Volker Heesen

Radio continuum emission traces star formation in galaxies, but cosmic-ray electron (CRE) propagation between the disk and halo complicates this connection. We present a wide-band, spatially resolved analysis of 19 nearby edge-on galaxies, combining LOFAR observations at 144 MHz with CHANG-ES VLA imaging at 1.575, 3.0, and 6.0 GHz, and hybrid H α +22 μ m star formation rate (SFR) maps. All maps are convolved to a common 2.1 kpc resolution for pixel-by-pixel analysis of the nonthermal spectral index α _nth and the SFR surface density Σ _SFR . All 19 galaxies exhibit statistically significant nonthermal spectral flattening with increasing Σ _SFR , with a sample median slope of 0.1 9 − 0.03 + 0.09 that traces the contrast between CRE injection in the star-forming disk and cooling in the halo. The galaxy-to-galaxy scatter in this slope shows no dependence on global galaxy properties and instead appears to be modulated by the local dynamical environment, with the largest α _nth –Σ _SFR slopes appearing in tidally perturbed systems. The resolved radio–SFR slope steepens from 0.57 at 144 MHz to 0.95 at 6 GHz, approaching linearity at high frequencies. This trend is consistent with energy-dependent CRE cooling and transport, with the deviation from linearity most pronounced at low frequencies, where the edge-on line of sight blends midplane and halo CRE populations. These results are robust against thermal contamination, beam smearing, and spectral curvature. Together, our analysis establishes α _nth as an observational probe of projected disk-to-halo CRE transport in edge-on galaxies and quantifies how this transport reshapes the frequency dependence of resolved radio–SFR calibrations.

Context . Face-on spiral galaxies offer a favorable geometry for studying magnetic-field structures and their impact on cosmic-ray propagation, as projection effects and structural overlap are significantly reduced. Aims . We aim to investigate the transport of cosmic-ray electrons (CREs) in the nearby face-on spiral galaxy NGC 2442 and assess how the galactic environment and magnetic-field structure influence their propagation. Methods . We combined radio continuum (RC) observations from ASKAP at 943 MHz, MeerKAT at 1.28 and 1.7 GHz, and ATCA at 5 GHz with optical H α and infrared data to characterize the galactic environment. We then compared these observations with 2D CRE transport simulations to evaluate the role of the magnetic field and the surrounding environment in shaping CRE propagation. Results . NGC 2442 exhibits an integrated RC spectrum that is steeper than those of most nearby galaxies, with spectral indices of α = −0.96 ± 0.04 for the total emission and α _nt = −1.21 ± 0.04 for the synchrotron emission over 408 MHz–5 GHz. The spectrum shows a break near 1 GHz, indicating substantial radiative aging of the CRE population. Under the equipartition assumption, we derive a mean magnetic-field strength of 10.8 µG. The RC–star formation rate smoothing analysis indicates effective CRE propagation lengths of ∼0.65–0.89 kpc at 943–1700 MHz and ∼0.44 kpc at 5 GHz, corresponding to effective diffusion coefficients on the order of 10 ²⁸ cm ² s ⁻¹ . We identify a steep-spectrum synchrotron “island” in the southeastern region, with an average synchrotron spectral index of α ∼ −1.09 and no clear H α , infrared, far-ultraviolet or near-ultraviolet counterpart, indicating that the CREs are unlikely to be injected in situ. Our 2D simulations using CRPropa show that anisotropic diffusion along ordered magnetic fields allows CREs to reach the island region much more efficiently than isotropic diffusion. Conclusions . NGC 2442 provides evidence that the combined effects of environmental disturbances and ordered magnetic fields can strongly regulate CRE propagation in disturbed spiral galaxies.

We present Data Release 5 (DR5) of the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES), including select data products. These data were observed with the Karl G. Jansky Very Large Array (VLA) in C configuration, in the L band (1.575 GHz) and C band (6.0 GHz) for all 35 edge-on galaxies in the CHANG-ES sample. We combine the new C-configuration L-band images with our previously released D-configuration C-band data, together with H α and Wide-field Infrared Survey Explorer 22 μ m infrared maps, to separate the thermal and nonthermal radio continuum emission across all 34 galaxies for which H α data are available. This yields spatially resolved maps of the thermal fraction, total and nonthermal band-to-band spectral index, and star formation rate at 15″ resolution. Together with total intensity images at two weightings, these data products are included in an atlas presented here and made publicly available at https://projects.canfar.net/changes . We find that nonthermal spectral indices flatten in star-forming regions and also that radio power increases with star formation rate.

