Information on resource 'The Apertif Science Verification Campaign'

The characteristics of the polarised radio sky are a key ingredient to constrain evolutionary models of magnetic fields in the Universe and their role in feedback processes. The origin of the polarised emission and the characteristics of the intergalactic medium in the line-of-sight can be investigated using large samples of polarised sources. Ancillary infrared and optical data can be used to study the nature of the emitting objects. We analyse five early science datasets from the APERture Tile in Focus (Apertif) phased array feed system to verify the polarisation capabilities of Apertif in view of future larger data releases. We aim to characterise the source population of the polarised sky in the L-Band using polarised source information in combination with infrared and optical data. We use automatic routines to generate full field-of-view Q- and U-cubes and perform RM-Synthesis, source finding and cross-matching with published radio, optical and infrared data to generate polarised source catalogues. All sources were inspected individually by eye for a verification of their infrared and optical counterparts. SED-fitting routines were used to determine photometric redhifts, star-formation rates and galaxy masses. Infrared colour information was used to classify sources as AGN or star-forming dominated and early- or late-type. We surveyed an area of 56 deg^2 and detected 1357 polarised source components in 1170 sources. The fraction of polarised sources is 10.57% with a median fractional polarisation of 4.70 ± 0.14%. We confirmed the reliability of the Apertif measurements by comparing them with polarised cross-identified NVSS sources. Average Rotation Measures of the individual fields lie within the error of the best Milky Way foreground measurements. All of our polarised sources were found to be dominated by AGN-activity in the radio regime with most of them being radio-loud (79%) and of the FRII-class (87%). Our polarised source sample is dominated by late-type spiral galaxies. The contribution of star-formation to the radio emission is on the order of a few percent for ≈10% of the polarised sources while for ≈90% it is completely dominated by the AGN. We identified a possible decrease of fractional polarisation towards higher star-formation rates of the AGN host galaxies. The Apertif system is suitable for large area high sensitivity polarised sky surveys. The data products of the polarisation analysis pipeline can be used to investigate the Milky Way magnetic field on projected scales of several arcminutes as well as the origin of the polarised emission in AGN and the properties of their host galaxies.