2.2. Observational overview of large scale riiagneticfields in spiral galaxies
2.2.1. Current instrumental capabilities.
The improved sensitivity and polarimetric
capability of both radio and optical telescopes has revealed a great
deal about the morphology and extent of magnetic fields in galaxies over
the past five to ten years. In this section we explain the observational
methods (well described by
Krause 1990), and
illustrate how they can be used to reveal magnetic field morphology and
strength in relatively nearby galaxies. The latter have typical
projected sizes of a few to a few tens of arcminutes, so that typically
5 to 15 linear resolution elements can be obtained with
large, single-dish radio telescopes at 2 cm
20 cm -
the usable wavelength range for this purpose. Good, and well calibrated
beam characteristics are required for polarization of these extended
objects, where the telescope's beam is embedded in an extended polarized
source. The 100 m Effelsberg telescope of the Max-Planck-Institut für
Radioastronomie (resolution
= 4.0'(
/ 10 cm)) has
been a prime instrument for such measurements at the shorter
s. At the
longer wavelengths and for more distant galaxies
the higher resolution of radio arrays is necessary; notably the
NRAO VLA and the
Westerbork Synthesis Radio Telescopes. There is a complicating
difficulty with interferometers for imaging diffuse polarized
emission in nearby galaxies which results from their inherent tendency
to filter out the largest scale components of diffuse polarized
emission. It is also sometimes difficult to deconvolve the latter from
polarized sidelobe structure of the individual array antennas, since the
largest images - e.g. M31 - are
sometimes larger than the beam size of a single interferometer element,
2.2.2. Current knowledge of spiral galaxy disk
magnetic fields.
Figure 1 shows a recent
2.8 cm (10.7 GHz)
Effelsberg 100 in telescope 'magnetic field'
image of the 6 Mpc distant, barred spiral galaxy Messier 83 at 68"
resolution
(Neininger et al 1991).
The large scale coherence of the magnetic field is striking, and this is
typical of the ca 20 nearby galaxies mapped thus far at some
radio wavelength. A derivation of the true
projected magnetic field orientation normally requires polarization
images at two, and preferably more radio wavelengths, so that Faraday
rotation (equation (1.3)) can be removed at each image point. At the
relatively short radio wavelength of 2.8 cm a Faraday rotation of
2.4° (relatively insignificant) corresponds to a Faraday rotation
measure (RM) of 55 rad m-2 which, from our general knowledge of
galaxy disk RMs, is unlikely to he exceeded at this resolution for a
relatively `top-on' view of the disk. Thus,
a single short
polarization map such as in figure 1 gives a
good approximation to the intrinsic magnetic field orientation over the
galaxy.
![]() |
Figure 1. A radio-optical superposition of
M83, in which the normals to the maximum E-vector
direction at
|
The fact that the polarized (synchrotron) component of the diffuse radio
emission becomes fainter with increasing frequency (S /
,
- 0.6) underlines the
`state-of-the-art' nature of the image in
figure 1 which, at
= 10.7 GHz, is already
sensitivity limited. A higher angular resolution would further reduce
the flux density per beam
and hence reduce the polarized signal. An advantageous consequence of
the low Faraday rotation at higher radio frequencies near 10 GHz is that
differential Faraday rotation is minimized, which would otherwise cause
depolarization along each bundle of sight-lines within the galaxy. Thus,
the galaxy is not `Faraday opaque', which can be the
case at longer
s,
e.g. at
20 cm (cf
Neininger et al 1993
for the case of M83).
A similar large scale, organized field is revealed by polarimetry of the optical surface brightness, as illustrated in the recent image in figure 2 of NGC 1068. The overall magnetic field alignment is striking, as in the radio polarization maps. Although no information about the sign of the B-direction can be inferred from optical polarization images, their resolution is limited only by the sensitivity of the detector/telescope combination, down to the optical `seeing' limit of resolution, which is of order 1 arcsecond for a ground-based telescope. Because of the multiple causes of optical polarization, and problems of optical extinction in inclined galaxy disks, the most yielding of optical information on the magnetic field morphology are nearby, face-on galaxies like the one shown.
![]() |
Figure 2. The prevailing magnetic field orientation in the optical V-band for the galaxy NGC 1068 (S. M. Scarrott, private communication). Typical degrees of optical polarization lie in the range 0% to 5%. |
Not shown in figure 1 is the Faraday rotation
measure (RM), which can also tell us, at each polarization pixel,
whether the field has a component which is directed into
or out of the plane of the sky. Whether or not there are global
reversals of prevailing magnetic field sense is an interesting
question, which bears directly on the field amplification mechanism, and
possibly on how the protogalactic
axis was coupled (if it
was) to a primeval intergalactic field. R M observations can
distinguish whether the global galaxy disk field has an
axisymmetric,
or bisymmetric form, as illustrated schematically in
figure 3. Observational discrimination between
these two possibilities can be
made with multi-
polarization images, using a straightforward technique first suggested
by Tosa and Fujimoto
(1978).
