6. SUMMARY AND OPEN QUESTIONS
Table 3 gives a concise description of the
general structures of ordered magnetic fields in various galaxy types as
observed from radio polarization observations. For most galaxy types
only a few cases were observed so far. –
“Turbulent” stands for “isotropic turbulent or
tangled”, “ordered” for “anisotropic
turbulent” or “regular”. “Regular
field” refers to additional measurements of Faraday rotation.
For a list of radio polarization observations of galaxies, see
Beck &
Wielebinski (2013)
and updated versions on arXiv.
Radio images of total intensity and polarization are compiled on:
http://www.mpifr-bonn.mpg.de/atlasmag.
Table 3. General field structures in nearby
galaxies
|
Galaxy type |
Magnetic field structure |
Regular field |
|
|
Sc with strong |
Spiral at inner arm edge and in |
Strong, |
density wave |
interarm regions, turbulent in arms |
fluctuating |
Sb or Sc with weak |
Spiral in interarm regions, |
Moderate, |
or moderate density wave |
turbulent + ordered in arms |
large-scale |
Barred Sc |
Ordered + turbulent along bar, |
Moderate |
|
spiral outside bar |
Sa |
Ordered + turbulent |
Detected |
S0 |
Not detected |
Not detected |
Interacting spiral |
Ordered, asymmetric |
Weak |
Spiral with nuclear jets |
Ordered along jet |
? |
Flocculent Sc or Sd |
Spiral + turbulent in disk |
Weak |
Irregular |
Turbulent in star-forming regions |
Weak |
|
+ segments of ordered field |
Starburst dwarf |
Turbulent in star-forming regions |
Not detected |
Spheroidal dwarf |
Not detected |
Not detected |
E without active nucleus |
Not detected |
Not detected |
|
The open questions raised in this review may inspire future work:
(1) Morphology:
- How far do turbulent and
regular magnetic fields extend radially in the disk and vertically above
the disk?
- How well are magnetic pitch angles aligned with
the orientations of gaseous spiral arms?
- How do magnetic pitch angles vary radially in the
disk?
- Do dominating bisymmetric spiral patterns
(azimuthal mode m=1) of the regular field exist in galaxies?
- Do azimuthal modes m > 2 exist and how
strong are these?
- Which galaxy types host magnetic arms between
gaseous spiral arms?
- How common are field loops generated by the
Parker instability?
- Do regular fields in galaxies have large-scale
reversals?
- Are the X-shaped fields in galaxy halos related
to outflows (anisotropic turbulent fields) or do they show
large-scale patterns in Faraday rotation (regular fields)?
(2) Origin and amplification:
- What was the main source of
seed fields needed for the amplification of galactic fields in young
galaxies?
- Are small-scale fields generated by the
small-scale dynamo or by tangling of regular fields?
- What is the origin of magnetic arms between
gaseous spiral arms?
- What determines the efficiency and saturation
level of the α − Ω dynamo?
- Is the mean helicity of the large-scale magnetic
fields in galaxy disks non-zero, as predicted by α −
Ω dynamo action?
- Are small-scale fields with non-zero mean
helicity expelled from the disk to keep the α − Ω
dynamo alive?
(3) Dynamical importance:
- Where is energy equipartition
between total magnetic field and total cosmic rays valid and where does
it fail?
- What is the dominant propagation mechanism of
CREs in galaxy disks and halos and how does this depend on magnetic
field strength and structure?
- How do magnetic fields interact with spiral
density waves?
- How does the relation between total magnetic
field strength and gas density vary between galaxies and within
galaxies?
- How do magnetic fields modify the star-formation
rate and the star-formation efficiency?
- Is amplification of small-scale fields in
star-forming regions the reason for the radio–IR
correlation?
- Is the magnetic energy density larger than that
of gas turbulence in the outer parts of galaxies?
- Can magnetic fields affect the general rotation
of gas?
- Can galactic outflows magnetize the intergalactic
medium?