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Measuring the magnetic field in cosmic filaments reveals how the Universe is magnetized and the process that magnetized it. Using the Rotation Measures (RM) at 144 MHz from the LoTSS DR2 data, we analyse the rms of the RM extragalactic component as a function of redshift to investigate the evolution with redshift of the magnetic field in filaments. From previous results, we find that the extragalactic term of the RM rms at 144 MHz is dominated by the contribution from filaments (more than 90 per cent). Including an error term to account for the minor contribution local to the sources, we fit the data with a model of the physical filament magnetic field, evolving as |$B_f = B_{f,0}\, (1+z)^\alpha$| and with a density drawn from cosmological simulations of five magnetogenesis scenarios. We find that the best-fitting slope is in the range α = [ − 0.2, 0.1] with uncertainty of σα = 0.4–0.5, which is consistent with no evolution. The comoving field decreases with redshift with a slope of γ = α − 2 = [ − 2.2, −1.9]. The mean field strength at z  = 0 is in the range B f , 0 = 39–84 nG. For a typical filament gas overdensity of δ g  = 10 the filament field strength at z  = 0 is in the range |$B_{f,0}^{10}=8$| –26 nG. A primordial stochastic magnetic field model with initial comoving field of B Mpc = 0.04–0.11 nG is favoured. The primordial uniform field model is rejected. [ABSTRACT FROM AUTHOR]

We used the rotation measure (RM) catalogue derived from the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz to measure the evolution with redshift of the extragalactic RM (RRM: Residual RM) and the polarization fraction (p) of sources in low-density environments. We also measured the same at 1.4 GHz by cross-matching with the NRAO VLA Sky Survey RM catalogue. We find that RRM versus redshift is flat at 144 MHz, but, once redshift-corrected, it shows evolution at high significance. Also, p evolves with redshift with a decrement by a factor of ∼8 at z ∼ 2. Comparing the 144-MHz and 1.4-GHz data, we find that the observed RRM and p are most likely to have an origin local to the source at 1.4 GHz, while a cosmic web filament origin is favoured at 144 MHz. If we attribute the entire signal to filaments, we infer a mean rest-frame RRM per filament of RRM |$_{\rm 0,f} = 0.71 \pm 0.07 \, \, \rm rad\, m^{-2}$| and a magnetic field per filament of B f = 32 ± 3 nG. This is in agreement with estimates obtained with a complementary method based on synchrotron emission stacking, and with cosmological simulations if primordial magnetic fields are amplified by astrophysical source field seeding. The measurement of an RRM0, f supports the presence of diffuse baryonic gas in filaments. We also estimated a conservative upper limit of the filament magnetic turbulence of |$\sigma _{\rm RRM_{\rm 0,f}} =0.039 \pm 0.001 \, \, \rm rad\, m^{-2}$|⁠ , concluding that the ordered magnetic field component dominates in filaments. [ABSTRACT FROM AUTHOR]

Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting these filaments could yield information on the magnetic field strength, cosmic ray population, and temperature of intercluster gas; yet, the faint and large-scale nature of these bridges makes direct detections very challenging. Using multiple independent all-sky radio and X-ray maps we stack pairs of luminous red galaxies as tracers for cluster pairs. For the first time, we detect an average surface brightness between the clusters from synchrotron (radio) and thermal (X-ray) emission with ≳5σ significance, on physical scales larger than observed to date (⁠|${\ge}3$|  Mpc). We obtain a synchrotron spectral index of α ≃ −1.0 and estimates of the average magnetic field strength of |$30\,\mathrm{ nG} \le B \le 60 $|  nG, derived from both equipartition and inverse-Compton arguments, implying a 5–15 per cent degree of field regularity when compared with Faraday rotation measure estimates. While the X-ray detection is inline with predictions, the average radio signal comes out higher than predicted by cosmological simulations and dark matter annihilation and decay models. This discovery demonstrates that there are connective structures between mass concentrations that are significantly magnetized, and the presence of sufficient cosmic rays to produce detectable synchrotron radiation. [ABSTRACT FROM AUTHOR]

Large-scale coherent magnetic fields in the intergalactic medium (IGM) are presumed to play a key role in the formation and evolution of the cosmic web, and in large-scale feedback mechanisms. However, they are theorized to be extremely weak, in the nano-Gauss regime. To search for a statistical signature of these weak magnetic fields, we perform a cross-correlation between the Faraday rotation measures (RMs) of 1742 radio galaxies at z > 0.5 and large-scale structure at 0.1 < z < 0.5, as traced by 18 million optical and infrared foreground galaxies. No significant correlation signal was detected within the uncertainty limits. We are able to determine model-dependent 3σ upper limits on the parallel component of the mean magnetic field strength of filaments in the IGM of ∼30 nG for coherence scales between 1 and 2.5 Mpc, corresponding to a mean upper bound RM enhancement of ∼3.8 rad m−2 due to filaments along all probed sightlines. These upper bounds are consistent with upper bounds found previously using other techniques. Our method can be used to further constrain intergalactic magnetic fields with upcoming future radio polarization surveys. [ABSTRACT FROM AUTHOR]

