Context. The existence of magnetic fields in the circumgalactic medium (CGM) is largely unconstrained. Their detection is important as magnetic fields can have a significant impact on the evolution of the CGM, and, in turn, the fields can serve as tracers for dynamical processes in the CGM. Aims. Using the Faraday rotation of polarised background sources, we aim to detect a possible excess of the rotation measure in the surrounding area of nearby galaxies. Methods. We used 2461 residual rotation measures (RRMs) observed with the LOw Frequency ARray (LOFAR), where the foreground contribution from the Milky Way is subtracted. The RRMs were then studied around a subset of 183 nearby galaxies that was selected by apparent B-band magnitude. Results. We find that, in general, the RRMs show no significant excess for small impact parameters (i.e., the perpendicular distance to the line of sight). However, if we only consider galaxies at higher inclination angles and sightlines that pass close to the minor axis of the galaxies, we find significant excess at impact parameters of less than 100 kpc. The excess in |RRM| is 3.7 rad m⁻² with an uncertainty between ±0.9 rad m⁻² and ±1.3 rad m⁻² depending on the statistical properties of the background (2.8σ–4.1σ). With electron densities of ∼10⁻⁴ cm⁻³, this suggests magnetic field strengths of a few tenths of a microgauss. Conclusions. Our results suggest a slow decrease in the magnetic field strength with distance from the galactic disc, as expected if the CGM is magnetised by galactic winds and outflows.

Context. Cosmic rays and magnetic fields are key ingredients in galaxy evolution, regulating both stellar feedback and star formation. Their properties can be studied with low-frequency radio continuum observations that are free from thermal contamination. Aims. We define a sample of 76 nearby (< 30 Mpc) galaxies with rich ancillary data in the radio continuum and infrared from the CHANG-ES and KINGFISH surveys, which will be observed with the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz. Methods. We present maps for 45 of them as part of the LoTSS data release 2 (LoTSS-DR2), where we measure integrated flux densities and study integrated and spatially resolved radio spectral indices. We investigate the radio–star formation rate (SFR) relation using SFRs derived from total infrared and Hα + 24-μm emission. Results. The radio–SFR relation at 144 MHz is clearly super-linear with L_144 MHz ∝ SFR^1.4−1.5. The mean integrated radio spectral index between 144 and ≈1400 MHz is ⟨α⟩= − 0.56 ± 0.14, in agreement with the injection spectral index for cosmic ray electrons (CREs). However, the radio spectral index maps show variation of spectral indices with flatter spectra associated with star-forming regions and steeper spectra in galaxy outskirts and, in particular, in extra-planar regions. We found that galaxies with high SFRs have steeper radio spectra; we find similar correlations with galaxy size, mass, and rotation speed. Conclusions. Galaxies that are larger and more massive are better electron calorimeters, meaning that the CRE lose a higher fraction of their energy within the galaxies. This explains the super-linear radio–SFR relation, with more massive, star-forming galaxies being radio bright. We propose a semi-calorimetric radio–SFR relation that employs the galaxy mass as a proxy for the calorimetric efficiency.

Context. Galactic winds play a key role in regulating the evolution of galaxies over cosmic time. In recent years, the role of cosmic rays (CRs) in the formation of the galactic wind has increasingly gained attention. Therefore, we use radio continuum data to analyse the CR transport in edge-on galaxies. Aims. With newly reduced radio continuum data of five edge-on galaxies (NGC 891, NGC 3432, NGC 4013, NGC 4157, and NGC 4631), we plan to set new constraints on the morphology of radio halos and the physical properties of galactic winds driven by stellar feedback. By distinguishing between the central and outer regions of the galaxies, our study setup allows us to search for variations in the radio halo profile or CR transport along the galactic disk. Methods. Data from the LOFAR Two-metre Sky Survey (LoTSS) Data Release 2 at 144 MHz (HBA) and reprocessed data from the Very Large Array (VLA) at 1.5 GHz (L band) from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES) enable us to increase the extent of the analysed radio continuum profile significantly (up to a factor of 2) compared to previous studies. We computed thermal emission maps using a mixture approach with Hα and near-infrared data, which were then subtracted to yield radio synchrotron emission maps. Then we compiled non-thermal spectral index maps and computed intensity profiles using a box integration approach. Lastly, we performed 1D CR transport modelling. Results. The non-thermal spectral index maps show evidence that the LoTSS maps are affected by thermal absorption in star-forming regions. The scale height analysis reveals that most of the galaxies are equally well fitted with a one-component instead of a two-component exponential profile. We find a bi-modality within our sample. While NGC 3432 and NGC 4013 have similar scale heights in the L band and HBA, the low-frequency scale heights of NGC 891, NGC 4157, and NGC 4631 exceed their high-frequency counterpart significantly. The 1D CR transport modelling shows agreement between the predicted magnetic field strength and the magnetic field strength estimates of equipartition measurements. Additionally, we find an increasing difference in wind velocities (with increasing height over the galactic disk) between the central and outer regions of the analysed galaxies.

