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Magnetic fields in external galaxies can be observed with the same methods as in the Milky Way, except for extragalactic pulsars which have been found so far only in the Magellanic clouds. Naturally the spatial resolution of the telescopes is much worse in galaxies, and the detailed structure of extragalactic fields on scales below about 100 pc is still invisible. On the other hand, the large-scale field properties, like the overall pattern and the total extent, can be best measured in external galaxies. Observations in the Milky Way and in external galaxies are complementary.

4.1. Optical polarization, infrared polarization, and Zeeman effect

Weak linear polarization (generally below 1%) is the result of extinction by elongated dust grains in the line of sight which are aligned in the interstellar magnetic field (the Davis-Greenstein effect, see section 2.1). Optical polarization surveys yielded the large-scale structure of the field in the local spiral arm of our Milky Way (section 3.1). The first extragalactic results by Hiltner (1958) were based on starlight polarization of globular clusters in M31 and showed that the magnetic field is aligned along the galaxy's major axis. Polarization of starlight in the LMC also gave evidence for ordered fields near 30 Dor (Mathewson & Ford 1970b) and possibly along the Magellanic stream (Schmidt 1976).

Polarization from diffuse optical light was used to search for large-scale magnetic fields, though some unknown fraction of the polarized light is due to scattering on dust particles. A survey of 70 nearby galaxies revealed degrees of polarization of ≤ 1% (Jones et al. 2012). Indications for ordered fields along the spiral arms were found in M82 (Elvius 1962), M51 and M81 (Appenzeller 1967; Scarrott et al. 1987), in NGC1068 (Scarrott et al. 1991) and in NGC6946 (Fendt et al. 1998). The pattern in M51 (Fig. 17) agrees well with the radio polarization results (see Fig. 23) in the inner spiral arms but large differences are seen in the outer arms and in the companion galaxy, which is unpolarized in the radio image. In the Sa-type edge-on Sombrero galaxy M104 and the Sb-type edge-on NGC4545, optical polarization indicates a field along the prominent dust lane and vertical fields above the plane (Scarrott et al. 1990), in agreement with the results from radio polarization (section 4.7).The polarization of the Sc-type edge-on galaxies NGC891 (Fig. 18), NGC5907 and NGC7331 shows fields near the galaxy plane which are predominantly oriented perpendicular to the plane (Fendt et al. 1996), possibly aligned along the vertical dust filaments observed in these galaxies. Radio continuum polarization, on the other hand, traces the magnetic fields in the diffuse medium which are mostly oriented parallel to the plane and have significant vertical components only beyond some height above the plane (section 4.7).

Figure 17

Figure 17. Spiral galaxy M51. E-vectors of the optical polarization of diffuse light which trace the spiral magnetic field orientation (Scarrott et al. 1987). Compare with the radio polarization map in Fig. 23.

Correction of the diffuse optical polarization for scattering effects is difficult and has never been attempted so far. Instead, polarization techniques were developed in the infrared where scattering is negligible. Near-IR polarization in a dust lane of the edge-on galaxy NGC4565 indicates a plane-parallel field (Jones 1989), similar to that seen at radio wavelengths. In the far-IR and sub-mm ranges, the emission of aligned dust grains is intrinsically polarized and the degrees of polarization can reach several %. The galaxy M82 was observed at 850 µm (Greaves et al. 2000), but the derived bubble-type field pattern is in contrast to the radio data indicating a field that is oriented radially outwards (Reuter et al. 1992), while near-IR polarimetry shows a vertical field (Jones 2000). Potential differences between IR, sub-mm and radio polarization data should be investigated with the polarimeters at the JCMT, APEX, ALMA and SOFIA telescopes.

Figure 18

Figure 18. Edge-on spiral galaxy NGC891. E-vectors of optical polarization of the diffuse light, indicating vertical magnetic fields (Fendt et al. 1996). Compare with the radio polarization map in Fig. 41.

Zeeman measurements in external galaxies are still very rare. Robishaw et al. (2008) detected the effect in the OH megamaser line at 18 cm in 5 distant starburst galaxies and derived field strengths in these dense gas clouds between 0.5 mG and 18 mG. Measurements in nearby galaxies will become possible with the Square Kilometre Array (section 5).

4.2. Magnetic field strengths

The dynamical importance of the total magnetic field B may be estimated by its energy density which is proportional to B2. Due to its vector nature, the dynamical effect of the magnetic field also depends on its structure and degree of ordering (section 4.4). The average strength of the component B of the total field and Bord,⊥ of the resolved ordered field in the plane of the sky can be derived from the total and polarized radio synchrotron intensity, respectively, if energy-density equipartition between total cosmic rays and total magnetic field B is valid (section 2.2). The field strengths B are given by the mean surface brightness (intensities) of synchrotron emission, hence do not depend on the distance to the galaxy.

The observed radio emission from galaxies has a contribution of thermal emission from ionized gas (and at frequencies beyond about 50 GHz also from dust) which needs to be subtracted to obtain the pure synchrotron part. The mean thermal fraction is about 10% at 21 cm and about 30% at 3 cm, but may increase to ≥ 50% in star-forming regions. A proper subtraction of the radio thermal intensity needs an independent thermal template, e.g. the Halpha intensity corrected for extinction with help of a dust model based on far-infrared data (Tabatabaei et al. 2007). For a crude separation of thermal and synchrotron intensity components, comparison of the observed radio spectral index with an assumed synchrotron spectral index is sufficient.

The average equipartition strength of the total field (corrected for inclination) for a sample of 74 spiral galaxies is B = 9 ± 2 µG (Niklas 1995). The average strength of 21 bright galaxies observed since 2000 is B = 17 ± 3 µG (Fletcher 2010). Dwarf galaxies host fields of similar strength as spirals if their star-formation rate per volume is similarly high. Blue compact dwarf galaxies are radio bright with equipartition field strengths of 10-20 µG (Klein et al. 1991). Spirals with moderate star -forming activity and moderate radio surface brightness like M31 (Fig. 26) and M33 (Fig. 36), our Milky Way's neighbors, have B ≈ 6 µG. In "grand-design" galaxies with massive star formation like M51 (Fig. 23), M83 (Fig. 24) and NGC6946 (Fig. 25), B ≈ 15 µG is a typical average strength of the total field.

In the density-wave spiral arms of M51 the total field strength B is 25-30 µG (Fig. 19). Field compression by external forces like interaction with other galaxies may amplify the fields (section 4.8). The strongest fields in spiral galaxies (50-300 µG) are found in starburst galaxies like M82 (Adebahr et al. 2013; Lacki & Beck 2013), the "Antennae" NGC4038 / 39 (Fig. 44), in nuclear starburst re gions, like in the nuclear ring of NGC1097 (Fig. 35) and of other barred galaxies, and in nuclear jets (Fig. 49).

Figure 19

Figure 19. Spiral galaxy M51. Total equipartition magnetic field strengths (in µG), corrected for the inclination of the galaxy (Fletcher et al. 2011).

If energy losses of cosmic-ray electrons are significant in starburst regions or massive spiral arms, the equipartition values are lower limits (section 2.2). The average equipartition field strength in normal spirals is proportional to the average gas surface density, but this relation is no longer valid for starburst galaxies (Thompson et al. 2006). Due to strong energy losses of the cosmic-ray electrons and even protons, the equipartition field strength is probably underestimated by a factor of a few. Field strengths of 0.5-18 mG were detected in starburst galaxies by the Zeeman effect in the OH megamaser emission line at 18 cm (Robishaw et al. 2008). However, these values refer to highly compressed gas clouds and are not typical for the interstellar medium.