For the bisymmetric field case in figure 3(a),
we can model the global magnetic field as a spiral (as justified from
figures 1
and 2), and plot the observed RM as a function
of azimuth angle
(
) for a suitable radial
zone (router - rinner)
within the de-projected plane of the galaxy's disk. For an inclination
angle i > 0°
(face-on = 0°), a plot of RM against
has four
zero-crossings for a bisymmetric field
galaxy, as opposed to two for an axisymmetric (non-reversing) global
field morphology. This is illustrated in
figure 3(c), along with data
from two galaxies. Additional geometric
details of the disk field can be specified, such as the pitch angle,
p, of the spiral field direction, and position angle,
, of the point of
maximum RM on the circle, in which case the model
RM-
curve can be
expressed as
![]() |
(2.1) |
(Tosa and Fujimoto 1978, Sofue et al 1986). A quicker observational discriminant between asymmetric and bisymmetric structure, proposed by Sofue et a1 (1985), is simply to plot the observed RM(r) along the projected major axis of the galaxy disk, from which the `signatures' of an axisymmetric and bisymmetric structure can be distinguished. This is illustrated in figure 3(b). Better still would be a detailed 2D RM map over the entire galaxy surface. Among the very few such images at the time of writing is that of NGC 6946 by Ehle and Beck (1993), which reveals a global, galaxy-scale `flip' of the RM sign between the two halves of this galaxy's disk - thus indicating an overall axisymmetric field configuration. It should be noted that the model curves in figure 3 assume purely toroidal fields in an infinitely thin disk - which is now generally realized to be unrealistically simple (cf sections 2.3, 5, 6).
![]() |
Figure 3. (a) Face-on view ora model
axisymmetric (left) and bisymmetric (right) magnetic
field distribution in a galactic disk. (b) RM(r) measured
along the major axis of the projected galaxy for the axisymmetric
(left), and bisymmetric (right) case,
respectively. (c) The form of the
RM( |
From the available subset of nearby galaxies having
RM()
measurements analyzed as just described, both axisymmetric and
bisymmetric configurations appear to exist. It is also increasingly
apparent that some galaxies do not conform to either, including
M83 in figure 1. In this
connection we note that these classifications
may be sensitive to the linear resolution available, and also
possibly to the galaxy's inclination or disk
thickness (z-height). Thus galaxy magnetic field configurations, and
the classification scheme are subject to future modification. In
general, global disk fields appear more aligned in the inter-am regions,
and less so in the visible arms
(Beck 1991),
and the degree of field
uniformity is anticorrelated with the intensity of CO line emission,
which comes from the denser, hence starforming regions - i.e. the visible
spiral arms
(Bajaja et al 1990).
As more detail emerges, the situation
becomes more complex; for example in M31 (the Andromeda galaxy), where
good linear resolution obtains due to its proximity, the patterns of
Faraday rotation and field ordering do not conform on smaller
scales to either a simple AS or BS model
(Krause et al 1989,
Beck et al 1989).
A more complex pattern has also emerged in detailed studies of M83
(Sukumar and Allen
1989,
1990,
Neininger et al 1993).
While the magnetic
field structure is generally better aligned in the outermost
regions of
spiral galaxies, the simple model assumptions of equation (2.1) and
figure 3 are oversimplified at smaller
galactocentric radii. More specifically, the form of the curves in
figure 3(c) requires that the
pitch angle, p , be constant over the galaxy's disk. This may not
normally be so, as is already evident in M31 and M83. We discuss further
observational evidence and physical reasons below, for magnetic fields
directed out of a galaxy's plane. Also, systematic field reversals
may be occurring which may not conform to a large scale bisymmetric
field, but which may rather be due to effects which are local, and possibly
transient. Thus, the field models suggested in
figure 3(a) may, with
future data prove to be very oversimplified; in particular they
do not incorporate halo or out-of-plane fields (cf
sections 2.3.5,
2.3.6).
2.2.3. Observations of magnetic fields in halos of
galaxies A datum of interest is the
magnetoionic strength above the disk of our own galaxy, and in the
Galactic Halo - which we can compare with the `out-of-disk' and halo
magnetic fields of other spiral galaxies.
Simard-Normandin and
Kronberg (1980)
used the (l, b) distribution of
EGRS RMs to estimate the Milky Way's magnetoionic scale height, and
determined a value of 1.4.kpc (full width), which is relatively
insensitive to the field reversal scale.
This is similar to the galactic scale height of the hot gas,
1 kpc (cf
Kulkarni and Heiles
1987,
Reynolds 1990).
It also indicates that any Faraday RM in the halo of our galaxy
is small compared with that in the disk. Information is just beginning
to appear on the Faraday rotation in the higher halos of nearby spiral
galaxies, however recent attempts to detect synchrotron-emitting `halos'
of galaxies seen edge-on indicate full widths (to
the 1% level of surface brightness) of 2-4 kpc
(Klein et at 1984,
Hummel 1990). This
result is not inconsistent with the scale height quoted above. We note,
though, that a few galaxies, those with increased star formation
activity, reveal pronounced and larger halos at lower radio frequencies,
in which polarized emission traces the magnetic field
out to large distances above their galactic planes. These are discussed
separately below (section 3.1).
Improved resolution and sensitivity to radio synchrotron emission off
the disks of some normal galaxies - which were observationally
selected to be edge-on for the purpose of investigating the out-of-plane
field geometry - has recently provided valuable
new information on the z-component of disk fields, and the
orientation and degree of
ordering of these fields out to large z-heights. A case in point is
the edge-on galaxy
NGC 891 (which is largely similar to our Milky Way).
NGC 891 shows
polarized synchrotron radiation up to 4 kpc into the halo, in which the
field ordering first increases, then decreases with z
(Hummel et a1 1991).
Another edge-on galaxy, NGC 4631 shows a similar behavior
(Hummel et a1 1988,
1991),
but with a radially aligned
magnetic field direction (the sense is not measured) out to
7
kpc from the disk - see figure 7
(section 3.1 below).
For the same galaxy, another scale height has been
measured, namely that of the ionized gas layer by imaging the diffuse
optical H
emission, giving a scale height of
1.1 kpc
(Rand et al 1990,
Dettmar 1990).
The H
data
agree with a similar scale height of
0.9 kpc estimated from
Faraday dispersion due to thermal ionized gas embedded within the
polarized halo emission
(Hummel et a1 1991).