Cosmic magnetic fields – as observed in the Universe, in galactic dynamos, and in the Milky Way
Introduction
Magnetism always had a mysterious appeal, a “Je ne sais quoi”! The goal of this review is to focus on what is important about magnetism in each type of objects in the Universe, summarizing the main results of our observational knowledge on magnetism, and the main interpretive model in each type of objects.
Here, this observational approach emphasizes the discoveries, moving from Nature to interpretations. Earlier reviews with a similar approach on magnetism have been given for the Milky Way (e.g., Vallée, 1997a; Heiles, 1996a, Heiles, 1996b; Wielebinski, 1995) and for nearby spiral galaxies and cosmology (e.g., Han and Wielebinski, 2002; Vallée, 1997b; Beck et al., 1996; Kronberg, 1994; Vallée, 1984a).
Elsewhere, other reviews have used a theoretical approach, moving from theoretical expectations to Nature. Such reviews were made on nearby galaxies and cosmology (e.g., Widrow, 2002; Kulsrud, 1999; Lesch and Chiba, 1997; Zweibel, 1996; Kahn and Brett, 1993; Ferrière, 1993a, Ferrière, 1993b; Parker, 1992; Kahn, 1992; Asseo and Sol, 1987).
Conversion between units. For magnetic strength conversion, one has 1 μT=10 mG (or 1 T=104 G). For distance conversion, one has 1 pc=3.2 light-years=3.1 × 1016 m. For interstellar distances, I use R0=7.5 kiloparsec for the distance of the Sun to the Milky Way's galactic center. For cosmological distances, I use a Hubble parameter H0=70 km/s/Mpc and a deceleration parameter q0=1. Hence a redshift z of light corresponds to a rough distance , so for z=1400 one gets D≈4.3 gigaparsecs (e.g., Harrison, 1993).
This review starts with magnetic effects inferred from astroparticles at different energies (Section 2), then discusses magnetic techniques employed at different electromagnetic wavelengths (Section 3), and continues with a summary of our knowledge of magnetism in objects from the largest ones down to those with a size of 10 pc (4 Cosmological scales, 5 Universal scales, 6 Superclusters and voids, 7 Elliptical galaxies, quasars, 8 Spiral galaxies, Seyfert galaxies, 9 Irregular galaxies, and small compact galaxies, 10 The Milky Way as a whole, 11 Interstellar superbubbles and supershells (50–500 pc), 12 Large interstellar objects in our Galaxy, 13 The center of the Milky Way galaxy (∼7.5 kpc away)). Magnetism in much smaller objects was recently reviewed in Vallée (2003).
Section snippets
Astroparticle energies
Particle astronomy is a relatively new area of research. Cosmic rays [CR] are measured directly by balloons in the upper atmosphere of the earth, or indirectly through secondary particles or light on the ground (mesons and ultraviolet rays). Balloon data taken at the top-of-the-atmosphere, for particle energy near 1 GeV, indicate the following composition: a proton flux near 1000, a helium nuclei flux near 100, and an antiproton flux near 0.01, all expressed in particles m−2 sr−1 s−1 GeV−1 (Boezio
Electromagnetic wavelengths
Techniques. The electromagnetic wave can be described by its amplitude, phase, wavelength, and polarization. For an observer, the wave can be described by the Stokes parameters I, Q, U, V, where I is the total intensity, Q is the linear polarization in the direction of the position angle 0°, U is the linear polarization at 45°, and V is the circular polarization. Linear polarization position angle is measured Eastward starting at true North.
An electromagnetic (EM) photon carries a spin in its
Cosmological scales
Although the seed for the cosmological magnetic field is unknown, some general models have been suggested, which can be classified according to the time of their onset: near the time of inflation; near recombination time, z≈1100; near the time of protogalaxy formation; or continuously over time.
Universal scales
Galaxies and quasars have been observed up to a limited distance from the Earth, the so-called observable Universe. Since our technological sensitivity and the size of our light-gathering instruments both increase with time, we continuously expand the dimension of our observable Universe. With each advance, new objects have come into view.