The relative importance of various competing forces in the interstellar medium can be estimated by comparing the corresponding energy densities. In the local Milky Way, the energy densities of the stellar radiation field, turbulent gas motions, cosmic rays, and total magnetic fields are similar (Boulares & Cox 1990). The mean energy densities of the total magnetic field and of the cosmic rays in NGC6946 and M33 are ≈ 10-11 erg cm-3 and ≈ 10-12 erg cm-3, respectively (Beck 2007; Tabatabaei et al. 2008), similar to that of the turbulent motions of the cold, neutral gas with density across the star - forming disk (Fig. 20). The turbulent energy may be underestimated if vturb is larger than 7 km/s or if the warm gas also contributes. The energy density of the warm ionized gas Eth with electron density ne is one order of magnitude smaller than that of the total magnetic field EB, which means that the ISM in spiral galaxies is a low-beta plasma (beta = Eth / EB), similar to that of the Milky Way (Boulares & Cox 1990). Hot gas also contributes to Eth, but its contribution is small. The overall dominance of turbulent energy is surprising because the supersonic turbulence should heat the gas, but is also derived from numerical ISM simulations (de Avillez & Breitschwerdt 2005). Further investigations with higher resolution are needed.

The radial distribution of synchrotron intensity in many spiral galaxies is well described by an exponential decrease with a scalelength lsyn of about 4 kpc. In case of equipartition between the energy densities of magnetic fields and cosmic rays, the scalelength of the total field is (3 - alpha) ≈ lsyn 16 kpc (where alpha ≈ -1 is the synchrotron spectral index). The scalelength of the ordered field is even larger (Fig. 20). These are still lower limits because energy losses of cosmic-ray electrons increase with increasing distance from their origin in the galaxy's star-forming regions, and a lower density of cosmic-ray electrons needs a stronger field to explain the observed synchrotron intensity. Fields in the outer disk of galaxies can be amplified by the alpha - Omega dynamo even without star-formation activity because turbulence can be generated by the magneto-rotational instability (MRI, section 2.6). The typical scalelengths of the density of neutral and ionized gas are only about 3 kpc, so that the magnetic field energy dominates over the turbulent energy in the outer region of galaxies if a constant turbulent velocity is assumed (Fig. 20). The speculation that magnetic fields may affect the global gas rotation (Battaner & Florido 2000; Ruiz-Granados et al. 2010) needs testing by future radio observations with higher sensitivity and at low frequencies where energy loss of electrons is smaller.

Figure 20

Figure 20. Spiral galaxy NGC6946. Radial variation of the energy densities of the total magnetic field EB (B2 / 8pi), the ordered (mostly regular) magnetic field (Breg2 / 8pi), the turbulent motion of the neutral gas Eturb (0.5 rhon vturb2, where vturb ≈ 7 km/s), the thermal energy of the ionized gas Eth (0.5 ne k Te) and the thermal energy of the molecular gas En (0.5 rhon k Tn), determined from observations of synchrotron and thermal radio continuum, and the CO and HI line emissions (Beck 2007).

In spiral arms of galaxies the typical degree of radio polarization is only a few %. The total field B in the spiral arms must be mostly isotropic turbulent with random orientations within the telescope beam, which typically corresponds to a few 100 pc at the distance of nearby galaxies. The typical ratios of isotropic turbulent fields to resolved ordered fields are ≥ 5 in spiral arms and circumnuclear starburst regions, 0.5-2 in interarm regions and 1-3 in radio halos. Turbulent fields in spiral arms are probably generated by turbulent gas motions due to supernovae (de Avillez & Breitschwerdt 2005) or by spiral shocks (Dobbs & Price 2008), driving a small-scale dynamo (section 2.6).

Magnetic turbulence occurs over a large spectrum of scales. The maximum scale of the turbulence spectrum in the Milky Way derived from the dispersion of rotation measures of pulsars is d ≈ 50 pc (Rand & Kulkarni 1989). This scale can also be derived from the depolarization by the superposition of emission from turbulent fields at centimeter wavelengths (section 2.3). For a typical degree of polarization of 1% in spiral arms, 500 pc resolution in nearby galaxies and 1 kpc pathlength through the turbulent medium, d ≈ 40 pc f1/3 where f is the filling factor of the ionized medium. At decimeter radio wavelengths the same turbulent field causes Faraday dispersion (section 2.4). Typical depolarization of 50% at 20 cm, an average electron density of the thermal gas of 0.03 cm-3 and an average strength of the turbulent field of 10 µG yields d ≈ 10 pc / f. The two estimates agree for d ≈ 30 pc and f ≈ 0.3, consistent with the results derived with other methods.

Faraday dispersion can also be used to measure the strength of isotropic turbulent magnetic fields. However, the achievable accuracy is limited because the ionized gas density has to be determined from independent measurements. The increase of the mean degree of polarization at 1.4 GHz with increasing distance from the plane of edge-on galaxies can constrain the parameters and, for NGC891 and NGC4631, yields strengths of the isotropic turbulent magnetic fields in the plane of 11 µG and 7 µG and scale heights of 0.9 kpc and 1.3 kpc, respectively (Hummel et al. 1991).

The strength of the resolved ordered (regular and/or anisotropic turbulent) fields Bord in spiral galaxies is determined from the total equipartition field strength and the degree of polarization of the synchrotron emission. Present-day observations with typical spatial resolutions of a few 100 pc give average values of 1-5 µG. The ordered field is generally strongest in the regions between the optical spiral arms with peaks of about 12 µG e.g. in NGC6946, is oriented parallel to the adjacent optical spiral arms, and is stronger than the tangled field. In several galaxies like in NGC6946 the field forms coherent magnetic arms between the optical arms (Fig. 25). These are seen at all wavelengths and hence cannot be the effect of weak Faraday depolarization in the interarm regions. Magnetic arms are probably signatures of higher modes generated by the alpha - Omega dynamo (section 4.4). In galaxies with strong density waves some of the ordered field is concentrated at the inner edge of the spiral arms, e.g. in M51 (Fig. 23), but the arm-interarm contrast of the ordered field is small, much less than that of the isotropic turbulent field. The ordered field in M51 is smoothly distributed.

The regular (coherent) component of the ordered field can in principle be determined from Faraday rotation measures (section 2.4), if the mean electron density is known. In the Milky Way, the pulsar dispersion measure is a good measure of the total electron content along the pathlength to the pulsar. Only 19 extragalactic radio pulsars have been found so far, all in the LMC and SMC. In all other galaxies, the only source of information on electron densities of the warm ionized medium comes from thermal emission, e.g. in the H line. However, thermal emission is dominated by the HII regions which have a small volume filling factor, while Faraday rotation is dominated by the diffuse ionized emission with a much larger filling factor. If the average electron density of the diffuse ionized medium in the Milky Way of 0.03-0.05 cm-3 is assumed also for other galaxies, Faraday rotation measures yield regular field strengths of a few G. The strongest regular field of 8 µG was found in NGC6946 (Beck 2007), similar to the strength of the ordered field, hence most of the ordered field is regular in this galaxy. The similarity between the average regular (RM-based) and the ordered (equipartition- based) field strengths in NGC6946 and several other galaxies demonstrates that both methods are reliable and hence no major deviations from equipartition occur in this galaxy on scales of a few kpc (but deviations may occur locally).

The situation is different in radio-bright galaxies like M51, where the average regular field strength is several times smaller than the ordered field (section 4.4). The total field is strong, so that the energy loss of cosmic-ray electrons is high and the equipartition field is probably underestimated (section 2.2). This even increases the discrepancy between the two methods because the RM is not affected. The high-resolution observations of M51 indicate that anisotropic turbulent fields related to the strong density waves contribute mostly to the ordered field.

4.3. The radio - infrared correlation

The highest total radio intensity (tracing the total, mostly turbulent field) generally coincides with the strongest emission from dust and gas in the spiral arms. The total radio and far-infrared (FIR) or mid- IR (MIR) intensities are highly correlated within galaxies. The exponent of the correlation is different in the spiral arms and the interarm regions (Dumas et al. 2011; Basu et al. 2012). The magnetic field and its structure play an important role to understand the correlation (Tabatabaei et al. 2013a). The scale-dependent correlations (using wavelets) between the radio synchrotron and IR emissions are strong at large spatial scales, but break down below a scale of a few 100 pc, which can be regarded as a measure of the electron diffusion length that seems to depend on the degree of field ordering (Tabatabaei et al. 2013b). Differences in typical electron ages between galaxies may also play a role (Murphy et al. 2008).

