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masses of galaxies predicted by this simulation at about one-third the universe’s
“Our result is strong evidence that,
for reasons we as yet do not understand,
the process of galaxy assembly at early
times was much more rapid and efficient
than the [dark matter model] in the simulations would have us believe,” Collins
says. Although the dark matter scenario
for galaxy formation accurately predicts
many features over a wide range of cosmic
history, “it seems that in these extreme
cluster environments, something else is
In the dense regions examined by
Collins’ team, the simulations do predict
extremely rapid growth. But even in these
regions, the masses of the galaxies are
much heavier than the model allows.
Over the past few years, other astronomers have peered even further back in
time and also found some monster galaxies among the newborns (SN: 10/8/05,
p. 235). These heavyweights, although
less massive than the ones found by Collins and his colleagues, would have had
much less time to bulk up and could have
put even tighter limits on models of galaxy formation. However, theorists argue
that dark matter models allow a few statistical oddballs. Modelers explain away
this handful of early massive galaxies as
extremely rare objects that happened to
Billions of years ago
02 4 6
drum (SN: 3/22/08, p. 186).
His team’s high-resolution
Log (stellar masses)
simulations show that some
gas funnels toward the center of the galaxy before the
gas heats up and can therefore make stars. That would
mean that galaxies could
have bulked up more efficiently in the past.
this chart predicts how the mass of the brightest galaxy in a
cluster grows with time, looking back some 9 billion years. the
red points reveal five such recently discovered galaxies that
are heavier than theory predicts (black points and gray lines).
“Dekel’s stuff points to
the underlying difficulties of forming large galaxies quickly and suggests a
nice possible way out, but
be in the densest dark matter regions,
In contrast, massive galaxies don’t seem
rare in clusters. And the rapid growth rate
of galaxies in clusters is already included
in the Millennium Simulation.
even here it may not be the last word,”
Collins says. “I think our data will stimulate more theoretical work.”
One reason that the dark matter model
may fail to produce massive galaxies rapidly is that in high-density regions gas
that is gravitationally snared by a young
galaxy would be compressed quickly
and heated. Hot gas, which cannot form
stars, would likely loiter in the halo of the
young galaxy rather than sinking toward
the center to add to the system’s mass.
In their models, theorists could also
attempt to ramp up the rate at which gas
turns into stars in the brightest members
of galaxy clusters, Evrard suggests. However, he cautions that it could be difficult
to fatten up only the brightest members
while leaving neighboring galaxies svelte.
“The unintended consequence could be
gigantic galaxies in today’s universe that
aren’t seen, and they certainly would be
easy to see,” he says.
A report in the Jan. 22 Nature by
Avishai Dekel of the Hebrew University
of Jerusalem and his colleagues offers a
possible solution to this cosmic conun-
Speaking of the distance to the observed
galaxies, Evrard says, “The observers have
laid out a 10 billion light-year tightrope
and challenged the theorists to balance
on it. It may not be easy.” s
Back Story | IN LEADING THEORY, COALESCENCE BUILDS GALAXIES
1. Fluctuations in the early universe cause
dark matter to collapse, forming halos. this
computer simulation shows dark matter wells
about 150 million years after the Big Bang.
soon after the wells form, visible matter, such
as gas, will begin to fall into them.
2. as densities within those wells become
high enough, stars begin to form. White
crosses (within red areas) denote stars forming in halos around 220 million years after
the Big Bang. as matter continues to fall in,
the stars and halos coalesce.
3. Galaxies form as dark matter pulls in more
stars and gas. in this simulation, the galaxy
takes on a more detailed structure around
440 million years after the Big Bang. Galaxies
grow and merge until jets and winds resist the
buildup of matter, slowing accumulation.
From top: C. Collins ET AL./NATURE; GreiF, Johnson, Klessen,
Bromm, texas advanCed ComputinG Center (all three)