Large scale voids in space (size ≈200 Mpc)
Medina Tanco (1998) modelled the propagation of high-energy cosmic particles inside sheets/walls and filaments threaded by a parallel magnetic field near 0.1 μG. He predicted that the particles are emerging anisotropically from the sheets by about 3 orders of magnitudes, most of the particles are following the direction of the sheet magnetic field – thus cosmic particles would be copious when looking towards the inside of a wall; and that any information regarding the injection direction or
Individual galaxies and quasars (∼200 kpc)
Most of the radio galaxies and quasars studied are located beyond 20 Mpc. The linear polarization of distant elliptical galaxies and quasars comes from synchrotron radiation emitted by relativistic electrons moving in the magnetic fields. These magnetic fields are located in the nucleus of the galaxy or quasar, in the jets pointing to two radio lobes (∼100 kpc long), and in the radio lobes located on both sides of the optical galaxy or quasar. The circular polarization of quasars comes from the
Methodology – different methods to obtain the magnetic field strength
Most of the galaxies studied here are located nearby, well within 20 Mpc of the Milky Way. Most studies showed that the magnetic field strength in late-type galaxies is close to the “equipartition magnetic field” value Beq. Three main methods often employed to give magnetic field strengths are the Faraday rotation method, the equipartition method, and the cosmic ray method.
Large irregular galaxies (organized fields)
Large irregular galaxies may have a large-scale regular magnetic field near 5 μG, much like that in normal spiral galaxies. However, their slow rotation prevents the classical dynamo, which needs a large differential rotation. However, large irregular galaxies can use a `modified dynamo', driven by localized Parker-type magnetic instabilities (Hanasz and Lesch, 2000).
0523 − 69. Linear polarization of the Large Magellanic Cloud (LMC) was presented by Haynes et al. (1991). RM analysis was
The Milky Way as a whole
The ISM pressure is composed of the sum of: the gaseous turbulent pressure (1.8 × 10−12 dyne/cm2), the CR pressure (1 × 10−12 dyne/cm2), the magnetic field pressure (1 × 10−12 dyne/cm2), and the gaseous thermal pressure (0.3 × 10−12 dyne/cm2), see Boulares and Cox (1990). Thus in the ISM the magnetic pressure amounts to one-quarter of the total pressure. The total pressure of the large-scale diffuse interstellar medium (ISM) does support the weight of the hot X-ray galactic corona, which is ∼4 × 10−12
Interstellar superbubbles and supershells (50–500 pc)
In addition to the large-scale (regular and random) components of the galactic magnetic field, there are localized deviations (or detours) of the magnetic field, occasioned by interstellar magnetized superbubbles around OB associations and hypernovae, with typical superbubble diameters of 200–250 pc. The first such magnetic detour was observed by Vallée and Kronberg (1973) around Loop I.
Fig. 21 shows the sky distribution of 674 quasars and galaxies with a measured Faraday RM. Data are from
Magnetized high-latitude, high-velocity HI filaments (∼30–100 pc)
Elongated neutral hydrogen (HI) High-Velocity Clouds (HVC) at high-galactic latitudes that are moving towards the Galactic Plane can locally perturb the galactic magnetic field. The bouncing-cloud model of Santillán et al. (1999) finds that the collision induces a magnetic tension capable of reversing the motion of the HVC, but there is no mass exchange between cloud and disk. During the collision of a HVC with the Galactic Plane, the thick magnetized gaseous galactic disk can prevent the cloud
Within 3 kpc of the galactic center
As mentioned earlier, there is a long bar straddling the galactic nucleus, oriented in the first quadrant about 20° from the Sun-GC line, as derived from analysis of HI, CO and CS molecular data, diffuse NIR background and others (Binney et al., 1991). It ends near 3 kpc on both sides of the GC (Freudenreich, 1998).
Within 500 pc of the galactic center
Some filaments are seen in total continuum emission (Stokes I) in this region.
Near 225 pc. There is a 18-pc Pelican filament located at ∼225 pc from the Galactic Center aligned
Conclusion
The pace of progress in observational polarimetry is restrained mainly by angular resolution (aperture) and flux sensitivity (detector), both of which are improving steadily. Here, polarimetric observations to date have been reviewed along with a physical analysis in terms of the magnetic fields on large scales (cosmological down to10 pc in dimension). Magnetic complexity can remind us poetically of life: “The magnetic field exists in the Universe as an organism, feeding on the general energy
Acknowledgements
I thank Tom Landecker (National Research Council of Canada, Penticton) for providing Fig. 25. Many of the figures were created with the PGPLOT drawing software.
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