Synchrotron intensity depends on the density of cosmic-ray electrons, which are accelerated in supernova remnants and diffuse into the interstellar medium, and on about the square of the strength of the total magnetic field B (section 2.2). Infrared intensity between wavelengths of about 20 µm and 70 µm (emitted from warm dust particles in thermal equilibrium, heated mainly by UV photons) is a measure of the star-formation rate. (Below about 20 µm wavelength, large PAH particles and stars contribute; emission beyond about 70 µm comes from cold dust which is heated by the general radiation field.) Hence, the radio-infrared correlation can be interpreted as a correlation between the strength of the isotropic turbulent field and the star-formation rate (Fig. 21). In NGC6946 this correlation has a smaller exponent of 0.16 ± 0.01 (Tabatabaei et al. 2013a). In contrast, the ordered field in NGC4254 is uncorrelated with the star-formation rate and weakly anticorrelated in NGC6946 where the ordered field is strongest in interarm regions with low star-formation rates (Tabatabaei et al. 2013a, see also section 4.4).

Figure 21

Figure 21. Spiral galaxy NGC4254. Correlation between the strength of the isotropic turbulent equipartition field and the star-formation rate per area (determined from the 24 µm infrared intensities) within the galaxy, plotted on logarithmic scales. The slope of the fitted line gives an exponent of 0.26 ± 0.01 (Chyzy 2008).

The radio-IR correlation requires that magnetic fields and star-formation processes are connected. In the "calorimeter" model, valid for starburst galaxies with s trong fields where energy losses of the cosmic-ray electrons are strong, B2 is assumed to increase with the infrared luminosity to obtain a linear radio-FIR correlation (Lisenfeld et al. 1996). In galaxies with low or medium star-formation rate (SFR), the cosmic-ray electrons can leave the galaxy and a combination of several processes with self-regulation is needed to explain the correlation within galaxies. If the dust is warm and optically thick to UV radiation, the IR intensity is proportional to the local SFR. Then, a possible scenario is the coupling of magnetic fields to the gas clouds (B ~ rhoa, where is the neutral gas density), the Schmidt- Kennicutt law of star formation (SFR ~ rhob) (Niklas & Beck 1997). Depending on the values of the exponents a and b and whether or not equipartition between the energy densities of magnetic fields and cosmic rays is valid, a linear or nonlinear radio-IR correlation is obtained (Dumas et al. 2011; Basu et al. 2012).

The radio-IR correlation also holds between the integrated luminosities of galaxies, which is one of the tightest correlations known in astronomy. Its explanation involves many physical parameters. The tightness needs multiple feedback mechanisms which are not yet understood (Lacki et al. 2010). The correlation holds for starburst galaxies up to redshifts of at least 4, and the average radio/IR ratio increases towards high redshifts (Murphy 2009). The detection of strong radio emission in distant galaxies (which is at least partly of synchrotron origin) demonstrates that magnetic fields existed already in the early Universe. A breakdown of the correlation is expected when the Inverse Compton loss of the cosmic-ray electrons dominates the synchrotron loss; the redshift of the breakdown gives information about the field evolution in young galaxies (Schleicher & Beck 2013).

Future radio telescopes like the SKA will allow the investigation of magnetic fields in young galaxies and search for their first fields (section 5). Faraday rotation of polarized QSO emission in intervening galaxies also reveals magnetic fields in distant galaxies if they are regular on the spatial scale corresponding to the angular size of the background source. With this method, significant regular fields of several µG strengths on scales of about 10 kpc were discovered in galaxies up to redshifts of about 2 (Bernet et al. 2008; Kronberg et al. 2008). Detection of regular fields in young galaxies is a critical test of alpha - Omega dynamo models (section 5).

4.4. Magnetic field structures in spiral galaxies

4.4.1. Ordered fields

At wavelengths ≤ 6 cm, Faraday rotation of the polarized synchrotron emission is generally small (except in central regions), so that the B-vectors directly trace the orientations of the ordered field (which can be regular or anisotropic turbulent, see section 2.3). Spiral patterns were found in almost every galaxy, even in those lacking optical spiral structure like the ringed galaxy NGC4736 (Fig. 22) and flocculent galaxies, while irregular galaxies show at most some patches of spiral structure (sections 4.6 and A.2). Spiral fields are also observed in the nuclear starburst regions of barred galaxies (section 4.5). Galaxies of type Sa and S0 and elliptical galaxies without an active nucleus have little star formation and hence produce only few cosmic rays that could emit synchrotron emission. The only deep observation of a Sa galaxy, M104 with a prominent dust ring, revealed weak, ordered magnetic fields (Krause et al. 2006).

Figure 22

Figure 22. Ring galaxy NGC4736. B-vectors of polarized radio intensity at 8.46 GHz (3.5 cm), observed with the VLA (Chyzy & Buta 2008). The background H image is from Johan Hendrik Knapen (Inst. Astr. de Canarias).

The gas flow in "smooth" galaxies (no bar, no tidal interaction, no strong density wave) is almost circular, while the field lines are spiral and do not follow the gas flow. If large-scale magnetic fields were frozen into the gas, differential rotation would wind them up to very small pitch angles. The observed smooth spiral patterns with significant pitch angles (10°- 40°, see Fletcher 2010) indicate a general decoupling between magnetic fields and the gas flow due to magnetic diffusivity, which is a strong indication for alpha - Omega dynamo action (section 2.6). There is no other model to explain the magnetic spiral patterns in many types of galaxies.

However, the spiral pattern of magnetic fields cannot be solely the result of alpha - Omega dynamo action. In gas-rich galaxies with strong density waves, the magnetic spiral pattern generally follows the spiral pattern of the gas arms. In the prototypical density-wave galaxy M51, for example, the pitch angle of the magnetic lines is mostly similar to that of the cold gas in the inner galaxy, but deviations occur in the outer parts of the galaxy, where the tidal effects of the companion galaxy are strong (Patrikeev et al. 2006). In dynamo theory, the pitch angle of the magnetic lines depends on global parameters (section 2.6) and is difficult to adjust to the pitch angle of the spiral structure of the gas. In the outer galaxy, ordered fields coincide with the outer southern and south-western spiral arms; these are possibly tidal arms with strong shear. The north-eastern field deviates from the gas arm and points towards the companion, signature of the interaction.

If the beautiful spiral pattern of M51 seen in radio polarization (Fig. 23) is due to a regular field, it should be accompanied by a large-scale pattern in Faraday rotation, which is not observed. This means that most of the ordered field is anisotropic turbulent and probably generated by compression and shear of the non-axisymmetric gas flow in the density-wave potential. From an analysis of dispersions of the radio polarization angles at 6.2cm in M51, Houde et al. (2013) measured a ratio of the correlation lengths parallel and perpendicular to the local ordered magnetic field of 1.83 ± 0.13. The anisotropic field is strongest at the positions of the prominent dust lanes on the inner edge of the inner gas spiral arms, due to compression of isotropic turbulent fields in the density-wave shock. Anisotropic fields also fill the interarm space, without signs of compression, probably generated by shearing flows. Regular fields also exist in M51 but are much weaker (see below).

Figure 23

Figure 23. Spiral galaxy M51. Total radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), combined from observations with the VLA and Effelsberg 100-m telescopes (Fletcher et al. 2011). The background optical image is from the HST (Hubble Heritage Team). Graphics: Sterne und Weltraum.

M83 (Fig. 24) and NGC2997 (Han et al. 1999) are cases similar to M51, with enhanced ordered (anisotropic) fields at the inner edges of the inner optical arms, ordered fields in interarm regions and ordered fields coinciding with the outer optical arms. Density-wave galaxies with less star-formation activity, like M81 (Krause et al. 1989b) and NGC1566 (Ehle et al. 1996), show little signs of field compression and the ordered fields occur mainly in the interarm regions.

