The Economist Newspaper and The Economist Group. All rights reserved.
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