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Largest
3D Map of Galaxies
Tuesday, 03 October
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Source / Credit: University of Nottingham
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Source / Credit: Caltech
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Source / Credit: RAS
The
findings are presented in this paper entitled “Reconstructed
Density and Velocity Fields from the 2MASS Redshift Survey”,
which has been accepted for publication by the journal
Monthly Notices of the Royal Astronomical Society.
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2006 A team of American,
Australian and British astronomers has released maps from the
largest full-sky, three-dimensional survey of galaxies ever
conducted.
Their detailed maps show the ‘local’
cosmos out to a distance of 600 million light years, identifying
all the major superclusters of galaxies and voids. They also
provide important clues regarding the distribution of the
mysterious ‘dark matter’ and ‘dark energy’
which are thought to account for up to 96% of the apparent mass
of the Universe.
Within this vast volume, the most massive
galaxy supercluster is 400 million light years away. It was named
after its identifier, the American astronomer Harlow Shapley. The
Shapley supercluster is so big that it takes light at least 20
million years to travel from its one end to the other. However,
Shapley is not the only massive supercluster in our vicinity.
The Great Attractor supercluster, which is three times
closer than Shapley, plays a bigger role in the motion of our
Galaxy. According to the team, our Milky Way galaxy, its sister
galaxy Andromeda and other neighbouring galaxies are moving
towards the Great Attractor at an amazing speed of about a
million miles per hour. The researchers also established that the
Great Attractor is indeed an isolated supercluster and is not
part of Shapley.
The new maps are based on the
observation that, as the Universe expands, the colours of
galaxies change as their emitted light waves are stretched or
“redshifted”. By measuring the extent of this
redshift, astronomers are able to calculate approximate distances
to galaxies.
The new survey, known as the 2MASS Redshift
Survey (2MRS), has combined two dimensional positions and colours
from the Two Micron All Sky Survey (2MASS), with redshifts of
25,000 galaxies over most of the sky. These redshifts were either
measured specifically for the 2MRS or they were obtained from an
even deeper survey of the southern sky, the 6dF Galaxy Redshift
Survey (6dFGS).
The great advantage of 2MASS is that it
detects light in the near-infrared, at wavelengths slightly
longer than the visible light. The near-infrared waves are one of
the few types of radiation that can penetrate gases and dust and
that can be detected on the Earth’s surface. Although the
2MRS does not probe as deeply into space as other recent
narrow-angle surveys, it covers the entire sky.
Galaxy
redshift surveys are only able to detect luminous matter. This
luminous matter accounts for no more than a small fraction of the
total matter in the Universe. The remainder is composed of a
mysterious substance called ‘dark matter’ and an even
more elusive component named ‘dark energy’.
“We
need to map the distribution of dark matter rather than luminous
matter in order to understand large-scale motions in our
Universe,” explained Dr. Pirin Erdogdu (Nottingham
University), lead author of the paper. “Fortunately, on
large scales, dark matter is distributed almost the same way as
luminous matter, so we can use one to help unravel the other.”
Her collaborator, Dr. Thomas Jarrett from Caltech, added,
“The other advantage of observing in the near-infrared
wavelength is the fact that it traces directly the luminous
matter, and thus the dark matter, as well.”
“Our
nearly two decade effort has produced the absolute best ever map
of the nearby Universe,” said Prof. John Huchra of Harvard
University. “With this we hope to elucidate the nature and
disposition of dark matter and understand much, much more about
our cosmological model and about galaxies themselves.”
In
order to map the dark matter probed by the survey, the team used
a novel technique borrowed from image processing. The method was
partly developed by Prof. Ofer Lahav, a co-author of the paper
and head of the astrophysics group at University College London.
The technique utilizes the relationship between galaxy velocities
and the total distribution of mass.
“It is like
reconstructing the true street map of London just from a
satellite image of London taken at night. The street lights, like
the luminous galaxies, act as beacons of the underlying roads,”
said Prof. Lahav.
"This extraordinarily detailed map
of the Milky Way’s cosmic neighbourhood provides a
benchmark against which theories for the formation of structure
in the Universe can be tested,” commented Prof. Matthew
Colless, director of the Anglo-Australian Observatory and leader
of the 6dF Galaxy Survey.
“In the near future, the
predicted motions derived from this map will be confronted with
direct measurements of galaxies’ velocities obtained by the
6dF Galaxy Survey, providing a new and stringent test of
cosmological models.”
This work is based the Two
Micron All Sky Survey, which is a joint project of the University
of Massachusetts and the Infrared Processing and Analysis Center
/ California Institute of Technology, funded by the National
Aeronautics and Space Administration and the U.S. National
Science Foundation. This research has also made use of the NASA /
IPAC Extragalactic Database (NED) which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration,
and the SIMBAD database, operated at CDS, Strasbourg, France.
Source
/ Credit: RAS / Contributed by Peter Bond
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