Figure 24

Figure 24. Barred galaxy M83. Polarized radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), combined from observations with the VLA and Effelsberg telescopes (Beck et al., unpublished). The background optical image is from Dave Malin (Anglo Australian Observatory).

Observations of another gas-rich spiral galaxy, NGC6946, revealed a surprisingly regular distribution of polarized emission with two symmetric magnetic arms located in interarm regions, with orientations parallel to the adjacent optical spiral arms and no signs of compression at the inner edge of the gas arms (Fig. 25). Their degree of polarization is exceptionally high (up to 50%); the field is almost totally ordered and mostly regular, as indicated by Faraday rotation measures. With the higher sensitivity at 20 cm wavelength, more magnetic arms appear in the northern half of NGC6946, extending far beyond the optical arms, but located between outer HI arms. Magnetic arms have also been found in M83 (Fig. 24), NGC2997 and several other gas-rich spiral galaxies. Magnetic arms can be explained in the framework of dynamo models (section 2.6).

Figure 25

Figure 25. Spiral galaxy NGC6946. Polarized radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), combined from observations with the VLA and Effelsberg 100-m telescopes (Beck 2007). The background Halpha image is from Anne Ferguson. Graphics: Sterne und Weltraum.

Ordered magnetic fields may also form spiral features that are disconnected from the optical spiral pattern. Long, highly polarized filaments were discovered in the outer regions of IC342 where only faint arms of HI line emission exist (Krause et al. 1989a). More recent observations at 20 cm revealed a system of such features extending to large distances from the center (Fig. 27).

In the highly inclined Andromeda galaxy, M31 (Fig. 26), the spiral arms are hard to distinguish due to the insufficient angular resolution. Star formation activity is concentrated to a limited radial range at around 10 kpc distance from the center (the "ring"). The ordered fields are strongest in the massive dust lanes where the degree of polarization is about 40%. The fie ld follows the "ring" with a coherent direction (Fig. 29).

Figure 26

Figure 26. Spiral galaxy M31. Total radio intensity (colors) and B-vectors (corrected for Faraday rotation) at 4.75 GHz (6.3 cm), observed with the Effelsberg telescope (Berkhuijsen et al. 2003).

At wavelengths of around 20 cm, a striking asymmetry of the polarized emission occurs along the major axis of all 12 spiral galaxies observed so far with sufficiently high sensitivity that have inclinations of less than about 60°. The emission is almost completely depolarized by Faraday dispersion, e.g. in IC342 (Fig. 27) on one side of the major axis, which is always the kinematically receding one (positive radial velocities). In strongly inclined galaxies, both sides of the major axis become Faraday-depolarized at around 20 cm, as a result of the long pathlength. The asymmetry is still visible at 11 cm, but disappears at smaller wavelengths. This tells us that, in addition to spiral fields in the disk, fields in the halo are needed, as predicted by alpha - Omega dynamo models (Urbanik et al. 1997; Braun et al. 2010; see section 4.7). The effect of such halo fields becomes prominent at 20 cm because most of the polarized emission from the disk is Faraday-depolarized (section 2.4). Testing by observations at longer wavelengths will soon become possible with LOFAR (section 5).

Figure 27

Figure 27. Spiral galaxy IC342. Polarized radio intensity at 1.46 GHz (20.5 cm), combined from VLA C- and D-array observations. The field size is about 30' × 28'. The inner and north-western parts are depolarized at this wavelength (Beck, unpublished).

4.4.2. Regular fields

Ordered magnetic fields as observed by polarized emission can be anisotropic (see above) or regular (with a coherent direction). Faraday rotation measures (RM) are signatures of such regular fields. RM is determined from multi-wavelength radio polarization observations (section 2.4). Spiral dynamo modes (section 2.6) can be identified from the periodicity of the azimuthal variation of RM in inclined galaxy disks (Fig. 28), where the RM can be determined from diffuse polarized emission (Krause 1990) or from RM data of polarized background sources (Stepanov et al. 2008). If several dynamo modes are superimposed, Fourier analysis of the RM variation is needed. The resolution of present-day observations is sufficient to identify not more than 2-3 modes.

Figure 28

Figure 28. Azimuthal RM variations (measured from the major axis) for axisymmetric spiral (ASS) and bisymmetric spiral (BSS) fields in inclined galaxies (Krause 1990).

The disks of a few spiral galaxies indeed reveal large-scale RM patterns giving strong evidence for modes generated by the alpha - Omega dynamo. M31 is the prototype of a dynamo-generated magnetic field (Fig. 29). The discovery became possible thanks to the large angular extent and the high inclination of M31. The polarized intensity at 6 cm is largest near the minor axis where the field component B is largest (Fig. 30a), while the maxima in |RM| are observed near the major axis where the line-of-sight field component B|| is strongest (Fig. 30b). This single-periodic RM variation is a clear signature of a dominating axisymmetric spiral (ASS) disk field (dynamo mode m = 0) (Fletcher et al. 2004), which extends to at least 25 kpc distance from the center when observed with an RM grid (see below) (Han et al. 1998).

Figure 29

Figure 29. Spiral galaxy M31. Total radio intensity at 4.75 GHz (6.3 cm) (contours), B-vectors and Faraday rotation measures between 4.75 GHz (6.3 cm) and 2.7 GHz (11.1 cm) (colors), derived from observations with the Effelsberg telescope (Berkhuijsen et al. 2003). The average rotation measure of about -90 rad/m2 is caused by the foreground medium in the Milky Way.

Figure 30

Figure 30. Spiral galaxy M31. (a) Polarized intensity and (b) Faraday rotation measures between 4.75 GHz (6.3 cm) and 2.7 GHz (11.1 cm) along the azimuthal angle in the plane of the galaxy, counted counterclockwise from the northern major axis (left side in Fig. 29) (Berkhuijsen et al. 2003).

Other galaxies with a dominating ASS disk field are the nearby spiral IC342, the Virgo galaxy NGC4254, the almost edge-on galaxies NGC253, NGC891 and NGC5775, the irregular Large Magellanic Cloud (LMC) and a few further candidates (see Appendix).

By measuring the signs of the RM distribution and the velocity field on both sides of a galaxy's major axis, the inward and outward directions of the radial component of the ASS field can be easily distinguished (Fig. 31). Dynamo models predict that both signs have the same probability, which is confirmed by observations. The ASS fields of M31, IC342, NGC253 and the ASS field component in NGC6946 point inwards, while those of NGC4254, NGC5775 and the ASS component of the disk field in M51 point outwards.

Figure 31

Figure 31. The sign of the Faraday rotation measure RM and the sign of the rotation velocity component vr along the line of sight, measured near the major axis of a galaxy, are opposite in the case of the inward direction of the radial component of an ASS-type field, while the signs are the same for the outward field direction. Trailing spirals are assumed (Krause & Beck 1998).

M81, M83 and an intervening galaxy at a redshift of 0.4 in front of the quasar PKS1229-021 (Kronberg et al. 1992) are the only candidates so far for a bisymmetric spiral (BSS) field (m = 1), characterized by a double-periodic RM variation, but the data quality is limited in all these cases. Dominating BSS fields are rare, as predicted by dynamo models. It was proposed that tidal interaction can excite the BSS mode, but no preference for BSS was found even in the most heavily interacting galaxies in the Virgo cluster (section 4.8). The idea that galactic fields are wound-up primordial intergalactic fields that are of BSS type (section 2.6) can also be excluded from the existing observations.

Faraday rotation in NGC6946 (Fig. 32) and in other similar galaxies with magnetic arms can be described by a superposition of two azimuthal dynamo modes (m = 0 and m = 2) with about equal amplitudes, where the quadrisymmetric spiral (QSS) m = 2 mode is phase shifted with respect to the density wave (Beck 2007). This model is based on the RM pattern of NGC6946 that shows different field directions in the northern and southern magnetic arm (Fig. 32). A weaker QSS mode superimposed onto the dominating ASS mode is indicated in the disk of M51 and in the inner part of M31. A superposition of ASS and BSS modes can describe the fields of M33 and NGC4254, while three modes (ASS+BSS+QSS) are needed for several other galaxies (Fletcher 2010; Appendix, Table 5).

