Galactic magnetic fields
| Rainer Beck (2007), Scholarpedia, 2(8):2411. | doi:10.4249/scholarpedia.2411 | revision #18419 [link to/cite this article] |
Contents |
Introduction
Magnetic fields are a major agent in the interstellar medium (ISM) of spiral, barred and irregular galaxies. They contribute significantly to the total pressure which balances the ISM against gravity. They may affect the gas flows in spiral arms and around bars. Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud during its collapse. MHD turbulence distributes energy from supernova explosions within the ISM. Magnetic reconnection is a possible heating source for the ISM and halo gas. Magnetic fields also control the density and distribution of cosmic rays in the ISM.
Cosmic-ray electrons in galaxies, accelerated in supernova remnants or in jets and spiralling around interstellar magnetic field lines with almost the speed of light, emit synchrotron emission over a large range of radio wavelengths. The most energetic electrons can even emit synchrotron infrared or optical light. The intensity of synchrotron emission is a measure of the density of cosmic-ray electrons and of the strength of the total magnetic field component in the sky plane. The degree of linear polarization of synchrotron emission can be as high as 75% in a completely regular field, which is a field with a constant direction within the volume traced by the telescope's beam. Any variation of the field direction within the beam reduces the degree of polarization. Regular fields are generated e.g. by a dynamo (see below). Polarized emission can also emerge from anisotropic fields which are generated from turbulent fields by a compressing or shearing gas flow and frequently reverse their field direction by 180o on scales smaller than the telescope beam. Unpolarized synchrotron emission indicates fields with random directions which have been tangled by turbulent gas flows. Hence, the total field consists of three components: regular, anisotropic and random.
At short radio wavelengths the orientation of the observed polarization vectors is perpendicular to the field orientation. The orientation of the polarization vectors is changed in a magnetized plasma by Faraday rotation. The rotation angle increases with the plasma density, the strength of the component of the regular field along the line of sight and the square of the observation wavelength. Fields directed towards us cause an anticlockwise sense of rotation, fields directed away from us a clockwise rotation. Anisotropic fields do not Faraday-rotate. For typical plasma densities and regular field strengths in the interstellar medium of galaxies, Faraday rotation becomes significant at wavelengths larger than a few centimeters. Measurements of the Faraday rotation from multi-wavelength observations allow to determine the strength and direction of the regular field component along the line of sight. Combination with the polarization vectors yields a fully three-dimensional picture of the magnetic field.
The most sensitive instruments for radio polarization measurements is the 100-m single-dish telescope in Effelsberg (Germany) and the synthesis (interferometer) telescopes in Westerbork (Netherlands), the Very Large Array (USA) and the Australia Telescope Compact Array.
The Origin of Galactic Magnetic Fields
The origin of the first magnetic fields in the Universe is still a mystery (Widrow 2002). Protogalaxies probably were already magnetic due to field ejection from the first stars or from jets generated by the first black holes. However, a primordial field in a young galaxy is hard to maintain because the galaxy rotates, so that field lines get strongly wound up (in contrast to observations, see below) and field lines with opposite polarity may cancel via magnetic reconnection. This calls for a mechanism to sustain and organize the magnetic field.
The most promising mechanism is the dynamo which transfers mechanical energy into magnetic energy (Beck et al. 1996, Rüdiger & Hollerbach 2004, Brandenburg & Subramanian 2005). With a suitable configuration of the fluid or gas flow, a strong magnetic field with a stationary or oscillating configuration can be generated from a weak seed field. In astronomical objects like stars, planets or galaxies, an efficient dynamo needs turbulent motions and non-uniform (differential) rotation and is called alpha-Omega dynamo. It generates large-scale regular fields, even if the seed field was turbulent ("order out of chaos").
The regular field structure obtained in dynamo models is described by modes of different azimuthal symmetry in the disk and vertical symmetry perpendicular to the disk plane. Several modes can be excited in the same object. In spherical bodies like stars or planets, the strongest mode is a torus near the equator with a reversal across the equatorial plane, surrounded by a dipole field (odd vertical symmetry). In flat objects like galactic disks, the strongest mode is of axisymmetric spiral shape in the plane without reversals, surrounded by a weak quadrupole field (even vertical symmetry). The next strongest mode is of bisymmetric spiral shape with two field reversals in the disk, followed by more complicated modes. Such modes can be identified from the pattern of polarization angles and Faraday rotation in multi-wavelength radio observations.
Results from Radio Observations
Magnetic Field Strengths in Galaxies
Total magnetic field strengths can be determined from the intensity of total synchrotron emission, assuming energy balance (equipartition) between magnetic fields and cosmic rays. This assumption seems valid on large spatial and time scales, but deviations probably occur on local scales in galaxies. The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.5 G or 50 μT. Radio-faint galaxies like M 31 (Fig.3) and M 33, our Milky Way's neighbours, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51 (Fig.1), M 83 and NGC 6946 (Fig.2), have 15 μG on average. In prominent spiral arms the total equipartition field can be up to 30 μG strong, in regions where also cold gas and dust are concentrated. The strongest total equipartition fields (50-100 μG) were found in starburst galaxies, like M 82 and the "Antennae", and in nuclear starburst regions, like in the centers of NGC 1097 and other barred galaxies.
