In the following I will describe some of the results for
individual galaxies. The order of the galaxies is firstly size,
but later some of the types will be described collected in
groups. The description of the magnetic field structure will
follow the ideas developed from the early observations namely
that fields are either axisymmetic [ASS] or bisymmetric [BSS]
spirals. The analysis of the magnetic fields, which was
originally developed by
Tosa and Fujimoto
(1978),
involves the
study of the Rotation Measure as a function of azimuthal angle
and is illustrated in figure 2. Further details
of such studies can be found in
Sofue et al. (1985)
and Krause et al.
(1989a,
b).
|
Figure 2. The basic mode
configurations. Rotation Measure as
a function of Azimuth |
LMC Optical observations of the Large (and Small) Magellanic Cloud (Schmidt, 1970; Mathewson and Ford, 1970) showed the presence of magnetic fields in both galaxies. The initial interpretation in terms of a 'Pan-Magellanic field' was questioned by Schmidt (1976) who pointed out that the local (foreground) field seems to be also aligned in the LMC-SMC direction. A detailed study of the LMC was recently given by Klein et al. (1989). Radio polarization studies of the LMC (Haynes et al., 1990) show that magnetic fields are seen as a series of filaments originating in 30 Doradus nebula. This in fact agrees with the recent results in HI, CO, FIR, UV etc. These results would suggest that 30 Doradus is the nucleus of the LMC. The filamentary structure of the young components is indeed baffling. These filaments could be spiral arms. This should lead to a reclassification of the LMC to be a 'Spiral' rather then the present classification as 'Irregular'.
SMC The Small Magellanic Cloud is considered to be the nearest dwarf galaxy. As such the magnetic field structure is of great interest. Optical studies showed some vectors aligned with the 'body' of the SMC, others to be directed towards the LMC (Schmidt 1976). Radio data (Loiseau et al., 1987) shows aligned field in the southern 'body', in agreement with optical data. This indicates a field along the 'body'. However the field is weak, possibly less then 3 µG, as expected in a dwarf galaxy.
M31 This northern spiral has been
a subject of
extensive study giving possibly the best information of any galaxies
to date. The Effelsberg
11 and 6cm data have now been
supplemented with multi-field VLA observations at
20cm. The
polarized intensity is concentrated to a 'ring' with minima in
the direction of the major axis (where Faraday depolarization is
expected to be greatest). Due to its inclination the Faraday
rotation is strong and can thus be measured with some accuracy.
M31 has the prototype 'axisymmetric' spiral field
structure (see
Beck, 1982 and
Beck et al., 1989
and figures 3 and 4).
However in detail small wave-like field perturbations are observed.
M33 Multifrequency observations of Buczilowski & Beck, (1987) have now been analyzed in some detail. The field in M33 is possibly 'bisymmetric' but this conclusion is only tentative because of problems of sensitivity in this rather low luminosity galaxy. It is difficult in general to determine Faraday rotation in face-on galaxies, in particular when the magnetic field strength is low. A regular field structure is seen in M33 in the northern spiral arm while in the south considerable perturbations are present.
NGC55 This large irregular galaxy is seen edge-on (e.g. Hummel et al., 1986). In the VLA observations no polarized emission was detected. Recent mapping with the Parkes telescope (Harnett et al., 1990) has shown some weak polarized emission in the nuclear area. This would be the second (after the SMC) dwarf galaxy with a confirmed magnetic field.
|
Figure 3. A low resolution map of the magnetic field in M31 (based on Effelsberg data from Beck et al., 1980 with correction for the Faraday rotation in our Galaxy only). |
|
Figure 4. A 'zoom' of a section of M31 with higher angular resolution (VLA data from Beck et al., 1989). |
M101 In M101 the giant HII regions, which have NGC designations themselves, dominate the structure. In spite of this the diffuse nonthermal emission shows two polarization maxima on opposite sides of the nucleus (Gräve et al. 1990). Magnetic fields on a grand scale are present also in this galaxy.
IC342 This galaxy was the subject of detailed studies by Krause et al. (1989a). Both Effelberg and VLA multifrequency data are available. The rotation measure analysis of this galaxy (see figure 5) shows an axisymmetric field. Higher angular resolution observations show that the symmetry on the two opposite sides of the galaxy is quite different. In the South-East a series of very extended filamentary arcs are observed. A polarization maximum in one arc shows zero Faraday rotation with rotation in the same direction on either side. We must be looking into an 'S' like magnetic field filament. In the North-West a very fine filamentary structure is seen with a number of maxima and minima. However the direction of the 'E' vector (i.e. magnetic field) does not change.
