July 21, 2003
Astronomers reveal the first detailed maps of
galaxy distribution in the early universe
By Tim Stephens
Peering back in time more than 7 billion years, a team of astronomers
using a powerful new spectrograph at the W. M. Keck Observatory in Hawaii
has obtained the first maps showing the distribution of galaxies in
the early universe.
|DEIMOS, shown here at the Lick
Observatory Laboratories on the UC Santa Cruz campus where it was
built, is mounted on the Keck II Telescope atop Mauna Kea, Hawaii.
The $10 million, 20,000-pound instrument is the most advanced optical
spectrograph in the world, capable of gathering spectra from 150
galaxies in a single exposure. Observations of distant galaxies
using DEIMOS will shed light on the formation and evolution of galaxies,
the expansion of the universe, and the influence of "dark matter"
on the large-scale structure of the universe. Photo:
UC Observatories/Lick Observatory
The maps show the clustering of galaxies into a variety of large-scale
structures, including long filaments, empty voids, and dense groups
These maps are among the first results from the DEEP2 Redshift Survey,
an ongoing three-year project designed to study galaxies in the distant
universe over a volume comparable to recent surveys of the local universe.
Using the new DEIMOS (Deep Extragalactic Imaging Multi-Object Spectrograph)
instrument at the 10-meter Keck II Telescope, this project is measuring
the properties of distant galaxies as well as mapping out their distribution
in space. DEIMOS, which was built precisely for this survey, allows
simultaneous, detailed observations of up to 150 galaxies at a time
Currents story). By studying galaxies whose light has taken billions
of years to reach the Earth, the astronomers are effectively looking
far back in time.
"For the first time, we are getting a map of the universe as it
was 7 billion years ago, when it was roughly half the age it is now.
Comparing these observations with local surveys will yield direct clues
to some of the most profound mysteries of the universe, such as the
nature of dark matter, the nature of dark energy, and the origins of
galaxies and quasars," said David Koo, professor of astronomy and
Koo is presenting the group's first findings July 17 at a meeting of
the International Astronomical Union (IAU) in Sydney, Australia. The
DEEP2 survey is a collaboration between astronomers at UC Santa Cruz,
UC Berkeley, and several other institutions, including the California
Institute of Technology and University of Hawaii. Marc Davis, professor
of astronomy at UC Berkeley, is principal investigator for the current
phase of the DEEP2 project. Sandra Faber, University Professor of astronomy
and astrophysics at UCSC, led the team that designed and built DEIMOS
at the Lick Observatory laboratories at UCSC.
"The DEIMOS data are so extensive compared to previous results
that we can construct maps showing considerable large-scale structure,
and we can see differences in the clustering of different types of galaxies,"
The structures seen in these maps of distant galaxies are similar to
structures seen in the local universe, although DEEP2 shows them at
an earlier stage of development.
"We are watching the galaxy population grow into what it is today,"
said Alison Coil, a graduate student at UC Berkeley who was the lead
researcher on the analysis of the maps.
The maps presented at the IAU meeting are the first detailed three-dimensional
pictures of the spatial distribution of galaxies in the early universe.
They show the locations of galaxies in the distant universe in three
of the four DEEP2 fields.
Although the maps do not cover much area of the sky (observations so
far cover an area about four times the size of the full moon), they
extend very far along the line of sight, probing deep into the universe
to reveal objects that are very distant in both space and time. This
is a different strategy from current local surveys such as the Sloan
Digital Sky Survey and the 2dF Survey, which cover very wide areas of
the sky but probe much shorter distances along the line of sight, Coil
"This survey is extremely deep. Not only are we seeing objects
more than 7 billion light-years away, but the survey itself covers 2.5
billion light-years, so we can see the evolution of the galaxy population
within the DEEP2 survey, in addition to comparing it with local surveys,"
Statistical tests indicate that the clustering seen in the DEEP2 maps
is not as strong as that seen in the local universe today, Coil said.
Galaxies cluster under the force of gravity, and this clustering grows
with time as structures get larger and exert stronger gravitational
pulls, causing them to grow even more. Regions of space with high densities
will exert their gravitational influence on surrounding galaxies, pulling
them in toward the dense regions, which become even more dense in the
process. While these dense regions are growing, empty voids are left
behind where very few, if any, galaxies reside.
At the same time, the galaxies themselves are evolving: creating stars,
forming black holes at their centers, falling in toward each other,
and often colliding and merging to form larger galaxies. All of these
processes can be seen in the DEEP2 data, Coil said.
DEEP2 observations will eventually map the locations of some 60,000
galaxies at distances of about 6 to 8 billion light-years. Seeing how
the large-scale structures made up of galaxies evolved over time is
just one aspect of the study.
The researchers are also investigating the properties of the individual
galaxies: their ages, how bright they are, how much matter they contain,
how quickly they are forming stars, and so on.
"Our ability to, in effect, weigh these distant galaxies is a
unique aspect of our survey," Koo said. "Galaxy mass is a
fundamental parameter for understanding the formation and evolution
of galaxies, and by having this dimension we can study these questions
with more precision than has been possible before."
Galaxies are represented on the maps with blue or red symbols. The
blue points are galaxies that appear to be forming new stars in abundance,
while the red points are galaxies that have little ongoing star formation,
which generally are also older and redder. These older galaxies are
much more strongly clustered in space than the star-forming galaxies.
The researchers also found that galaxies with red colors are more clustered
than bluer galaxies, and brighter galaxies cluster more strongly than
"These trends are similar to those seen in the local universe;
but for the first time we can see that they were already in place 7
billion years after the Big Bang, before our own Sun and solar system
had formed," Davis said.
This project was supported in part by grants from the National Science
Foundation (NSF). The DEIMOS spectrograph was funded by a grant from
CARA (Keck Observatory), by an NSF Facilities and Infrastructure grant,
by the Center for Particle Astrophysics, and by gifts from Sun Microsystems
and the Quantum Corporation. The DEEP2 Redshift Survey has been made
possible through the dedicated efforts of the DEIMOS staff at UCSC who
built the instrument and the Keck Observatory staff who have supported
it on the telescope.
Return to Front Page