The degree of radio polarization within the spiral arms is only a few %; hence the field in the spiral arms must be mostly tangled. The ordered (regular or anisotropic) fields traced by polarized synchrotron emission are generally strongest (10-15 μG) in the regions between the optical spiral arms. This can be explained by a dynamo wave which is phase shifted with respect to the density wave producing the spiral arms.
Magnetic Field Structure in Galaxies
The magnetic field forms nice spiral patterns in almost every galaxy, even in flocculent and bright irregular types which lack any spiral optical structure (Wielebinski & Beck 2005). This is regarded as a strong argument for the action of galactic dynamos. Spiral fields are also observed in the central regions of galaxies and in circum-nuclear rings of gas. In galaxies with massive spiral arms, the magnetic field lines run mostly parallel to the optical arms, but are concentrated at the inner edge of the spiral arms or between the spiral arms (as an example, see Fig.1). In several galaxies, the field forms independent magnetic arms between the arms, as in NGC 6946 (Fig.2). In galaxies with massive bars, the field pattern seems to follow the gas flow. As the gas rotates faster than the spiral or bar pattern of a galaxy, a shock occurs in the cold gas which has a small sound speed, while the warm, diffuse gas is only slightly compressed. As the observed compression of the field in spiral arms and bars is also small, the ordered field is coupled to the warm gas and is strong enough to affect the flow of the warm gas.
Large-scale patterns of Faraday rotation observed in a few spiral galaxies reveal regular fields with a large-scale constant direction, as predicted by dynamo models. The Andromeda galaxy M 31 (Fig.3) hosts a dominating axisymmetric field, the basic dynamo mode, which extends to at least 15 kpc distance from the centre (one kiloparsec (kpc) corresponds to 3260 light years). Other candidates for a dominating axisymmetric field are the nearby spiral IC 342 and the irregular Large Magellanic Cloud (LMC). The field structures in M 51 and NGC 6946 (Figs.1 and 2) can be described by a superposition of two dynamo modes. However, in most galaxies observed so far no clear patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be distinguished with the limited sensitivity and resolution of present-day telescopes, or most of the ordered fields traced by the polarization vectors are anisotropic (with frequent reversals), due to shearing or compressing gas flows.
Galaxies seen in edge-on view possess radio halos with exponential scale heights of 1-2 kpc. The magnetic field orientations are mainly parallel to the disk near the plane, but vertical components are visible at above and below the plane and also at large distances from the center (Fig.4). A prominent exception is the edge-on irregular galaxy NGC 4631 with the brightest and largest radio halo observed so far, composed of vertical magnetic spurs connected to star-forming regions in the disk. The observations support the idea of a galactic wind which is driven by star formation in the disk and transports gas, magnetic fields and cosmic-ray particles into the halo.
An atlas of magnetic field structures observed in polarized radio intensity can be found on: http://www.mpifr-bonn.mpg.de/staff/wsherwood/mag-fields.html.
Magnetic Field in the Milky Way
The average strength of the total magnetic field in the Milky Way is about 6 μG near the Sun and increases to 20-40 μG in the Galactic center region. Radio filaments near the Galactic center host fields of mG strength. Outside the central region, the field is mostly parallel to the plane of the Galactic disk. Faraday rotation measurements from the polarized emission of pulsars with known distances allow to investigate the structure of the Milky Way's magnetic field in three dimensions with much higher resolution than in external galaxies. The overall field structure follows the optical spiral arms, like in external galaxies, but several large-scale field reversals and distortions near star-forming regions were discovered (Wielebinski & Beck 2005). Radio polarization surveys of the Milky Way revealed a wealth of parsec-scale structures in the magnetized interstellar medium (Uyaniker et al. 2004, Reich 2006).
Future Radio Telescopes
Present-day radio polarization observations are limited by sensitivity and angular resolution. The best available spatial resolution is 100-300 pc (one parsec (pc) corresponds to 3.26 light years) in the nearest spiral galaxies and 10 pc in the nearest galaxy, the Large Magellanic Cloud. The Expanded Very Large Array (EVLA, http://www.aoc.nrao.edu/evla), under construction, and the Square Kilometre Array (SKA, http://www.skatelescope.org), planned for 2015-2020, will have much improved sensitivity at centimetre and decimetre wavelengths (Carilli & Rawlings 2004). The SKA will allow to study magnetic field structures at resolutions more than 10 times better than today. The SKA will discover thousands of new pulsars in the Milky Way which will enormously increase the number of Faraday rotation measurements and hence provide a detailed map of the magnetic field structure.
At long wavelengths of a few metres, a new-generation radio telescope, the LOw Frequency Array (LOFAR), is already under construction in the Netherlands (http://www.lofar.org), with extensions to Germany (http://www.lofar.de), the UK (http://www.lofar-uk.org), France (http://www.lesia.obspm.fr/plasma/Lofar) and possibly further European countries. Among many other observing possibilities, LOFAR will be able to trace radio synchrotron emission from low-energy cosmic rays in weak magnetic fields. This will allow to observe the outermost regions of galaxies which are only accessible via radio waves.
References
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