Faraday rotation signatures of fluctuation dynamos in young galaxies
  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 simulations 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.
