The University of Michigan News Service
Feb. 24, 2005
ANN ARBOR, Mich.—An international team of astronomers using the world’s
largest X-ray and optical telescopes have spotted the most distant
massive object ever detected, a cluster of galaxies 9 billion light
years distant from Earth.
The cluster of galaxies is so far away that the
light detected by the team is much older than the Earth itself. The
galaxy cluster, if it is even still there, would be at least 11 billion
years old now.
“By capturing this ancient, 9-billion-year-old
light, we have a snapshot of the universe at a youthful age of less
than 5 billion years, which is about 1/3 of the present age,” said
project leader Christopher Mullis, a research fellow in the University
of Michigan’s Department of Astronomy.
As exciting as it is to break a record, it’s also
an important cosmological finding. “Just a few years ago, astronomers
did not believe structures like this even existed at such an early
time,” Mullis said. This galaxy cluster, which is being seen as it
appeared about 2 billion years after its formation, is well-organized
and “mature,” he said. Although it is very far back in time, it looks
as if this structure had formed in a way that is consistent with more
recent structures.
“Even at this early stage in cosmic history, this
appears already as a mature, fully assembled structure which implies
that this is an old cluster in a young universe,” said European
Southern Observatory astronomer Piero Rosati, who collaborated on the
study.
The record-breaking galaxy cluster was also a
somewhat surprising find for the team, who were testing a new approach
to hunting distant objects. “Basically we stepped up to the plate for
our first time at bat with this new system, and we hit a home run,”
Mullis said.
Mullis and his colleagues started their search by
combing through archives of old images from the European Space Agency’s
orbiting X-ray observatory, XMM-Newton, looking for diffuse X-ray
sources that had not been previously studied. Cluster galaxies shine
brightly in optical light, but they also emit strong X-ray signals
resulting from very hot gas that envelopes the cluster.
The record-breaking cluster initially turned up, small but distinct, off center in an image made by another team.
The X-ray image of the distant cluster is comprised
of just 280 photons—individual parcels of light—collected over a
12.5-hour exposure. By comparison, on a sunny day the human eye is
flooded by about 10 quadrillion photons per second.
With this distant cluster candidate and dozens of
others culled from the X-ray archive, Mullis and his team then turned
to one of the world’s largest optical telescopes, the European Southern
Observatory’s Very Large Telescope, located in the Atacama Desert,
Chile. They took a series of relatively quick exposures of the
candidates with red and blue filters on the telescope.
What Mullis and his Italian and German
collaborators were looking for at each of the candidate spots were very
red galaxies, indicating light that has traveled for an extremely long
time to reach Earth. “The redder the better,” Mullis said. Almost
immediately, they turned up this cluster of red objects that seemed to
be beyond the previous distance record.
“I spent a full day rechecking my data before I
called any of the other scientists,” Mullis said. “It appeared to be
almost unbelievably distant.”
Subsequent, more detailed measurements on 12 major
galaxies in the cluster were used to confirm that they were equidistant
from Earth at about 9 billion light years. The entire cluster is
probably hundreds or even thousands of galaxies held together by
gravity, Mullis said
Collaborator Hans Bohringer of the Max Planck
Institute for Extraterrestrial Physics in Garching, Germany said the
discovery “encourages us to search for additional distant clusters
using the same efficient techniques used to locate the present cluster.”
Mullis and his team are going to broaden the search
to find more super-distant galaxy clusters with this new approach. They
also plan to go back and take longer optical and X-ray telescope
exposures of the record-setting cluster to get a better sense of its
features.
“Finding it is one thing,” Mullis said. “We also
need to go back in there and maximize that return.” With enough data on
this and other super-distant massive objects, Mullis expects to find
new answers to some fundamental questions of how the universe formed.
Mullis will be presenting this finding at an
international astronomy conference in Hawaii focused on connecting
galaxy clusters to the underlying physics of space time and gravity.
The meeting is being organized by U-M physics professor Gus Evrard, and
sponsored in part by the Michigan Center for Theoretical Physics.
“It’s special to live in the era of human history
when the terrain of the whole visible universe is being revealed,”
Evrard said.
A paper by Mullis and his team will also appear in an upcoming issue of The Astrophysical Journal.
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