Magnetic fields in Local Group dwarf irregulars
  We wish to clarify whether strong magnetic fields can be effectively
generated in typically low-mass dwarf galaxies and to assess the role of dwarf
galaxies in the magnetization of the Universe. We performed a search for radio
emission and magnetic fields in an unbiased sample of 12 Local Group (LG)
irregular and dwarf irregular galaxies with the 100m Effelsberg telescope at
2.64 and 4.85GHz. Magnetic fields in LG dwarfs are three times weaker than in
the normal spirals (<4.2+-1.8muG). The production of total magnetic fields
appears to be regulated mainly by the star-formation surface density, with the
power-law exponent of 0.30+-0.04, or by the gas surface density (with the
exponent 0.47+-0.09). In addition, we find systematically stronger fields in
objects of higher global star-formation rate. The dwarf galaxies follow a
similar far-infrared relationship (with a slope of 0.91+-0.08) to that
determined for high surface brightness spiral galaxies. The magnetic field
strength in dwarf galaxies does not correlate with their maximum rotational
velocity, indicating a small-scale rather than a large-scale dynamo process. If
magnetization of the Universe by galactic outflows is coeval with its metal
enrichment, we show that more massive objects (such as Lyman Break Galaxies)
can efficiently magnetize the intergalactic medium with a magnetic field
strength of about 0.8nG out to a distance of 160-530kpc at redshifts 5-3,
respectively. Several times weaker fields and shorter magnetization distances
are expected from primordial dwarf galaxies. We also predict that most
star-forming local dwarfs might have magnetized their surroundings up to about
0.1muG within about 5kpc distance. Strong magnetic fields (>6muG) are observed
only in dwarfs of extreme characteristics while typical LG dwarfs are
unsuitable objects for the efficient supply of magnetic fields to the
intergalactic medium.
