ApPolLO: Apertif counterparts of Polarised LOFAR sources

Knowledge of the structure and evolution of cosmic magnetic fields is indispensable to our understanding of how primordial magnetic fields have been generated and amplified during the lifetime of the Universe. Polarisation studies over significant redshift ranges are an important tool in understanding cosmic magnetic fields. We aim to understand the structure and evolution of cosmic magnetic fields by utilising the polarised spectra and depolarisation characteristics of (background) radio sources. Using radio polarisation data at two different frequencies from the LOFAR Rotation Measure (RM) grid catalogue at 144,MHz and new Apertif data at 1.4,GHz, we investigated the source statistics of a common sample of polarised sources, deriving the depolarisation ratio between these two frequencies. We utilised this approach to disentangle depolarising effects local to the source from those along the line of sight. We compiled a catalogue of 598 polarised radio components, most of which are located in low-density environments. We find that the fractional polarisation decreases for both wavelengths with redshift. We attributed the main depolarisation at 1.4,GHz to effects occurring close to the sources, while the depolarisation measured at 144,MHz is most likely generated by cosmic filaments along the line of sight. We were also able to get a first redshift-dependent upper limit estimate of the RM dispersion in cosmic web filaments.