We present the third data release of the LOFAR Two-metre Sky Survey (LoTSS-DR3). The survey images cover 88% of the northern sky and were created from 12 950 h of data (18.6 PB) accumulated over 10.5 years. Producing the images took 20 million core hours of processing through direction-independent and direction-dependent calibration pipelines that correct for instrumental effects as well as spatially and temporally varying ionospheric distortions. In our 120–168 MHz continuum mosaic images with an angular resolution of 6″ (9″ below declination 10°) we catalogue 13 667 877 sources, formed from 16 943 656 Gaussian components. The scatter in the astrometric precision approximately follows the expected noise-like behaviour but with an additional systematic component of at least 0.24″ that is likely due to calibration imperfections. The random flux density scale error is 6%, while the systematic offset was previously shown to be within 2%. The median sensitivity of our mosaics is 92 μJy beam ⁻¹ , improving to 68 μJy beam ⁻¹ at high observing elevations, but degrading to 183 μJy beam ⁻¹ at the celestial equator due to station area projection effects. Completeness simulations, accounting for realistic source models, time- and bandwidth-smearing effects, and astrometric errors, indicate that we detect more than 95% of compact sources with integrated flux densities exceeding 9 times the local root mean square (RMS) noise. However, the recovered source counts in a particular integrated flux density bin do not match the injected counts until flux densities exceed 45 times the local RMS noise. The Euclidean-normalised differential source counts derived from the survey constrain the radio source population over five orders of magnitude and are in good agreement with previous deep and wide-area surveys. All data products are publicly available, including catalogues, individual-field Stokes I , Q , U , and V images, mosaicked Stokes I images, and uv data with associated direction-dependent calibration solutions.

CHANG-ES (Continuum Halos in Nearby Galaxies—an EVLA Survey) is an ambitious project to target 35 nearby disk galaxies that are edge-on to the line of sight. The orientation permits both the disk and halo regions to be studied. The observations were initially at 1.5 GHz (L-band) and 6.0 GHz (C-band) in a variety of VLA array configurations, and in all four Stokes parameters, which allowed for spatially resolved images in total intensity plus polarization. The inclusion of polarization is unique to an edge-on galaxy survey and reveals the galaxies’ halo magnetic fields. This paper will summarize the results to date, some of which are new phenomena, never seen prior to CHANG-ES. For example, we see that ‘X-type’ fields, as well as rotation measure reversals, are common features of spiral galaxies. Further observations at 3.0 GHz (S-band) as well as future scientific opportunities will also be described.

Context. Magnetic fields, which regulate stellar feedback and star formation in galaxies, are key to understanding galaxy evolution. Aims. We probe the origin of magnetic fields in late-type galaxies, measuring magnetic field strengths and exploring whether magnetic fields are only passive constituents of the interstellar medium or whether, being part of the local energy equilibrium, they are active constituents. Methods. We measure equipartition magnetic field strengths in 39 galaxies from the second data release of the LOFAR Two-metre Sky Survey (LoTSS-DR2) at 144 MHz with 6 arcsec angular resolution (0.1–0.7 kpc). For a subset of nine galaxies, we obtain atomic and molecular mass surface densities using H I and CO(2-1) data from the THINGS and HERACLES surveys, respectively. These data are at 13 arcsec angular resolution, which corresponds to 0.3–1.2 kpc at the distances of our galaxies. We measure kinetic energy densities using H I and CO velocity dispersions. Results. We find a mean magnetic field strength of 3.6–12.5 μG with a mean of 7.9 ± 2.0 μG across the full sample. The magnetic field strength has the tightest and steepest relation with the total gas surface density, with B ∝ ΣH I+H₂^{0.309 ± 0.006}. The relations with the star-formation rate surface density and molecular gas surface density have significantly flatter slopes. After accounting for the influence of cosmic-ray transport, we find an even steeper relation of B ∝ ΣH I+H₂^{0.393 ± 0.009}. Conclusions. These results suggest that the magnetic field is regulated by a B–ρ relation, which has its origin in the saturation of the small-scale dynamo. This is borne out by an agreement of kinetic and magnetic energy densities, although local deviations do exist, in particular in areas of high kinetic energy densities where the magnetic field is sub-dominant.