Figure 32

Figure 32. Spiral galaxy NGC6946. Total radio intensity at 4.86 GHz (6.2 cm) (contours) and Faraday rotation measures between 8.46 GHz (3.5 cm) and 4.86 GHz (6.2 cm) (colors), derived from combined observations with the VLA and Effelsberg telescopes (Beck 2007). The average rotation measure of about +50 rad/m2 is caused by the foreground medium in the Milky Way.

In most galaxies observed so far, a spiral polarization pattern was found, but no large-scale RM pattern as a signature of regular fields. In many cases the available polarization data is insufficient to derive reliable RMs. In other cases the data quality is high but no large-scale RM patterns are visible. In density-wave galaxies, strong compression and shearing flows generate anisotropic fields (with frequent reversals) of spiral shape which are much stronger than the underlying regular field, like in M51 (see above). In galaxies without density waves, several dynamo modes may be superimposed but cannot be distinguished with the limited sensitivity and resolution of present-day telescopes. Another explanation is that the timescale for the generation of large-scale modes is longer than the galaxy's lifetime, so that the regular field is not fully organized and still restricted to small regions.

Large-scale field reversals were discovered from pulsar RMs in the Milky Way (section 3.5), but nothing similar has yet been detected in spiral galaxies, although high-resolution RM maps of Faraday rotation are available for many spiral galaxies. In M81 the dominating BSS field implies two large-scale reversals (Krause et al. 1989b). The disk fields of several galaxies can be described by a mixture of modes where reversals may emerge in a limited radial and azimuthal range of the disk, like in NGC4414 (Soida et al. 2002). However, no multiple reversals along the radial direction, like those in the Milky Way, were found so far in the disk of any external galaxy. A satisfying explanation is still lacking (section 3.5). Reversals on smaller scales are probably frequent but difficult to observe in external galaxies with the resolution of present-day telescopes. Only in the barred galaxy NGC7479, where a jet serves as a bright polarized background (Fig. 50), several reversals on 1-2 kpc scale were detected in the foreground disk of the galaxy (Laine & Beck 2008).

While the azimuthal symmetry of the dynamo modes is known for many galaxies, the vertical symmetry (even or odd) is much harder to determine. The RM patterns of even and odd modes are similar in mildly inclined galaxies. The toroidal field of odd modes reverses its sign above and below the galactic plane. Thus, in a mildly inclined odd field, half of the RM is observed compared to that in an even field, which cannot be distinguished in view of the large RM variations caused by ionized gas density and field strength. The symmetry type becomes only visible in strongly inclined galaxies, as a RM sign reversal above and below the plane. Only even-symmetry fields were found so far (in M31, NGC253, NGC891 and NGC5775), in agreement with the prediction of dynamo models (section 4.7).

If polarized emission is too weak to be detected, the method of RM grids towards polarized background QSOs can still be applied. This allows the determination of a large-scale field pattern in an intervening galaxy on the line of sight (Kronberg et al. 1992). Here, the distance limit is given by the polarized flux of the background QSO which can be much larger than that of the intervening galaxy, so that this method can be applied to much larger distances than the analysis of RM of the polarized emission from the foreground galaxy itself. At least 10 randomly distributed background sources behind the galaxy disk are needed to recognize simple patterns and several 1000 sources for a full reconstruction (Stepanov et al. 2008). Present-day observations are not sensitive enough, and one has to wait for the SKA and its precursor telescopes.

Ordered fields of nearby galaxies seen edge-on near the disk plane are preferably oriented parallel to the plane (section 4.7). As a result, polarized emission can be detected from distant, unresolved galaxies if they are symmetric (not distorted by interaction) and their inclination is larger than about 20° (Stil et al. 2009). This opens another method to search for ordered fields in distant galaxies. As the plane of polarization is almost independent of wavelength, distant spiral galaxies with known orientation of their major axis can also serve as background polarized sources to search for Faraday rotation by intergalactic fields in the foreground.

In summary, magnetic field structures in spiral galaxies are complex. The observations can best be explained as a superposition of dynamo-generated modes of regular fields coupled to the diffuse warm gas, plus anisotropic turbulent fields by shearing and compressing flows, plus isotropic turbulent fields coupled to the cold gas. The magnetic fields in barred galaxies behave similarly (section 4.5). For a more detailed model of the physics of the field-gas interaction, high-resolution data with future telescopes are required (section 5).

4.5. Magnetic fields in barred galaxies

Gas and stars in the gravitational potential of strongly barred galaxies move in highly noncircular orbits. Numerical models show that gas streamlines are deflected in the bar region along shock fronts, behind which the cold gas is compressed in a fast shearing flow (Athanassoula 1992). The compression regions traced by massive dust lanes develop along the edge of the bar that is leading with respect to the galaxy's rotation because the gas rotates faster than the bar pattern. The warm, diffuse gas has a higher sound speed and is not compressed. According to simulations, the shearing flows around a bar should amplify magnetic fields and generate complicated field patterns changing with time (Otmianowska-Mazur et al. 2002). The asymmetric gas flow may also enhance dynamo action and excite the QSS (m = 2) mode (Moss et al. 2001).

20 galaxies with large bars were observed with the Very Large Array (VLA) and with the Australia Telescope Compact Array (ATCA) (Beck et al. 2002; 2005a). The total radio luminosity (a measure of the total magnetic field strength) is strongest in galaxies with high far-infrared luminosity (indicating high star-formation activity), a result similar to that in non-barred galaxies. The average radio intensity, radio luminosity and star-formation activity all correlate with the relative bar length. Polarized emission was detected in 17 of the 20 barred galaxies. The pattern of the ordered field in the galaxies with long bars (NGC1097, 1365, 1559, 1672, 2442 and 7552) is significantly different from that in non-barred galaxies: Field enhancements occur outside of the bar (upstream), and the field lines are oriented at large angles with respect to the bar.

NGC1097 (Fig. 33) is one of the nearest barred galaxies and hosts a huge bar of about 16 kpc length. The total radio intensity (not shown in the figure) and the polarized intensity are strongest in the downstream region of the dust lanes (southeast of the center). This can be explained by a compression of isotropic turbulent fields in the bar's shock, leading to strong and anisotropic turbulent fields in the downstream region. The surprising result is that the polarized intensity is also strong in the upstream region (south of the center in Fig. 33) where RM data indicate that the field is regular. The pattern of field lines in NGC1097 is similar to that of the gas streamlines as obtained in numerical simulations (Athanassoula 1992). This suggests that the ordered (partly regular) magnetic field is aligned with the flow and amplified by strong shear. Remarkably, the optical image of NGC1097 shows dust filaments in the upstream region which are almost perpendicular to the bar and thus aligned with the ordered field. Between the region upstream of the southern bar and the downstream region the field lines smoothly change their orientation by almost 90°. The ordered field is probably coupled to the diffuse gas and thus avoids being shocked in the bar. The magnetic energy density in the upstream region is sufficiently high to affect the flow of the diffuse gas.

Figure 33

Figure 33. Southern half of the barred galaxy NGC1097. Total radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), observed with the VLA (Beck et al. 2005a). The background optical image is from Halton Arp (Cerro Tololo Observatory).

NGC1365 (Fig. 34) is similar to NGC1097 in its overall properties, but the polarization data indicate that the shear is weaker. The ordered field bends more smoothly from the upstream region into the bar, again with no indication of a shock. M83 is the nearest barred galaxy but with a short bar; it shows compressed ordered fields at the leading edges of the bar on both sides of the nucleus and some polarization in the upstream regions (Fig. 24). In all other galaxies observed so far (section A.2) the resolution is insufficient to separate the bar and upstream regions.

Figure 34

Figure 34. Barred galaxy NGC1365. Total radio intensity (contours) and B-vector at 4.86 GHz (6.2 cm), observed with the VLA (Beck et al. 2005a). The background optical image is from Per Olof Lindblad (ESO).

