Context :

Aims : 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 .

Methods : 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 100-m Effelsberg telescope at 2.64 GHz .
Three galaxies were detected .
A higher frequency ( 4.85 GHz ) was used to search for polarized emission in five dwarfs that are the most luminous ones in the infrared domain , of which three were detected .

Results : Magnetic fields in LG dwarfs are weak , with a mean value of the total field strength of < 4.2 \pm 1.8 \mu G , three times lower than in the normal spirals .
The strongest field among all LG dwarfs of 10 \mu G ( at 2.64 GHz ) is observed in the starburst dwarf IC 10 .
The production of total magnetic fields in dwarf systems appears to be regulated mainly by the star-formation surface density ( with the power-law exponent of 0.30 \pm 0.04 ) or by the gas surface density ( with the exponent 0.47 \pm 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 \pm 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 that a small-scale rather than a large-scale dynamo process is responsible for producting magnetic fields in dwarfs .
If magnetization of the Universe by galactic outflows is coeval with its metal enrichment , we show that more massive objects ( such us Lyman break galaxies ) can efficiently magnetize the intergalactic medium with a magnetic field strength of about 0.8 nG out to a distance of 160–530 kpc at redshifts 5–3 , respectively .
Magnetic fields that are several times weaker and shorter magnetization distances are expected for primordial dwarf galaxies .
We also predict that most star-forming local dwarfs might have magnetized their surroundings up to a field strength about 0.1 \mu G within about a 5 kpc distance .

Conclusions : Strong magnetic fields ( > 6 \mu G ) are observed only in dwarfs of extreme characteristics ( e.g .
NGC 4449 , NGC 1569 , and the LG dwarf IC 10 ) .
They are all starbursts and more evolved objects of statistically much higher metallicity and global star-formation rate than the majority of the LG dwarf population .
Typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium .