Faint polarised sources in the Lockman Hole field at 1.4 GHz
  We aim to study the nature of the faint, polarised radio source population
whose source composition and redshift dependence contain information about the
strength, morphology, and evolution of magnetic fields over cosmic timescales.
We use a 15 pointing radio continuum L-band mosaic of the Lockman Hole,
observed in full polarisation, generated from archival data of the WSRT. The
data were analysed using the RM-Synthesis technique. We achieved a noise of 7
{\mu}Jy/beam in polarised intensity, with a resolution of 15''. Using infrared
and optical images and source catalogues, we were able to cross-identify and
determine redshifts for one third of our detected polarised sources. We
detected 150 polarised sources, most of which are weakly polarised with a mean
fractional polarisation of 5.4 %. With a total area of 6.5 deg^2 and a
detection threshold of 6.25 {\sigma} we find 23 polarised sources per deg^2.
Based on our multi wavelength analysis, we find that our sample consists of AGN
only. We find a discrepancy between archival number counts and those present in
our data, which we attribute to sample variance. Considering the absolute radio
luminosty, to distinguish weak and strong sources, we find a general trend of
increased probability to detect weak sources at low redshift and strong sources
at high redshift. Further, we find an anti-correlation between fractional
polarisation and redshift for our strong sources sample at z{\geq}0.6. A
decrease in the fractional polarisation of strong sources with increasing
redshift cannot be explained by a constant magnetic field and electron density
over cosmic scales, however the changing properties of cluster environments
over the cosmic timemay play an important role. Disentangling these two effects
requires deeper and wider polarisation observations, and better models of the
morphology and strength of cosmic magnetic fields.
