using the South Pole Telescope report that they have discovered
the most massive galaxy cluster yet seen at a distance of
7 billion light-years. The cluster (designated SPT-CL J0546-5345)
weighs in at around 800 trillion Suns, and holds hundreds
- Astronomers using the South Pole Telescope report that they have
discovered the most massive galaxy cluster yet seen at a distance
of 7 billion light-years. The cluster (designated SPT-CL J0546-5345)
weighs in at around 800 trillion Suns, and holds hundreds of galaxies.
galaxy cluster wins the heavyweight title. It's among the most
massive clusters ever found at this distance," said Mark Brodwin,
a Smithsonian astronomer at the Harvard-Smithsonian Center for
Astrophysics. Brodwin is first author on the paper announcing
the discovery, which appeared in the Astrophysical Journal.
Redshift measures how light from a distant object has been stretched
by the universe's expansion. Located in the southern constellation
Pictor (the Painter), the cluster has a redshift of z=1.07. This
puts it at a distance of about 7 billion light-years, meaning
we see it as it appeared 7 billion years ago, when the universe
was half as old as now and our solar system didn't exist yet.
Even at that young age, the cluster was almost as massive as the
nearby Coma cluster. Since then, it should have grown about four
times larger. If we could see it as it appears today, it would
be one of the most massive galaxy clusters in the universe.
cluster is full of 'old' galaxies, meaning that it had to come
together very early in the universe's history - within the first
two billion years," stated Brodwin.
Galaxy clusters like this can be used to study how dark matter
and dark energy influenced the growth of cosmic structures. Long
ago, the universe was smaller and more compact, so gravity had
a greater influence. It was easier for galaxy clusters to grow,
especially in areas that already were denser than their surroundings.
could say that the rich get richer, and the dense get denser,"
quipped Harvard astronomer Robert Kirshner, commenting on the
As the universe expanded at an accelerating rate due to dark energy,
it grew more diffuse. Dark energy now dominates over the pull
of gravity and chokes off the formation of new galaxy clusters.
Brodwin and his colleagues spotted their quarry in the first 200
square degrees of data collected from the new South Pole Telescope.
The SPT is currently completing its pioneering millimeter-wave
survey of a huge swath of sky covering 2,500 square degrees.
They're hunting for giant galaxy clusters using the Sunyaev-Zel'dovich
effect - a small distortion of the cosmic microwave background
(a pervasive all-sky glow left over from the Big Bang). Such distortions
are created as background radiation passes through a large galaxy
Surveying for this effect has significant advantages over other
search techniques. It works just as well for very distant clusters
as for nearby clusters, which allows astronomers to find very
rare, distant, massive clusters. Further, it provides accurate
measurements of the masses of these clusters, which are crucial
to unraveling the nature of dark energy.
The main goal of the SPT survey is to find a large sample of massive
galaxy clusters in order to measure the equation of state of the
dark energy, which characterizes cosmic inflation and the accelerated
expansion of the universe. Additional goals include understanding
the evolution of hot gas within galaxy clusters, studying the
evolution of massive galaxies in clusters, and identifying distant,
gravitationally lensed, rapidly star-forming galaxies.
Once this distant cluster was found, the team studied it with
the Infrared Array Camera on the Spitzer Space Telescope to pinpoint
galaxies within the cluster. Detailed observations of the galaxies'
speeds with the Magellan telescopes in Chile proved that the galaxy
cluster was a heavyweight.
The team expects to find many more giant galaxy clusters lurking
in the distance once the South Pole Telescope survey is completed.
many years of effort, these early successes are very exciting.
The full SPT survey, to be completed next year, will rewrite the
book on the most massive clusters in the early universe," added
The South Pole Telescope is an NSF funded project run by an international
collaboration involving scientists at over a dozen institutions.
for more information.Headquartered in Cambridge, Mass., the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between
the Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists, organized into six research divisions,
study the origin, evolution and ultimate fate of the universe.
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