  We analyze the redshift evolution of the Rotation Measure (RM) in Taylor et
al. (2009) dataset, which is based on NVSS radio data at 21 cm, and compare
with results from our previous work (Kronberg et al. (2008), Bernet et al.
(2008),(2010)), based on RMs determined at lower wavelengths, e.g. 6 cm. We
find that, in spite of the same analysis, Taylor's dataset produces neither an
increase of the RM dispersion with redshift as found in Kronberg et al. (2008),
nor the correlation of RM strength with MgII absorption lines found in Bernet
et al. (2008). We develop a simple model to understand the discrepancy. The
model assumes that the Faraday Rotators, namely the QSO's host galaxy and the
intervening MgII host galaxies along the line of sight, contain partially
inhomogeneous RM screens. We find that this leads to an increasing
depolarization towards longer wavelengths and to wavelength dependent RM
values. In particular, due to cosmological redshift, observations at fixed
wavelength of sources at different redshift are affected differently by
depolarization and are sensitive to different Faraday active components. For
example, at 21 cm the polarized signal is averaged out by inhomogeneous Faraday
screens and the measured RM mostly reflects the Milky Way contributions for low
redshift QSOs, while polarization is relatively unaffected for high redshift
QSOs. Similar effects are produced by intervening galaxies acting as
inhomogeneous screens. Finally, we assess the performance of Rotation Measure
synthesis on our synthetic models and conclude that the study of magnetic
fields in galaxies as a function of cosmic time will benefit considerably from
the application of such a technique, provided enough instrumental bandwidth.
For this purpose, high frequency channels appear preferable but not strictly
necessary.