We present the largest ever sample of radio polarization properties for z > 4 sources, with 14 sources having significant polarization detections. Using wide-band data from the Karl G. Jansky Very Large Array, we obtained the rest-frame total intensity and polarization properties of 37 radio sources, nine of which have spectroscopic redshifts in the range 1 ≤ z ≤ 1.4, with the other 28 having spectroscopic redshifts in the range 3.5 ≤ z ≤ 6.21. Fits are performed for the Stokes I and fractional polarization spectra, and Faraday rotation measures are derived using rotation measure synthesis and QU fitting. Using archival data of 476 polarized sources, we compare high-redshift (z>3) source properties to a 15 GHz rest-frame luminosity matched sample of low-redshift (z < 3) sources to investigate if the polarization properties of radio sources at high redshifts are intrinsically different than those at low redshift. We find a mean of the rotation measure absolute values, corrected for Galactic rotation, of 50 ± 22 radm-2 for z > 3 sources and 57 ± 4 radm-2 for z < 3. Although there is some indication of lower intrinsic rotation measures at high-z possibly due to higher depolarization from the high-density environments, using several statistical tests we detect no significant difference between low- and high-redshift sources. Larger samples are necessary to determine any true physical difference. [ABSTRACT FROM AUTHOR]

This is the first of two papers describing the observations and cataloguing of deep 3-GHz observations of the Lockman Hole North using the Karl G. Jansky Very Large Array. The aim of this paper is to investigate, through the use of simulated images, the uncertainties and accuracy of source-finding routines, as well as to quantify systematic effects due to resolution, such as source confusion and source size. While these effects are not new, this work is intended as a particular case study that can be scaled and translated to other surveys. We use the simulations to derive uncertainties in the fitted parameters, as well as bias corrections for the actual catalogue (presented in Paper II). We compare two different source-finding routines, OBIT and AEGEAN, and two different effective resolutions, 8 and 2.75 arcsec. We find that the two routines perform comparably well, with OBIT being slightly better at deblending sources, but slightly worse at fitting resolved sources. We show that 30-70 per cent of sources are missed or fit inaccurately once the source size becomes larger than the beam, possibly explaining source count errors in high-resolution surveys. We also investigate the effect of blending, finding that any sources with separations smaller than the beam size are fit as single sources. We show that the use of machine-learning techniques can correctly identify blended sources up to 90 per cent of the time, and prior-driven fitting can lead to a 70 per cent improvement in the number of de-blended sources. [ABSTRACT FROM AUTHOR]

We present a study of diffuse extragalactic radio emission at 1.75 GHz from part of the ELAIS-S1 (European Large Area Infrared Space Observatory Survey – South 1) field using the Australia Telescope Compact Array. The resulting mosaic is 2.46 deg2, with a roughly constant noise region of 0.61 deg2 used for analysis. The image has a beam size of 150 arcsec × 60 arcsec and instrumental 〈σn〉 = (52 ± 5) μJy beam−1. Using point-source models from the Australia Telescope Large Area Survey, we subtract the discrete emission in this field for S ≥ 150 μJy beam−1. Comparison of the source-subtracted probability distribution, or P(D), with the predicted distribution from unsubtracted discrete emission and noise, yields an excess of (76 ± 23) μJy beam−1. Taking this as an upper limit on any extended emission, we constrain several models of extended source counts, assuming Ωsource ≤ 2 arcmin. The best-fitting models yield temperatures of the radio background from extended emission of Tb = (10 ± 7) mK, giving an upper limit on the total temperature at 1.75 GHz of (73 ± 10) mK. Further modelling shows that our data are inconsistent with the reported excess temperature of ARCADE2 to a source-count limit of 1 μJy. Our new data close a loop-hole in the previous constraints, because of the possibility of extended emission being resolved out at higher resolution. Additionally, we look at a model of cluster halo emission and two dark matter particle annihilation source-count models, and discuss general constraints on any predicted counts from such sources. Finally, we report the derived integral count at 1.4 GHz using the deepest discrete count plus our new extended-emission limits, providing numbers that can be used for planning future ultradeep surveys. [ABSTRACT FROM AUTHOR]