The central regions of barred galaxies are often sites of ongoing intense star formation and strong magnetic fields that can affect the gas flow. Radio emission from ring-like regions has been found in NGC1097, NGC1672, and NGC7552 (Beck et al. 2005b). NGC1097 hosts a bright ring with about 1.5 kpc diameter and an active nucleus in its center (Fig. 35). The ordered field in the ring has a spiral pattern and extends towards the nucleus. The orientation of the innermost spiral field agrees with that of the spiral dust filaments visible on optical images. Magnetic stress in the circumnuclear ring can drive mass inflow at a rate of dM / dt = -h / Omega (<br b> + Br BPhi), where h is the scale height of the gas, Omega its angular rotation velocity, b the strength of the turbulent field and B that of the ordered field, and r and Phi denote the radial and azimuthal field components (Balbus & Hawley 1998). For NGC1097, h ≈ 100 pc, v ≈ 450 km/s at 1 kpc radius, br ≈ bPhi ≈ 50 µG gives an inflow rate of several Modot / yr, which is sufficient to fuel the activity of the nucleus (Beck et al. 2005a).

Figure 35

Figure 35. Central star-forming ring of the barred galaxy NGC1097. Total radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), observed with the VLA (Beck et al. 2005a). The background optical image is from the Hubble Space Telescope.

In summary, the isotropic turbulent field in galaxies with massive bars is coupled to the cold gas and compressed in the bar's shock. The ordered field outside the bar region follows the general flow of the cold and warm gas, possibly due to shear, but decouples from the cold gas in front of the shock and goes with the diffuse warm gas. The polarization pattern in barred galaxies can be used a tracer of the flow of diffuse gas in the sky plane and hence complements spectroscopic measurements of radial velocities. Detailed comparisons between polarimetric and spectroscopic data are required, as well as MHD models including the back-reaction of the magnetic fields onto the gas flow.

Radio polarization data have revealed differences but also similarities between the behaviours of ordered magnetic fields in barred and non-barred galaxies. In galaxies without bars and without strong density waves the field lines have a spiral shape, they do not follow the gas flow and are probably amplified by dynamo action. In galaxies with massive bars or strong density waves the field lines mostly follow the flow of the diffuse warm gas. Near the shock fronts galaxies with strong bars and with strong density waves (section 4.4) reveal a similar behaviour: Isotropic turbulent fields are coupled to the cold gas, are shocked and become anisotropic turbulent, while regular fields are coupled to the warm diffuse gas and hence avoid the shock.

4.6. Flocculent and irregular galaxies

Flocculent galaxies have disks but no prominent spiral arms. Nevertheless, spiral magnetic patterns are observed in all flocculent galaxies, indicative that the alpha - Omega dynamo works independently of density waves. The multi-wavelength data of M33 and NGC4414 call for a mixture of dynamo modes or an even more complicated field structure (Appendix, Table 5). Ordered magnetic fields with strengths similar to those in grand-design spiral galaxies have been detected in the flocculent galaxies M33 (Fig. 36), NGC3521, NGC5055 and in NGC4414, and also the mean degree of polarization is similar between grand-design and flocculent galaxies (Knapik et al. 2000).

Figure 36

Figure 36. Flocculent galaxy M33. Total radio intensity (colors) and B-vectors of the flocculent galaxy at 8.35 GHz (3.6 cm), observed with the Effelsberg telescope (Tabatabaei et al. 2008).

Radio continuum maps of irregular, slowly rotating galaxies may reveal strong total equipartition magnetic fields, e.g. in the Magellanic-type galaxy NGC4449 (Fig. 37) and in IC10 (Fig. 38). In NGC4449 some fraction of the field is ordered with about 7 µG strength and a spiral pattern. Faraday rotation shows that this ordered field is partly regular and the alpha - Omega dynamo is operating in this galaxy. The total field is of comparable strength (10-15 µG) in starburst dwarfs like NGC1569 (Kepley et al. 2010) where star formation activity is sufficiently high for the operation of the small-scale dynamo (section 2.6). In these galaxies the energy density of the magnetic fields is only slightly smaller than that of the (chaotic) rotation of the gas and thus may affect the evolution of the whole system. The starburst dwarf galaxy NGC1569 shows polarized emission, but no large-scale regular field. In dwarf galaxies with very weak star-forming activity, no polarized emission is detected and the isotropic turbulent field strength is several times smaller than in spiral galaxies (Chyzy et al. 2011), sometimes less than 5 µG (Chyzy et al. 2003). The latter value may indicate a sensitivity limit of present -day observations or a threshold for small-scale dynamo action.

Figure 37

Figure 37. Magellanic-type galaxy NGC4449. Total radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), combined from VLA and Effelsberg observations (Chyzy et al. 2000). The background image shows the H emission.

Figure 38

Figure 38. Irregular galaxy IC10. Total radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), observed with the VLA (from Chris Chyzy, Kraków University). The background H image is from Dominik Bomans (Bochum University).

The Magellanic Clouds are the closest irregular galaxies and deserve special attention. Polarization surveys with the Parkes single-dish telescope at several wavelengths had low angular resolution and revealed weak polarized emission. Two magnetic filaments were found in the LMC south of the 30 Dor star-formation complex (Klein et al. 1993). ATCA surveys of an RM grid towards background sources show that the LMC probably contains a large-scale magnetic field similar to large spirals (Gaensler et al. 2005) and that the field of the SMC is weak and uniformly directed away from us, possibly part of a pan-Magellanic field joining the two galaxies (Mao et al. 2008).

4.7 Radio halos

Radio halos are observed around the disks of most edge-on galaxies, but their radio intensity and extent varies significantly. The halo luminosity in the radio range correlates with those in H and X - rays (Tüllmann et al. 2006), although the detailed halo shapes vary strongly between the different spectral ranges. These results suggest that star formation in the disk is the energy source for halo formation and the halo size is determined by the energy input from supernova explosions per surface area in the projected disk (Dahlem et al. 1995).

In spite of the different intensities and extents of radio halos, their exponential scale heights at 5 GHz are about 1.8 kpc (Dumke & Krause 1998; Heesen 2009a), with a surprisingly small scatter in the sample, ranging from one of the weakest halos, NGC4565, to the brightest ones known, NGC253 (Fig. 39) and NGC891 (Fig. 41). In case of equipartition between the energy densities of magnetic field and cosmic rays, the exponential scale height of the total field is at least (3 - alpha) times larger than the synchrotron scale height (where alpha ≈ -1 is the synchrotron spectral index), ≥ 7 kpc. The real value depends on the energy losses of the cosmic-ray electrons propagating into the halo (section 2.2). A prominent exception is NGC4631 with the largest radio halo observed so far (Fig. 42). With large scale heights, the magnetic energy density in halos is much higher than that of the thermal gas, while still lower than that of the dominating kinetic energy of the gas outflow.

Figure 39

Figure 39. Almost edge-on spiral galaxy NGC253. Total radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), combined from observations with the VLA and the Effelsberg telescope (Heesen et al. 2009b)

Figure 40

Figure 40. Synchrotron scaleheights of the northern radio halo of NGC253 at different distances from the center and at different wavelengths, as a function of synchrotron lifetime of cosmic-ray electrons. The slope of the linear fit corresponds to a bulk outflow speed of about 300 km/s (Heesen et al. 2009a).

Figure 41

Figure 41. Edge-on spiral galaxy NGC891. Total radio intensity (contours) and B-vectors at 8.35 GHz (3.6 cm), observed with the Effelsberg telescope (Krause 2009). The background optical image is from the CFHT.