M81 This 'grand design' spiral galaxy was subject of multifrequency studies by Krause et al. (1989b). It is the bisymmetric field prototype (see figure 5 and figure 6). However the symmetry is also not perfect. The South-West arm breaks up into two filaments aligned in the direction of the spiral arms. The highest degree of polarization is in the inter-arm region. This is a very significant result pointing to a tangled field in the arms.
|
Figure 5. Rotation measure studies for M81 and IC342. (from Krause et al., 1989a, b) |
NGC4258 This galaxy has posed a problem of interpretation in view of its 'anomalous arms' (van der Kruit et al., 1972). The fact that these anomalous arms are highly polarized (van Albada, 1978, Hummel et al., 1989) implies that magnetic fields are involved in the origin of the radio emission in this object. At low angular resolution the two arms show up as maxima of polarization symmetrically disposed about the nucleus. Rotation measure analysis by Hummel et al. (1989) implies that these arms are in the plane, or nearly in the plane, of the galaxy.
|
Figure 6. The magnetic field orientation in M81 (Krause et al., 1989b) |
NGC6946 This galaxy was one of the earliest to be mapped with polarization information at a high radio frequency (Klein et al., 1982). Subsequent multifrequency observation studies both at the VLA and in Effelsberg (Harnett et al., 1989a) showed that in spite of a regular field structure no decision between axisymmetric of bisymmetric field could be made. The local perturbations in this 'Arp' galaxy make any decision impossible.
M51 There are extensive data for this galaxy both in optical and radio domain. The original Westerbork data (Segalovitz et al., 1976) have been supplemented by Effelsberg and VLA observations. This galaxy was the first to be investigated by Tosa and Fujimoto (1978) for the presence of a bisymmetrical magnetic field. Also excellent optical polarization CCD maps have been made by Scarrott et al. (1987). There is a general agreement between the optical and radio data for most of the galaxy. In the South-West part of the galaxy the optical and radio data disagree (Beck et al., 1988). More recent radio data (Horellou et al., 1990) confirm the BSS magnetic field. The M51 data confirm that the optical and radio polarizations are due to the same magnetic field.
NGC4631 The question of the structure of the magnetic fields above the plane of a galaxy is of great interest. The closure of magnetic field lines is expected to occur in the halo. The classical edge-on galaxy with a thick disc halo is NGC4631 (Ekers & Sancisi, 1977; Wielebinski & von Kap-herr, 1977). The observations of this galaxy with the VLA (Hummel et al. 1988) showed the existence of halo fields. The orientation of the field is normal to the disc (see figure 7) on the assumption of low Faraday rotation. It seems that Parker instabilities or a galactic wind are pushing the magnetic field above the plane of this galaxy. This observation must however be treated with some caution since NGC4631 has the most extended synchrotron halo. The field strength in the halo may be as much as ~ 2 µG.
|
Figure 7. The 'B'-field orientation in NGC4631. (from Hummel et al., 1989; not corrected for Faraday effect) |
NGC891 This nearly perfect edge-on galaxy has been studied in radio continuum (e.g. Allen et al., 1978; Klein et al., 1984). Recent polarization studies by Dahlem, Beck, Hummel, Sukumar, Allen (private communication) indicate a magnetic field away from the plane but not as perpendicular to the plane as in NGC4631. Since data at two frequencies was obtained the confirmation of a low Faraday rotation in the halo (which was so far assumed for NGC4631) has been obtained.
M83 The barred galaxy M83 shows beautiful highly symmetric field structure (Sukumar et al., 1987). More recent VLA observations with higher angular resolution by Sukumar and Allen (1990) indicate that filamentary structure is seen, reminiscent of IC342. It is also interesting to note that the aligned magnetic field starts at the optical edge of M83, where the bar structure stops. In the inner parts of this galaxy the field must be quite turbulent.
NGC253, NGC4945 These large southern galaxies are seen nearly edge-on. Radio continuum observation showed in each galaxy two maxima distributed symmetically about the nucleus (Klein et al., 1983, Harnett et al., 1989). The question if this is only a geometrical effect, or a morphological feature are still open.
NGC3628 This edge-on galaxy show some polarization in the halo (M. Krause, private communication). Again the structure is emphasized by two maxima on opposite sides of the nucleus.
M104 The 'Sombrero' galaxy was known for a long time to have a compact (VLBI) source in the nucleus. Optical polarization studies by Scarrott et al. (1987) showed a field along the disc of this galaxy. The disc emission was finally detected by Bajaja et al. (1999) using the high dynamic mode of the VLA. In addition polarized emission perpendicular to the disc was seen in the nuclear area. Recent analysis of the optical polarization by Matsumura & Seki (1989) suggest also a Z-field in the nucleus superimposed on the azimuthal field in the disc. The rotation curve of M104 (e.g. Wagner et al., 1989) suggests a rotating ring surrounding the nucleus.
M82 This mildly active galaxy is the most studied object in all spectral ranges. Although detailed radio continuum observations were published (e.g. Kronberg et al., 1985) none contained polarization data. Optical polarization (Bingham et al., 1976) is dominated by the light scattered by dust, showing a circular vector distribution. Recent CCD observations of this object, after subtraction of the scattered light component, showed a Z-field in the nuclear area (Neininger, 1989). The radio continuum spectrum of M82 is very well studied (Klein et al., 1988) indicating that the magnetic field may have mean values of ~ 50µG, the highest of any galaxy. A rotating ring which is seen in all constituents (HI, OH, sub-mm continuum, CO etc.) has been interpreted to be instrumental for the production of this Z-field (Lesch et al., 1989).
Many other galaxies have been observed but not in such detail as those mentioned above. For the general scenario we can conclude that magnetic fields are azimuthal in the galaxies except in the nuclear area where fields are in the Z-direction. This is in agreement with the model shown for our Galaxy in figure 1.