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The Atlas as it has been realized in the following pages illustrates again that galaxies cannot be characterized as just assemblages of stars, radiation, and gravitation. The following Atlas pictures emphasize the importance of dust in some; they particularly imply a much more important role for the gas in general and point to the existence of either new forces or forces which previously have been little considered. For example, the twisted, distorted shapes and curious linkages pictured here attest to the fact that there are viscocity-like forces present that in some cases are dominant. Probably these forces are due to magnetic effects. Vorontsov-Velyminov has stressed in the past the probable magnetic nature of these effects. Magnetic forces are very difficult to study, but may be very important in our Universe. The recent radio-astronomy discoveries of violent events in galaxies reveal sources of energetic charged particles. These charged particles interact with magnetic fields and offer the hope of mapping, measuring, and understanding cosmic magnetic fields. Exploration of the connection between the plasmas observed with the radio telescope and the optical evidences of plasma effects picture in the present Atlas in now open to us.
Halton Arp, Atlas of Peculiar Galaxies, University of Chicago Press, Chicago, Illinois, 1966.
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Helicity and Filamentation, the Signature of a Magnetic Field. The data shown is on the extragalactic, or cosmic, scale. |
Magnetism is the fundamental force that determines the character, or motion or shape of ionized matter (plasma). The degree of ionization in interplanetary space and in other cosmic plasmas may vary over a wide range, from fully ionized to degrees of ionization of only a fraction of a percent.\footnote{The degree of ionization is defined as $n_p/ \left( {n_0+n_p} \right)$ where $n_p$ is the plasma density and $n_0$ is the density of neutral particles.} Even weakly ionized plasma reacts strongly to electromagnetic fields since the ratio of the electromagnetic force to the gravitational force is 39 orders of magnitude. For example, although the solar photospheric plasma has a degree of ionization as low as 10$^{-4}$, the major part of the condensable components is still largely ionized. The “neutral” hydrogen (HI) regions around galaxies are also plasmas, although the degree of ionization is only 10$^{-4}$. Most of our knowledge about electromagnetic waves in plasmas derives from laboratory plasma experiments where the gases used have a low degree of ionization, 10$^{-2}$-10$^{-6}$.
Because electromagnetic fields play such an important role in the electrodynamics of plasmas, and because the dynamics of plasmas are often the sources of electromagnetic fields, it is desirable to determine where within the universe a plasma approach is necessary. Of primary importance is the magnetic field. On earth, magnetic field strengths can be found from about 0.5 gauss ($0.5 \times 10^{-4}$ T) to $10^7$ gauss ($10^3$ T) in pulsed-power experiments; the outer planets have magnetic fields reaching many gauss, while the magnetic fields of stars are 30-40 kG (3-4 T). Large scale magnetic fields have also been discovered in distant cosmic objects. The center of the Galaxy has milligauss magnetic field strengths stretching 60 pc in length. Similar strengths are inferred from polarization measurements of radiation recorded for double radio galaxies. No rotating object in the universe, that is devoid of a magnetic field, is known.
In cosmic problems involving planetary, interplanetary, interstellar, galactic, and extragalactic phenomena, magnetohydrodynamics effects are appreciable. Neglecting lightning, planetary atmospheres and hydrospheres are the only domains in the universe where a nonhydromagnetic treatment of fluid dynamic problems is justified.
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The Helix Nebula |
The most basic difference between ionized and nonionized matter is the ability to carry electric current. This ability is also the reason why virtually all the matter in the universe is—and presumably has always been—magnetized. The presence of the magnetic field has important dynamical consequences since the magnetic force can locally be much greater than the gravitational force.
The ability of carrying current, which is the basis for the magnetization, is a property that is still not well known in the case of cosmical plasma. It can be different by many powers of ten from what classical theories predict.
In laboratory and space plasma electric currents tend to cause filamentation. Filamentary structures are also abundant in the cosmical plasma and make homogeneous models of astrophysical plasma very dubious. Correspondingly, homogeneous models are likely to be misleading when applied to large-scale astrophysical processes.
One of the notable characteristics of space plasma, revealed by satellites and space probes, is its tendency to form sharp boundaries between plasmas with different properties. This tendency towards "cellular structure" can have profound astrophysical implications such as generating electric fields in space and providing sources of energy for driving electric currents over very large distances.
A phenomenon called critical ionization velocity, first proposed in Alfven’s cosmogonic theory, has been observed in both laboratory and space plasmas. It implies, among other things, an exchange of momentum between plasma and neutral gases in the presence of magnetic fields. One of the surprising discoveries in the near-Earth plasma is the existence of previously unknown mechanisms that allow very efficient separation of different chemical species. Even more surprisingly, is the recent discovery through radiotelescope observations that interstellar space displays the same phenomena.
Plasma, which is rare in our close environment, is the dominating state in the universe as a whole. The Sun and stars as well as the diffuse matter between them and between the galaxies are all in this state, despite their great difference in density and temperature. The only exceptions are the cold celestial bodies such as the Earth and other planets, the satellites, asteroids, comets, meteoroids, and dust grains, all of which account for an extremely small fraction of the know matter in the universe. Furthermore, all the cosmical plasma is magnetized. From the nearest cosmical plasma—the Earth’s ionosphere—out to the most distant intergalactic regions all cosmical plasma are penetrated by magnetic fields that influence their physical properties in various, often dramatic, ways.
In diffuse matter, which forms a major part of the universe, the motion of each individual particle is strongly controlled by the magnetic field. For example, a hydrogen ion in the solar wind with a thermal velocity of 20 kilometers per second in the interplanetary magnetic field of 5 nanotesla experiences a magnetic force of that is about 10^7 times stronger than the gravitational force from the Sun. Even in the other extreme, for example, in stellar interiors, where gravitational forces dominate, the magnetic field makes possible wave modes (such as Alfvén waves) that are not possible in nonionized media.
Except in very limited circumstances, all cosmical plasmas carry electric currents that constitute the sources of the magnetic field.
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Contours of ‘neutral’ hydrogen regions superposed on
an optical image of the galaxy NGC 4151. The Contours show two hydrogenic
plasma density ‘peaks’ (left and right) situated about a void at the center
of the galaxy.
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Magnetic field derived from galaxy simulation overlaid on the galaxy NGC 4151. The blue ‘ribbons’ are components of a vertical magnetic field while the green arrows depict both the axisymmetric and bisymmetric magnetic fields observed in galaxies of this morphological type. |
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Left: Simulation magnetic energy density superimposed on simulation galaxy. A ‘horse-shoe’ shaped cusp, opening towards a spiral arm surrounds a magnetic field field/HI minima core. Right: HI distribution superimposed on an optical photograph of the galaxy NGC 4151. |
One of the most compelling pieces of evidence for the existence of supercluster-sized Birkeland currents came from the discovery of faint supercluster-scale radio emission at 326 MHz between the Coma cluster of galaxies and the Abell 1367 cluster. The radiation’s synchrotron origin implies magnetic field strengths of 0.3-0.6 microgauss stretching for 1.5 megaparsecs. This corresponds to a classical galactic Birkeland current of nearly 10^19 amperes.
The photographs in the Atlas of Peculiar Galaxies are completely familiar to high energy density plasma experimentalists For example, the physicist Winston Bostick could sequence the photographs in the Atlas according to type and time evolution, based on his observation of millimeter sized colliding plasmas in pulsed power generators. The shapes are, indeed, dependent on the magnetic field as can be demonstrated in three-dimensional electromagnetic-gravitation particle-in-cell simulations.
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