Faint polarised sources in the Lockman Hole field at 1.4 GHz

Abstract

Context. In the context of structure formation and galaxy evolution, the contribution of magnetic fields is not well understood. Feedback processes originating from active galactic nucleus (AGN) activity and star formation can be actively influenced by magnetic fields, depending on their strength and morphology. One of the best tracers of magnetic fields is polarised radio emission. Tracing this emission over a broad redshift range therefore allows an investigation of these fields and their evolution. Aims. 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. Methods. We use a 15-pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the Westerbork Synthesis Radio Telescope. The data were analysed using the rotation measure synthesis technique. We achieved a noise of 7 μJy beam−1 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. Results. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4%. No source was found with a fractional polarisation higher than 21%. With a total area of 6.5 deg2 and a detection threshold of 6.25σ, we find 23 polarised sources per deg2. 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 (i.e. large-scale structures). Considering the absolute radio luminosity, we find a general trend of increased probability of detecting weak sources at low redshift and strong sources at high redshift. We attribute this trend to a selection bias. Further, we find an anti-correlation between fractional polarisation and redshift for our strong-source sample at z ≥ 0.6. Conclusions. 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 cosmic time may play an important role. Disentangling these two effects requires deeper and wider polarisation observations as well as better models of the morphology and strength of cosmic magnetic fields.

The Apertif science verification campaign

“…The blue, green, and red dashed lines represent the 1%, 2%, and 3% fractional polarisation levels, respectively. Berger et al (2021) found fractions of 10.6%, 14.2%, 5.9%, 2.6%, and 8.8%, respectively, and so our values are consistent with the previously reported ones.…”

“…Literature values for the fraction of polarised sources are scattered between ≤2.5% and 14.1% (see Hales et al 2014a;Grant et al 2010;Taylor et al 2007;Stil et al 2014;Subrahmanyan et al 2010;O'Sullivan et al 2015). Berger et al (2021) showed that the FP of sources is dependent on the completeness of the source sample as well as on its redshift distribution. In addition to this, we show that the FP of a source is dependent on whether it is compact or resolved with approximately 20% more polarised emission originating from resolved sources.…”

“…In addition, even when only investigating the AGN sample, certain types of AGN with different characteristics dominate polarised samples. O' Sullivan et al (2015) and Berger et al (2021) showed that a marginal difference exists between the FP of FRI and FRII sources, and also between radio-loud and radio-quiet sources. Hammond et al (2012) found a difference between the FP of SDSS galaxies and radio-loud QSOs.…”

Magnetic field strength in cosmic web filaments

“…Furthermore, simulations predict that the intensity of filament magnetic fields can help discriminate between the possible scenarios that have magnetised these cosmic structures (e.g., Vazza et al 2015). The time evolution can also inform us about the evolution of extragalactic sources themselves, such as a change in the physical conditions of the source and its environment (e.g., Kronberg et al 2008;Berger et al 2021).…”

First release of Apertif imaging survey data

“…Such an analysis also provides information on the morphology and turbulence of the magnetic fields, not only in the host objects, where the polarised emission is generated, but also along the line-of-sight to the observer. When using additional redshift information this enables an analysis of the evolution of cosmic magnetic fields (Berger et al 2021).…”