2.4. How well does the galactic dynamo theory conform to the observations?
The answer to this question is mixed; the large scale magnetic field
geometry of `well-behaved' spiral galaxies agrees with some, though not
all, of the straightforward predictions and model simulations of linear
dynamo theory. The affirmative side of the
answer is that the predominant field modes do seem to be the simplest
ones, as illustrated
by recent data shown for M83 and NGC 1068 in
figures 1 and
2. This is
probably the most significant result, in that it shows that, if the
observed large scale fields were
amplified by the
-
dynamo in the disks,
the prominent modes are the low order ones,
an outcome which is not a priori obvious. However, the angular
resolution for many
of the galaxies is probably not good enough to confirm definitive
agreement with the standard
-
dynamo theory. Apart
from `geometrical'
tests, observation-based estimates of magnetic field strength indicate
that, if the observed fields were entirely
the effect of a galactic dynamo, the (non-linear) amplification must
saturate at 2-8 µG,
which is when the magnetic energy density
m(=
10-12(|B| / 5 µ G)2 erg
cm-3) becomes comparable with
cr
and
t.
Recent images of improved sensitivity and resolution are beginning to
elucidate some further details of the magnetic structure near the plane
of spiral galaxies. The highest degree of magnetic field ordering
generally occurs in the outermost regions of galaxy disks, and the
interarm regions. Conversely, field orientation is generally more
randomized in the star-forming complexes within spiral arms, and also
possibly at smaller galactocentric radii. Specifically, field
disordering tends to correlate with locally enhanced
H (seen optically) and
CO 1-0 (
2.7 mm) emission
(Bajaja et a1 1990),
and sometimes enhanced H I
(
21 cm) surface
brightness. Where
the field ordering is high, the field orientation usually conforms to
the galaxy's spiral pattern
(figures 1,
2).
Most or all of these properties and correlations have been described in
the analysis of recent surface polarimetry observations of NGC 6946
(Ehle and Beck 1993),
M83
(Sukumar and Allen
1990,
Neininger et al 1991),
and M51
(Beck et al 1987,
Scarrott et al 1987
(optical polarization), and
Neininger 1992).
The unprecedented detail of these recent observations has provided some
of the first clear evidence for local deviations or `disruptions' of the
otherwise ordered azimuthal disk component
(Sukumar and Allen
1990,
Horellou et al 1992).
In M31, the suggested detection of arc-like looping of
field lines between tie-down points
2 kpc apart, where
the field lines bulge out of the disk, is possibly due to a Parker-Jeans
instability
(Beck et al 1989).
Such phenomena may be the nascent beginnings of magnetic field expulsion.
On a simple energetic calculation, where more intense than normal star
formation occurs, a `breakout' of field into the local halo and IGM will
result. This type of evidence reinforces the notion that such
instabilities will `snuff out' the slow field strength build-up by the
conventional
-
dynamo process in an
assumed ever-quiescent galaxy disk. This, potentially, puts the
conventional
-
dynamo theory
in great difficulty if the
dynamo is required to explain the amplification of very weak seed
fields in the early universe up to present-day µG
levels. Parker's modified
-
dynamo
(section 2.3.6),
although not yet incorporated in a detailed numerical model, might
overcome this shortcoming, by allowing for a fast dynamo amplification
of weaker seed fields, if indeed the protogalactic fields were much weaker.
Another test of dynamo models which require a slow field build-up over
cosmic time is to estimate the magnetic field strengths in low-mass and
dwarf irregular galaxies,
whose lower mass and rotation might be expected to cause field
regeneration to a different, lower value than large spiral
galaxies. Observations of two such galaxies, the
Large and Small Magellanic Clouds (LMC and SMC), indicate (using the
assumption of equipartition) interstellar field strengths at the few
microgauss level
(Loiseau et al 1987
(SMC),
Klein et al 1989,
1993 (LMC)).
Allowing for uncertainties in the estimates,
these field strengths are essentially the same as measured in the Milky
Way using pulsars, and in other large spiral galaxies. The similarity of
the magnetic field strengths in these low mass, slowly rotating
galaxies thus casts doubt on the requirement of a slow-acting
-
dynamo to regenerate an
initially weak
primordial field to a few microgauss over several billion years.