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Astronomers
Construct Largest-ever 3D Map of a Million Galaxies
15 May 2006
An international team of
astronomers has released new results on the Cosmos, based on
the largest map of the heavens ever produced.
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The
central galaxies is these images are closer than any of
the galaxies used in the sample (which is why they are so
detailed). They are, however, the same type of galaxy
that was used in the survey and would have been in the
sample if the team had studied the very local
Universe.
Image credit: David W. Hogg, Michael R. Blanton,
and the SDSS Collaboration
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This massive atlas
emphatically confirmed recent findings that the Universe is full
of 'dark energy', a mysterious substance that makes up
three-quarters of our Universe, together with 'dark matter' which
accounts for most of the remaining quarter. Understanding this
composition is now one of the most important problems facing the
whole of science.
"We now have a precise
view of what makes up our Universe, but little idea as to why,”
said Prof. Ofer Lahav, a member of the international team and the
Head of the Astrophysics Group at University College London. “It
is intriguing that the ordinary matter our bodies are made of and
that we experience in everyday life only accounts for a few
percent of the total cosmic budget."
Our Universe
contains billions of galaxies of all shapes and sizes. In recent
years astronomers have used increasingly large surveys to map out
the positions of these galaxies, stepping their way out into the
Cosmos.
The new cosmic map unveiled
today is the largest to date -- a three-dimensional atlas of over
a million galaxies spread over a distance of more than 5 billion
light years. The findings confirm that we live in a Universe
filled with mysterious dark matter and dark energy.
"We have analyzed the
patterns in this map and discovered waves of structure over a
billion light years across," said Dr. Chris Blake of the
University of British Columbia, principal author of the study.
"These waves were generated billions of years ago and have
been vastly stretched in size by the expanding Universe."
Construction of the cosmic
atlas was led by co-author Dr. Adrian Collister of the University
of Cambridge, as part of his PhD work, using a novel Artificial
Intelligence technique he developed with his supervisor Prof.
Ofer Lahav.
"By using very accurate distances of
just 10,000 galaxies to train the computer algorithm we have been
able to estimate reasonably good distances for over a million
galaxies," said Collister. "This novel technique is the
way of the future."
The original 2-dimensional
positions and colours of the one million galaxies were from the
Sloan Digital Sky Survey. The precise observations of the
10,000 galaxy distances were made as part of an international
collaboration between U.S., U.K. and Australian teams using data
from the Sloan Digital Sky Survey and the Anglo-Australian
Telescope.
By measuring the positions of galaxies,
astronomers can unravel the balance of forces that govern our
Universe: the force of gravity which pulls everything together,
and the competing effect of the expanding Universe which smoothes
things out. These cosmic forces have arranged the galaxy
distribution into a complex network of clusters, filaments and
voids.
"The galaxy map can tell us the amount of
ordinary 'baryonic' matter relative to the amount of mysterious
'dark matter',” said co-author Dr. Sarah Bridle of
University College London. "We have confirmed that over 80%
of the material in the Universe consists of an invisible dark
matter whose nature is not yet understood."
The
cosmic atlas of a million galaxies will shortly be made freely
available on the World Wide Web for the benefit of other
researchers. This free exchange of data is an important feature
of modern astronomy, since many discoveries are only possible
when different observations are combined.
The key problem in mapping the
cosmos is determining the distance to each galaxy. Researchers
can measure these distances because as the Universe expands, the
colour of each galaxy changes as their emitted light waves are
stretched or ‘redshifted’.
Traditionally, astronomers have
needed to take a "spectrum" of each galaxy to determine
this distance, splitting its light into many components to reveal
sharp features with which to measure the amount of redshifting.
This requires a time-consuming, individual observation of each
galaxy.
The new cosmic map has been
constructed using a novel technique focusing on a special class
of galaxy whose intrinsic colour is very well known. For these
‘Luminous Red Galaxies’ researchers can measure the
amount of colour distortion, and hence the approximate distance
of the galaxy, just by looking at digital images of the sky,
without the need to obtain a full spectrum.
All that is
needed to exploit the technique is accurate observations of a
small sample of the galaxies. In this case, precise measurements
of just 10,000 galaxies were used to produce the atlas of over a
million galaxies. These techniques will be very important for
future large astronomical projects such as the international Dark
Energy Survey, scheduled to start in 2009, in which University
College London and the universities of Portsmouth, Cambridge and
Edinburgh are key partners.
Source
/ Credit: RAS
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