STUDIES OF LIGHT
AND SHADOW
Spotlighting dark matter and galaxy formation
BY RON COWEN
New surveys of the sky are examining the two
facets of galaxy formation. One focuses on the
glitter. The other examines the gloom. A census of both the light and the dark parts of the
cosmos is essential for understanding how
today’s universe came to be, the scientists say.
One of the new surveys looks at the panorama of galaxy shapes,
masses, colors, and sizes that existed when the cosmos was about half
its current age—a critical time when galaxies began to take the form
they’re in today. The other survey also sizes up galaxies but uses some
gravitational sleight of hand to expose the dark side—the vast concentrations of dark matter that provide the framework for all the visible stars and gas in the universe. Dark matter, though unseen, makes
up more than 90 percent of the mass
of the universe, theorists say.
Together, the new surveys paint a
more detailed portrait of galaxies
and their origin than astronomers
have ever had.
any large concentration of mass—visible or not—acts as a gravitational lens, bending the light coming to Earth from galaxies that
lie directly behind that concentrated mass. The effect distorts the
galaxies’ observed shapes.
By measuring the shapes of half a million distant galaxies
recorded with the sharp eye of Hubble’s Advanced Camera for
Surveys, Caltech astronomer Richard Massey and his collaborators inferred the distribution of matter that resides directly in
front of those galaxies, as seen from Earth. The researchers
describe their work in the Jan. 18 Nature. They also presented
their findings in January at a meeting of the American Astronomical Society in Seattle.
Astronomers don’t know the original, undistorted shape of any
particular galaxy, notes Massey. But by analyzing the images of hundreds of thousands of galaxies, he and his colleagues found departures from the expected pattern of
galactic shapes and, from those, estimated the distortions imprinted by
dark matter.
DARK LANDSCAPE The Hubble Space Telescope’s Cosmic Evolution Survey (COSMOS) is the largest
galaxy study the telescope has ever
conducted. It covers an area nine
times the size of the full moon on the
sky. That’s big enough not only to map
the distribution of several hundred
thousand galaxies, but also to unveil
the arrangement of dark matter in
unprecedented detail.
“It’s a milestone achievement to
see where the dark matter lies,” says
COSMOS astronomer Richard
Ellis of the California Institute of
Technology (Caltech) in Pasadena.
Without this invisible stuff, he says, the universe as we know it
wouldn’t exist. According to the leading model for the formation
of cosmic structure, dark matter coalesced earlier in the universe’s history than ordinary, visible matter did. It’s the tug of dark
matter that pulled stars and gases into galaxies.
As the cosmos evolved, dark matter formed an irregular scaffold
across the cosmos. Galaxies continue to concentrate in the densest
regions of this network, forming the tapestry of galaxy clusters and
superclusters seen today. Dark matter also maintains the shape of
those clusters against the diluting effect of cosmic expansion, which
would otherwise wash out cosmic structures.
Dark matter betrays its presence through its gravitational effect
on visible matter. As first predicted by Albert Einstein in the 1930s,
NEW DIMENSION To add a critical third dimension to their work,
the COSMOS researchers used the
large Subaru telescope on Hawaii’s
Mauna Kea to record the colors of
the galaxies observed by Hubble.
The colors indicate approximate
distance because the expansion of
the universe shifts the light emitted
by the most remote galaxies to the
reddest wavelengths. The more distant the galaxy, the longer its light
must travel to reach Earth, and the
farther back in time observers see
the galaxy.
Treating the survey as if it were an
archaeological dig, the team divided
the observable galaxies into slices
from three different cosmic epochs. Gravity-generated distortions
in images of the galaxies from each time slice then enabled the
researchers to map the distribution of dark matter as it appeared
3.5 billion, 5 billion, and 6.5 billion years ago. The earliest map represents the cosmos at nearly half its current age. The maps demonstrate how dark matter weaves a cosmic web, with filaments growing in size and density as the universe ages, Ellis says.
“It’s very reassuring” that the dark matter structure revealed by
COSMOS matches models that theorists have touted for more than
2 decades, notes Ellis. Mapping dark matter using the gravita-tional-lens technique dates back to the 1980s, but the distribution
“has never been seen very clearly” or on such a large scale, he adds.
The Hubble study covers a smaller area of sky than searches for
MASSEY ET AL., NASA, ESA
DARK CLUMPS — The Hubble Space Telescope’s
COSMOS survey revealed these images of dark matter as
distributed (left to right) at 3.5 billion, 5 billion, and
6.5 billion years ago. As the dark matter coalesced into
clumps, the densest areas formed the framework for
clusters of galaxies.
