Fig. 1: Radio map of the
Milky Way. This map shows the amount of electrons with nearly the speed
of light times combined with the magnitude of the transverse component
of the galactic magnetic field, projected along each line of sight
through the Milky Way. © MPA
The magnetic fields of the Milky Way cause
electrons with nearly the speed of light to rotate and to emit radio
waves. As consequence, this radiation should also "rotate" in some way,
i.e. the polarization of the radiation will change. This very weak
circular polarization of the Milky Way, however, has not been observed
so far. Researchers at the Max Planck Institute for Astrophysics and
colleagues have now predicted some properties of this polarization and
created a "wanted poster" to allow targeted searches. A measurement of
the circular polarization would provide important insights into the
structure of the galactic magnetic fields and confirm that electrons -
and not positrons - are the source of this radio emission in the Milky
Way.
The vast space in between the Milky Way stars is not empty; it
is filled with gas, dust, magnetic fields, and particles with almost the
speed of light - the so-called cosmic radiation. This consists of
atomic nuclei, electrons and small amounts of antimatter, especially
positrons and antiprotons. Part of the cosmic radiation reaches the
earth directly, but it can also be detected indirectly. The ultra-fast
electrons and positrons emit radiation, which has already been detected
and measured (Fig. 1). So far, however, it is almost impossible to
distinguish whether this radio emission originates from electrons or
positrons.
The circularly polarized radiation could tip the scientists off,
since electrons and positrons rotate in opposite directions. However,
this radiation is less than one thousandth of the galactic radio
emission; researchers therefore have been unable to detect it. Moreover,
astronomers do not have a clear idea of what patterns to look for in
the sky, they do not know what this radiation should look like. This gap
has now been filled by Torsten Enßlin and his colleagues. The
astrophysicists show that the current information about the magnetic
field of the Milky Way is enough to estimate the circular polarization.
Three conditions must be fulfilled for a region in space to radiate
circular polarized light. First, there must be an excess of electrons
(or positrons) with almost the speed of light, so that the rotation of
these particles in the magnetic field will be in a preferred direction.
Second, the magnetic field has to be at least partially aligned with the
observer so that the direction of rotation is visible in the sky
projection. And third, the magnetic field must not be completely in the
direction of the line of sight, since the radio waves are mainly emitted
transversely to the magnetic field.
Information about both the amount of electrons and positrons with
almost the speed of light and the transverse component of the magnetic
field is given by the radio map of the Milky Way (
Fig. 1). In general,
it is assumed that this emission is generated mainly by electrons with
only a small contribution by positrons.
Fig. 2: Map of the Faraday
effect in the Milky Way. This map shows the line-of-sight component of
the galactic magnetic field weighted with the amount of thermal
electrons, projected along each line of sight through the Milky Way.
Regions where the magnetic field is mainly directed at us are red and
regions in which it points away from us are blue. © MPA
Information on the line of sight component of the magnetic field
comes from measurements of the so-called Faraday effect. Linearly
polarized light, radiated from radio-galaxies outside the Milky Way, is
being rotated as it traverses the galactic magnetic field. This rotation
depends on both the intensity and the orientation of the magnetic field
along the line of sight. Radio waves interact with slow thermal
electrons in the galactic gas, which perform circular motions in the
magnetic fields. The rotation of the linear polarization of the
lightwaves is in the opposite direction as the rotation of these
electrons. Since the magnitude of the Faraday effect varies with the
frequency of the radiation, it can be detected and mapped. In this way, a
Faraday map of the sky was produced already in 2012 by Niels Oppermann
working with Torsten Enßlin (
Fig. 2,
MPA Highlight November 2012). This shows the summed up magnetic field component that is aligned with any given line-of-sight.
Thus, all three necessary components are known: the number of
electrons at nearly the speed of light, and the strengths of the two
magnetic field components involved. The information from observations,
however, is always given only as a projection along a line of sight. For
an accurate prediction of the circular polarization, further data is
needed to describe how these three components are distributed along the
lines of sight.
Fig. 3: Map of the intensity
and direction of rotation of the circular polarization of the radio
emission. Regions in which the polarization is predominantly clockwise
are red, and regions with counterclockwise polarization are blue. This
prediction was made by combining the intensity maps of galactic radio
emission (Figure 1), the Faraday effect (Figure 2), and a rough model of
the 3D distribution of galactic electrons. The details of the true
circular polarization will differ, but the map should show the correct
direction of rotation more often than not if our knowledge of particles
at near the speed of light in the Milky Way is more or less correct. © MPA
For an estimation of this distribution in the third dimension,
Torsten Enßlin used both known and plausible statistical properties of
turbulent magnetic fields. Thus, he was able to show that the exact
details of the statistics do not have much influence on the results, as
long as the magnetic fields do exhibit any improbable structure. The PhD
student Sebastian Hutschenreuter then made a prediction of the circular
polarization using a coarse model of the distribution of both thermal
and highly energetic electrons, as well as the magnetic energy contained
in the Milky Way from observed radio and Faraday maps (Figure 3).
The details of the prediction will not be accurate in all details as
there were uncertain assumptions. However, the map should indicate the
preferred direction of rotation of the actual circular polarization more
often than not. This statistical prediction therefore is suitable in
searches for the extremely weak circular polarization signal.
The next step will be to look for the predicted small circular
polarization pattern in the data of both existing and soon-to-come
terrestrial radio telescopes. If astronomers were able to actually
detect the "rotating radiation", astronomers could draw important
conclusions about the galactic magnetic field and confirm that electrons
and not positrons are the source of this radiation in the Milky Way.
Original Publication
1. Torsten A. Enßlin, Sebastian Hutschenreuter, Valentina Vacca, and Niels Oppermann
The Galaxy in circular polarization: all-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays
submitted
Source
Authors
Enßlin, Torsten
Scientific Staff
Phone:
2243
Email: tensslin@mpa-garching.mpg.de
Links:
personal homepage (the institute is not responsible for the contents of personal homepages)
Hutschenreuter, Sebastian
PhD student
