Alignment of dust grains in magnetic fields
Dust grains in the interstellar medium range in size from 5 to 500 micrometers, and have an irregular, elongated structure. The orientation of dust grains in the interstellar magnetic field, which was discovered in 1949, is still not fully understood. Size, chemical composition, rotation speed, strength of the external magnetic field, and striking rates of gas atoms, photons, and cosmic ray particles all play a role. Interstellar iron combines with carbon chains to form molecules, but it does not exist in pure form; Therefore, there are no compass needles among the stars. Silicates with magnetic properties play the decisive role. Paramagnetic materials are magnetized and attracted to an external magnetic field, but without an external magnetic field they do not exhibit magnetic order.
According to the idea of Leverett Davis and Jesse Greenstein in 1951, dust grains are caused by the collision of gas atoms. The axis of rotation rotates about the direction of the magnetic field. The cumulative effect of the torque generated by randomly colliding particles leads to a slow alignment of the spin axis parallel to the direction of the magnetic field, i.e. an alignment of the major axis of the particles perpendicular to the magnetic field (Davis-Greenstein effect). . Unfortunately, this process turns out to be very inefficient and very slow. According to research by Alex Lazarian and Tim Huang in 2007, the torque is transmitted by photons from the asymmetric (anisotropic) radiation field of nearby stars. When irradiated by starlight, the dust grains spin at more than a million revolutions per second. This is called “superthermal,” which means that the rotational energy is greater than the thermal energy of the molecules. This speeds up the alignment process, which only takes a few tens of thousands of years. This theory could explain why small particles are misaligned: they are not irradiated by photons as often. Particles in dense molecular clouds are also misaligned because the radiation field there is very weak.
As a result of this alignment, the dust grains rotate so that their major axes are perpendicular to the magnetic field lines. Warm dust grains emit far-infrared radiation, the direction of linear polarization being observed perpendicular to the magnetic field component in the celestial plane. Dust grains also absorb light from stars that lie in line of sight behind the dust cloud, preferably perpendicular to the magnetic field. The direction of polarization of optical radiation is therefore parallel to the magnetic field in the plane of the sky (see “Polarization through dust”).
The alignment of dust particles is by no means perfect: not all particles are magnetized, the radiation field is not strong enough everywhere, and collisions of gas atoms or cosmic radiation particles can disrupt the alignment process; In addition, the directions of the magnetic field are sometimes perturbed. Therefore, it is not surprising that the observed degrees of polarization do not exceed a few percent. The average on a galaxy like NGC 891 is only about one percent.
© Rainer Beck; Processing: SuW drawing (detail)
Polarization by dust | In this diagram, optical radiation from the star strikes an elongated dust grain from the upper right and is linearly polarized. Here Ω indicates the axis around which the bead rotates (purple arrow) which is parallel to the direction of the magnetic field (black arrow). Warm dust grains emit their own far-infrared radiation, the polarization direction of which is perpendicular to the direction of the optical radiation.
The magnetic field strength in 9io9 can be estimated by assuming that the magnetic field and cold gas turbulence have similar energy densities. The resulting field strength at 9io9 is about 500 microgauss, which is 5 × 10-VIII The Tesla is much higher than that of nearby spiral galaxies such as NGC 891, but similar to that of nearby stellar galaxies such as Messier 82. The FIR method of measuring magnetic fields has long been successfully applied to dust clouds in the Milky Way and nearby galaxies. , for example with the HAWC+ polarimeter on board SOFIA. Now the leap to distant, young galaxies has been made.
Almost Einstein's ring | In the near-infrared range, Galaxy 9io9 can be seen as a reddish arc that appears to lie opposite the elliptical galaxy in the center of the image, which acts as a gravitational lens. It is a collection of images from the European Southern Observatory's VISTA survey and the French Canadian Hawaii Telescope (CFHT).
Origin of magnetic fields
How do the new results fit into our rudimentary understanding of the formation and evolution of cosmic magnetic fields? Weak seed fields can be created by systematic separation of electrical charges, for example in the vicinity of rotating, compact galactic nuclei (“Berman battery”) or in supersonic shock fronts in very young galaxies (protogalaxies). Phase transitions in the very young universe can leave behind weak seed fields. Field amplification of several orders of magnitude in young galaxies is then carried out by a small-scale dynamo, which converts part of the energy generated by turbulent gas motions on scales of less than 100 light-years into magnetic energy. This process takes only a few tens of millions of years. It is therefore not surprising that the smallest known radio quasars appear at redshifts greater than For example = 7.5 Only about 700 million years after the Big Bang did it already have such strong (turbulent) magnetic fields that it could emit intense synchrotron radiation.
© Jin Ah Kim, Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, USA. Kim et al.: Exploring the magnetic field geometry of NGC 891 using SOFIA/HAWC+. The Astronomical Journal 165, 2023; Processing: SuW drawing (detail)
Spiral galaxy NGC 891 in the infrared This part of the sky shows the trend of linearly polarized far-infrared radiation at a wavelength of 154 µm (black lines), observed with the HAWC+ polarimeter on board the SOFIA aircraft observatory. The background gradient represents the total far-infrared intensity at 160 µm as measured by the Herschel Space Telescope. The unit of Janski/pixel for spectral flux density is 10-26 Watts per hertz, per square meter and per pixel (3 x 3 arc seconds).
Only a small-scale dynamo produces turbulent, i.e., turbulent, magnetic fields. However, in relatively nearby galaxies such as NGC 891, we observe large-scale organized magnetic fields. The Faraday rotation measured in the Andromeda galaxy Messier 31 shows a field of uniform orientation over a large area. The theory of the large-scale dynamo, which operates on a scale of a few thousand light-years and is driven by turbulence and differential rotation, is used to explain this. The time scale until the complete directional regime is reached is a few hundred million years, and for large galaxies several billion years.
Radio map of NGC 891 | The image of the six-centimeter wavelength spiral galaxy was created from images collected from the Very Large Array (USA) and the Effelsberg radio telescope. The lines show the direction of linearly polarized synchrotron radiation. In addition, Faraday rotation measurements show that the magnetic field is ordered over a large area. The contour lines represent the far-infrared intensity at 170 µm, recorded by the Herschel telescope. The unit microjanski/antenna lobe for spectral flux density is 10-32 Watts per hertz, per square meter, and per antenna beam (twelve arc-second resolution).
Previously, in 2017, astronomer Sui An Mao and her team from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn observed distant magnetic fields with a large-scale orderly orientation in a galaxy with a cosmic redshift of For example = 0.44 found.
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