Radio halos grow in size with decreasing observation frequency. The extent is limited by energy losses of the cosmic-ray electrons, i.e. synchrotron, inverse Compton and adiabatic losses (Heesen et al. 2009a). The stronger magnetic field in the central region causes stronger synchrotron loss, leading to the "dumbbell" shape of many radio halos, e.g. around NGC253 (Fig. 39). From the radio scale heights of NGC253 at three frequencies and the electron lifetimes (due to synchrotron, inverse Compton and adiabatic losses) an outflow bulk speed of about 300 km/s was measured (Fig. 40). The similarity of the scale height of the radio halos around most edge-on galaxies observed so far, in spite of the different field strengths and hence different electron lifetimes, indicates that the outflow speed increases with the average strength of the total field and with the star-formation rate (Krause 2009). Outflows slower than the escape velocity are often called fountain flows, while escaping flows are galactic winds.

Radio polarization observations of nearby galaxies seen edge-on generally show a disk-parallel field near the disk plane (Dumke et al. 1995). High-sensitivity observations of several edge-on galaxies like NGC253 (Fig. 39), NGC891 (Fig. 41), NGC5775 (Tüllmann et al. 2000, Soida et al. 2011) and M104 (Krause et al. 2006) revealed vertical field components which increase with increasing height above and below the galactic plane and also with increasing radius, the so-called X-shaped halo fields. The X-pattern is even seen in NGC4565 with its low star-formation rate and a radio-faint halo, thus this pattern seems to be a general phenomenon.

The observation of X-shaped field patterns is of fundamental importance to understand the field origin in halos. The field is probably transported from the disk into the halo by an outflow emerging from the disk. The X-shaped halo field is consistent with the predictions from alpha - Omega dynamo models if outflows with moderate velocities are included (section 2.6). Numerical models (neglecting magnetic fields) indicate that global gas outflows from the disks of young galaxies can also be X-shaped due to pressure gradients (Dalla Vecchia & Schaye 2008). MHD models are still lacking.

The exceptionally large radio halos around the irregular and interacting galaxies M82 (Reuter et al. 1992) and NGC4631 (Fig. 42) exhibit X-shaped halo fields with almost radial orientations in their inner regions. This indicates that the wind transport is more efficient here than in spiral galaxies. The small gravitational potential of irregular galaxies or external forces by neighboring galaxies may be responsible for high outflow velocities. The alpha - Omega dynamo cannot operate under such conditions. The radio halos of M82 and NGC4631 were resolved into a few magnetic spurs, emerging from star- forming regions in the disk (Golla & Hummel 1994). These observations also support the idea of a fast galactic outflow which is driven by regions of star formation activity in the disk. The outflow cone of the starburst galaxy NGC253 hosts a helical magnetic field (Heesen et al. 2011).

Figure 42

Figure 42. Edge-on irregular galaxy NGC4631. Total radio intensity (contours) and B-vector at 8.35 GHz (3.6 cm), observed with the Effelsberg telescope (Krause 2009). The background optical image is from the Misti Mountain Observatory.

Polarization "vectors" do not distinguish between halo fields which are sheared into elongated loops or regular dynamo-type fields. A large-scale regular field can be measured only by Faraday rotation measures (RM) (section 2.4). RM patterns are very hard to measure in halos, because the field components along the line of sight are small. The detailed analysis of the multi-frequency observations of the highly inclined galaxy NGC253 (Fig. 39) allowed to identify an axisymmetric disk field with even symmetry and an X-shaped halo field, also of even symmetry (Fig. 43). The combined analysis of RMs of the diffuse emission and extragalactic sources revealed an even-symmetry halo field in the LMC (Mao et al. 2012). The polarization asymmetry along the major axis observed around 20 cm in all spiral galaxies with less than 60° inclination observed so far gives further evidence that galaxies host even-parity fields (Urbanik et al. 1997; Braun et al. 2010; Vollmer et al. 2013).

Figure 43

Figure 43. Model of the symmetric (outwards-directed) halo field of NGC253. The spiral disk field is also symmetric with respect to the plane (from Heesen et al. 2009b).

Dynamo models for thin galaxy disks predict fields of even symmetry, in the simplest case a poloidal component of quadrupolar shape (section 2.6). The vertical component of such a quadrupolar field is largest near the rotation axis and decreases with distance from the rotation axis. Such an effect is possibly seen in NGC4631 (Fig. 42), while in several other edge-on galaxies the vertical field component increases with increasing distance from the rotation axis, giving rise to X-shapes. Furthermore, the field strength of a pure quadrupole-type field decreases rapidly with distance from the center (e.g. Prouza & Smída 2003), while the observed radial profiles of polarized emission show a slow exponential decrease. The field structure cannot be a pure quadrupole. For example, dynamo models including winds can generate X-shaped fields (section 2.6).

In summary, the detection of X-shaped fields in all galaxies observed so far can be explained by dynamo action and/or outflows. If outflows are a general phenomenon in galaxies, they can magnetize the intergalactic medium (IGM). Starburst dwarf galaxies in the early Universe were especially efficient in magnetizing the IGM. The extent of magnetic fields into the IGM is not yet visible. Energy losses of the cosmic-ray electrons prevent the emission of radio waves beyond some height while magnetic fields may still exist much further outwards. Low-energy electrons live longer, can propagate further into the IGM and emit synchrotron emission at low frequencies (section 2.2). Forthcoming observations with the Low Frequency Array (LOFAR) may reveal larger radio halos (section 5).

4.8. Interacting galaxies

Gravitational interaction between galaxies leads to asymmetric gas flows, compression, shear, enhanced turbulence, and outflows. Compression and shear of gas flows can also modify the structure of galactic and intergalactic magnetic fields. In particular, fields can become aligned along the compression front or perpendicular to the velocity gradients. Such gas flows make turbulent fields highly anisotropic.

The classical interacting galaxy pair is NGC4038 / 39, the "Antennae" (Fig. 44). It shows bright, extended radio emission filling the volume of the whole system, with no dominant nuclear sources. In the interaction region between the galaxies, where star formation did not yet start, and at the northeastern edge of the system, the magnetic field is partly ordered, probably the result of compression and shearing motions along the tidal tail, respectively. Particularly strong, almost unpolarized emission comes from a region of violent star formation, hidden in dust, at the southern end of a dense cloud complex extending between the galaxies. In this region, highly turbulent magnetic fields reach strengths of ≈ 30 µG. The mean total magnetic field is stronger than in normal spirals, but the mean degree of polarization is unusually low, implying that the ordered field, generated by compression, has become tangled in the region with violent star formation. After an interaction, the magnetic field strength in a galaxy decreases again to its normal value (Drzazga et al. 2011).

Figure 44

Figure 44. "Antennae" galaxy pair NGC4038 / 39. B-vectors of polarized radio intensity at 4.86 GHz (6.2 cm), observed with the VLA (Chyzy & Beck 2004). The background optical image is from the Hubble Space Telescope.

Interaction with a dense intergalactic medium also imprints unique signatures onto magnetic fields and thus the radio emission. The Virgo cluster is a location of especially strong interaction effects, and almost all cluster galaxies observed so far show asymmetries of their polarized emission (Appendix, Table 7). In NGC4254, NGC4522 and NGC4535 (Fig. 45), the polarized emission on one side of the galaxy is shifted towards the edge of the spiral arm, an indication for shear by tidal tails or ram pressure by the intracluster medium. The heavily disrupted galaxy NGC4438 (Vollmer et al. 2007) has almost its whole radio emission (total power and polarized) displaced towards the giant elliptical M86 to which it is also connected by a chain of H-emitting filaments.

Figure 45

Figure 45. Spiral galaxy NGC4535 in the Virgo cluster. Polarized radio intensity (contours) and B-vectors at 4.75 GHz (6.3 cm), observed with the Effelsberg telescope (Wegowiec et al. 2007). The background optical image is from the Digital Sky Survey.

Interaction may also induce violent star-formation activity in the nuclear region or in the disk which may produce huge radio lobes due to outflowing gas and magnetic field. The lobes of the Virgo spiral NGC4569 reach out to at least 24 kpc from the disk and are highly polarized (Fig. 46). However, there is neither an active nucleus nor a recent starburst in the disk, so that the radio lobes are probably the result of nuclear activity in the past.

