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The sky map of the Faraday effect caused by the magnetic fields of the Milky Way. Red and blue colors indicate regions of the sky where the magnetic field points toward and away from the observer, respectively. The band of the Milky Way (the plane of the Galactic disk) extends horizontally in this panoramic view. The center of the Milky Way lies in the middle of the image. The North celestial pole is at the top left and the South Pole is at the bottom right. Image: Max Planck Institute for Astrophysics
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Scientists at the Naval Research Laboratory are part of an
international team that has pooled their radio observations into a database,
producing the highest precision map to date of the magnetic field within our
own Milky Way galaxy.
The team, led by the Max Planck Institute for Astrophysics
(MPA), used the database they created and were able to apply information theory
techniques to produce the map, explains NRL's Tracy Clarke, PhD, a member of
the research team. "The key to applying these new techniques is that this
project brings together over 30 researchers with 26 different projects and more
than 41,000 measurements across the sky. The resulting database is equivalent
to peppering the entire sky with sources separated by an angular distance of
two full moons." This incredible volume of data results in a new, unique
all-sky map that gives scientists the ability to measure the magnetic field
structure of the Milky Way in unparalleled detail.
The map shows scientists a quantity known as Faraday depth,
a concept that depends on magnetic fields along a specific line of sight. The
research team created the map by combining the more than 41,000 individual
measurements using a unique image reconstruction technique. The researchers at
MPA are specialists in the new discipline of information field theory. Clarke,
working in NRL's Remote Sensing Division, is part of the team of international
radio astronomers who provided the radio observations for the database. The
new, high-precision map not only shows the Galactic magnetic field's structure
on large scales, it also reveals small-scale features that help scientists
better understand turbulence in the Galactic gas.
The Milky Way, along with all other galaxies, possesses
magnetic fields. Until now, scientists have been puzzled over the origin of
these galactic magnetic fields. The assumption was that the magnetic fields
were created by processes where mechanical energy is converted into magnetic
energy. These same kinds of processes occur in the interior of the Earth and
the sun. The map that the team has created will give scientists valuable
knowledge about the structure of Galactic magnetic fields throughout the Milky
Way.
For 150 years, scientists have measured cosmic magnetic
field by observing the Faraday effect. They know that when polarized light
passes though a magnetized medium, the plane of polarization turns. This
concept is called Faraday rotation. The strength and direction of the magnetic
field governs the amount of rotation that occurs. So scientists observe the
rotation to investigate the magnetic fields' properties.
Radio astronomers study the polarized light from distant
radio source, passing through the Milky Way on the way to Earth, in order to
measure our Galaxy's magnetic field. By measuring the polarization of the light
sources at different frequencies, researchers can determine the amount of
Faraday rotation.
With these individual measurements, researchers gain data
about only a single path through the Galaxy. To gain a fuller picture of the
Milky Way's magnetic fields from the Faraday rotation measurements, researchers
have to observe many sources across the sky. To achieve this map, radio
astronomers from around the world have pooled data from 26 different projects,
collecting a total of 41,330 individual measurements. The map contains
approximately one radio source per square degree of sky.
Despite this large catalog of date, there are still some
large areas, especially in the southern sky, where only a few measurements have
been recorded. So to gain a realistic map of the entire sky, researchers have
to interpolate between the existing data points that they do have recorded.
There are some difficulties in obtaining the map data this
way. First, the accuracy of the various measurements varies greatly although
the more exact measurements should have the greatest influence. However
scientists are not certain exactly how reliable any single measurement is in
providing dependable information about the environment around it. Therefore
more accurate measurements are not always given the highest priority.
There is also the problem of the uncertainty of the
measurements simply because the process for obtaining the measurements is
highly complex. A seemingly small error can impact the data in a significant
way, leading to a distorted map.
To address these problems, the MPA scientists have developed
an algorithm used to reconstruct the images. This algorithm, called the
"extended critical filter," uses tools provided by the new discipline
known as information field theory. Information field theory, which uses logical
and statistical methods applied to fields, is an effective tool for dealing
with erroneous information. Besides astronomy these tools can be used in fields
such as medicine or geography for a range of image and signal-processing
applications.
While the new map is particularly important for studying our
own Galaxy, researchers will also be able to use it for future studies for
extragalactic magnetic fields. This is possible because the scientists will use
the new map to help them account for the Galactic contribution to observed
Faraday rotation. In the near future astronomers are looking toward a new
generation of radio telescopes, such as LOFAR, eVLA, ASKAP, MeerKAT, and the SKA
that will provide an abundance of measurements of the Faraday effect. With this
new data, researchers will be able to provide updates to the image of the
Faraday sky, and perhaps someday understand the origin of the galactic magnetic
fields.
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