Abstract:
Observations of Faraday rotation through high-redshift galaxies have revealed that they host
coherent magnetic fields that are of comparable strengths to those observed in nearby galaxies.
These fields could be generated by fluctuation dynamos. We use idealized numerical simula-
tions of such dynamos in forced compressible turbulence up to rms Mach number of 2.4 to
probe the resulting rotation measure (RM) and the degree of coherence of the magnetic field.
We obtain rms values of RM at dynamo saturation of the order of 45–55 per cent of the value
expected in a model where fields are assumed to be coherent on the forcing scale of turbulence.
We show that the dominant contribution to the RM in subsonic and transonic cases comes
from the general sea of volume filling fields, rather than from the rarer structures. However,
in the supersonic case, strong field regions as well as moderately overdense regions contribute
significantly. Our results can account for the observed RMs in young galaxies.