Figure 46

Figure 46. Spiral galaxy NGC4569 in the Virgo cluster. Polarized radio intensity (contours) and B-vectors at 4.75 GHz (6.3 cm), observed with the Effelsberg telescope (Chyzy et al. 2006). The background optical image is from the Digital Sky Survey.

Tidal interaction is also the probable cause of the asymmetric appearance of NGC3627 within the Leo Triplet (Fig. 47). While the ordered field in the western half is strong and precisely follows the dust lanes, a bright magnetic arm in the eastern half crosses the optical arm and its massive dust lane at a large angle. No counterpart of this feature was detected in any other spectral range. Either the optical arm was recently deformed due to interaction or ram pressure, or the magnetic arm is an out-of-plane feature generated by interaction.

Figure 47

Figure 47. Interacting spiral galaxy NGC3627. Polarized radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), combined from observations with the VLA and Effelsberg telescopes (Soida et al. 2001). The background optical image is from the Hubble Space Telescope.

In a few cases a radio and gaseous bridge has been found between colliding galaxies. The radio emission is due to relativistic electrons pulled out from the disks together with gas and magnetic fields. This phenomenon (called "taffy galaxies") seems to be rare because only 2 objects, UGC12914 / 5 and UGC813 / 6, were found so far (Condon et al. 2002; Drzazga et al. 2011). This may be due to the steep spectrum of the bridges, making them invisible at centimeter wavelengths in weaker objects.

In compact galaxy groups tidal interactions may trigger rapid star formation in one or more member galaxies, causing supersonic outflows of hot gas. Some compact groups have long HI tails, indicating strong, tidally-driven outflows of the neutral gas from the system. If the expelled gas was magnetized it might provide the supply of magnetic fields into the intergalactic space. Starburst galaxies (either dwarf and massive) constitute the basic source responsible for the enrichment of the intra-group medium with relativistic particles and magnetic fields. There are grounds to expect that the compact galaxy groups show diffuse radio emission, with a spectrum rapidly steepening away from the cosmic-ray sources in galactic disks.

The best studied example of a compact group is Stephan's Quintet (at a distance of 85 Mpc), with its pool of hot gas extending between the galaxies (Nikiel-Wroczyski et al. 2013b). It shows a huge, long filament visible in radio continuum. Strong polarization of this intra-group emission (Fig. 48) indicates a substantial content of ordered (probably shock-compressed) magnetic fields.

Figure 48

Figure 48. Stephan's Quintet of interacting galaxies. Total radio intensity (contours) and B-vectors at 4.86 GHz (6.2 cm), observed with the VLA (from Marian Soida, Kraków University). The background optical image is from the Hubble Space Telescope.

In summary, polarized radio emission is an excellent tracer of tidal effects between galaxies and of ram pressure in the intracluster medium. As the decompression and diffusion timescales of the field are very long, it keeps memory of events in the past, up to the lifetime of the illuminating cosmic-ray electrons. Low-frequency radio observations will trace interactions that occurred many Gyr ago and are no longer visible in other spectral ranges. Tidal tails from interacting galaxies may also constitute a significant source of magnetic fields in the intracluster and intergalactic media.

4.9. Galaxies with jets

Nuclear jets are observed in several spiral galaxies. These jets are weak and small compared to those of radio galaxies and quasars. Detection is further hampered by the fact that they emerge at some angle with respect to the disk, so that little interaction with the ISM occurs. Only if the accretion disk is oriented almost perpendicular to the disk, the jet hits a significant amount of ISM matter, cosmic-ray electrons are accelerated in shocks, and the jet becomes radio-bright. This geometry was first proven for NGC4258 by observations of the water maser emission from the accretion disk (Greenhill et al. 1995). This is why NGC4258 is one of the rare cases where a large radio jet of at least 15 kpc length is observed (van Albada & van der Hulst 1982; Krause & Löhr 2004). The total intensity map of NGC4258 (Fig. 49) reveals that the jets emerge from the Galactic center perpendicular to the accretion disk, which is oriented in east-west direction and is seen almost edge-on, and bend out to become the "anomalous radio arms", visible out to the boundaries of the spiral galaxy. The magnetic field orientation is mainly along the jet direction. The observed tilt with respect to the jet axis may indicate an additional toroidal field component or a helical field around the jet. The equipartition field strength is about 300 µG (at the resolution of about 100 pc), which is a lower limit due to energy losses of the cosmic-ray electrons and the limited resolution.

Figure 49

Figure 49. Spiral galaxy NGC4258 with two jets. Total radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), observed with the VLA (Krause & Löhr 2004). The background H image is from the Hoher List Observatory of the University of Bonn.

The barred galaxy NGC7479 also shows remarkable jet-like radio continuum features: bright, narrow, 12 kpc long in projection, and containing an aligned magnetic field (Fig. 50). The lack of any optical or near-infrared emission associated with the jets suggests that at least the outer parts of the jets are extraplanar features, although close to the disk plane. The equipartition strength is 35-40 µG for the total magnetic field and about 10 µG for the ordered magnetic field in the jets. According to Faraday rotation measurements, the large-scale regular magnetic field along the bar points towards the nucleus on both sides. Multiple reversals on scales of 1-2 kpc are detected, probably occurring in the galaxy disk in front of the eastern jet by anisotropic fields in the shearing gas flow in the bar potential. Highly polarized radio emission from kpc-sized jets has also been detected e.g. in NGC3079 (Cecil et al. 2001), with the field orientations perpendicular to the jet's axis), and in the outflow lobes of the Circinus Galaxy (Elmouttie et al. 1995).

Figure 50

Figure 50. Barred spiral NGC7479 with two jets. Total radio intensity (contours) and B-vectors at 8.46 GHz (3.5 cm), observed with the VLA (Laine & Beck 2008). The background shows a Spitzer/IRAC image at 3.6 µm (NASA/JPL-Caltech/Seppo Laine).

Jets in spiral galaxies may be more frequent than the available radio observations suggest. Future low-frequency observations may help, because they may show weak synchrotron emission from interface regions between the jets and the low-density halo gas.

4.10. Elliptical and dwarf spheroidal galaxies

Elliptical galaxies with active nuclei are among the brightest known radio sources. Their jets and radio lobes are generated by magneto-hydrodynamic processes which are discussed elsewhere. Radio emission from quiet elliptical and S0 galaxies is also associated with their nuclei (Fabbiano et al. 1987). Apart from the nuclear activity, elliptical galaxies are radio-faint because star-formation activity is very low and cosmic-ray electrons are rare. A few ellipticals form stars in their inner regions, but synchrotron emission and hence magnetic fields were not yet detected.

The existence of magnetic fields in the halos of non-active ellipticals is a matter of speculation. Regular fields are not expected in ellipticals because the lack of ordered rotation prevents the action of the alpha - Omega dynamo. Dwarf spheroidal galaxies have some ordered rotation, but lack turbulent gas. Turbulence in the hot gas of large ellipticals may drive a small-scale dynamo and generate turbulent fields with a few µG strength and turbulent scales of a few 100 pc (Moss & Shukurov 1996). However, there are no cosmic-ray electrons and hence no synchrotron emission. Detection of turbulent magnetic fields is only possible via the dispersion of Faraday rotation measures towards polarized background sources. Most large ellipticals are located in galaxy clusters where Faraday rotation will be dominated by the turbulent fields of the intracluster gas. For small ellipticals, the number of polarized background sources will only be sufficient with much more sensitive radio telescopes like the SKA. This leaves only isolated giant ellipticals for future studies.

Dwarf spheroidal galaxies are of interest to search for synchrotron emission from secondary electrons and positrons generated by the decay of dark-matter by WIMP annihilations, e.g. neutralinos (Colafrancesco et al. 2007). These galaxies do not generate thermal emission or primary electrons from star formation. Detection of radio emission would be of high importance, but all attempts failed so far. The main uncertainty is origin of magnetic fields in such systems (see above). If the field strength is a few µG, detection of synchrotron emission from dark-matter decay may be possible. Radio observations of several dwarf galaxies yielded only upper limits so far (Spekkens et al. 2013).

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