Context . Radio continuum emission has the potential to be useful as an extinction-free tracer of star formation. However, the relation between the radio continuum luminosity and star formation rate (SFR) is potentially limited by various effects such as cosmic-ray transport, free-free absorption, and cosmic-ray electron energy losses. Aims . We aim to calibrate the radio-SFR relation in a sample of nearby galaxies ranging from dwarf to spiral galaxies, covering nearly five orders of magnitude in terms of the SFR range. We include both global and local measurements from individual and spatially resolved, kiloparsec-sized galaxies, respectively. Methods . We measured the radio continuum luminosities at 144 MHz using observations with the LOw Frequency ARray (LOFAR) and measured the radio spectral indices using ancillary 1.4 GHz data. We selected 70 nearby (distance d < 30 Mpc) galaxies, 15 of which were used for local measurements. Taking advantage of their rich ancillary data, we carried out a study of the radio-SFR relation using the total infrared (TIR), mid-infrared (MIR), H α , and far-ultraviolet (FUV) as complementary SFR tracers. About a third of our sample is made up of at least moderately star-forming edge-on galaxies with the remaining ones chosen to form a representative sample of a wide range of morphological types and SFR values. Results . For the first time, we show that the radio luminosity ( L ₁₄₄ ), the SFR, and the radio spectral index ( α ) define a ‘fundamental plane’ in the [log( L ₁₄₄ ), log(SFR), α ] space. This enables us to define a unified radio-SFR relation that is applicable to both for global and local data when using the radio spectral index as a secondary parameter. Conclusions . A unified radio-SFR relation for both global and local data could serve as a litmus test for galaxy simulations that include the effect of cosmic rays and magnetic fields. It also strengthens the case for using the radio-SFR relation as an extinction-free tracer of star formation.

We present new Karl G. Jansky Very Large Array S -band (2–4 GHz) observations of the edge-on spiral galaxy NGC 891, complemented by C -band data, to investigate the structure of its radio continuum halo. Using rotation measure synthesis we detected an extended polarized halo, with most spatially extended polarized emission confined to Faraday depths within ±150 rad m ⁻² . We identified a localized region in the northeast side of the galaxy that shows an enhancement in polarized intensity (not in percentage polarization). By combining the radio data with H α and diffuse X-ray maps, we discuss a possible origin for this structure: a superbubble powered by clustered supernovae. Across the disk and halo, the percentage polarization decreases toward the midplane but shows a mild wavelength dependence, despite the edge-on orientation of NGC 891. This behavior implies that the depolarization cannot be dominated by small-scale Faraday rotation within the disk. Instead, it is possible that most of the observed polarized emission arises on the Earth-facing side of the galaxy. Our peak rotation measure map shows a smooth transition along the major axis, consistent with a large-scale axisymmetric magnetic field. Using H α and UV data, we analyzed the distribution of H ii regions and found that they are parts of different spiral arms. We also identified a faint, isolated H ii region at a galactocentric radius of 16.9 kpc, with both H α and far-UV counterparts, indicating star formation outside the thin disk.

Context. Galactic magnetic fields regulate star formation and cosmic-ray (CR) transport. Understanding their three-dimensional structure is key to constraining galactic CR transport. Aims. By stacking the linearly polarised signal from a sample of star-forming edge-on galaxies, we aim to analyse the average magnetic field structure of star-forming galaxies. Methods. Using synthetic data, we explored the validity of stacking Stokes Q and U spectra to infer the intrinsic polarisation characteristics of star-forming galaxies. Before stacking, we aligned, scaled, convolved, and reprojected C -band (6 GHz, λ = 0.05 m) Stokes Q and U cubes of 27 star-forming edge-on galaxies. For the stacked cubes, we performed an RM-synthesis and discussed the derived polarised intensity (PI), polarisation angle ( χ ₀ ), and RM maps. Results. Synthetic data tests demonstrate that stacking Stokes Q and U spectra is valid for tightly constrained underlying distributions of PI, χ ₀ , and RM. We identify the spread of the underlying RM distribution ( σ _RM ) as a critical parameter and establish a threshold of σ _RM ^thresh = 180 rad m ^–2 ( σ _RM ^thresh λ ² = 0.45 rad). For conditions that represent star-forming galaxies, stacking introduces a systematic uncertainty of δ _RM ^sys = 90 rad m ^–2 and underestimates the recovered PI. Stacking results reveal an X-shaped pattern, and polarised emission is detected up to 7 kpc above the galactic disc. We find higher PI on the approaching side of galaxies and a decrease in PI in the galactic halo of ∼60% near the galaxy’s minor axis. For an X-shaped halo field, this aligns with the fitted opening angle of these galaxies and suggests that there is energy equipartition between CRs and the magnetic field rather than a uniform CR density. A global RM pattern, as reported in a previous study, cannot be confirmed. Conclusions. We present stacking of Stokes Q and U cubes as an effective tool to recover faint polarised emission in the halo of nearby galaxies, if the underlying distributions of PI, χ ₀ , and RM are tightly constrained.

We present the Very Large Array C-array S -band (2–4 GHz) radio continuum observations of the nearby edge-on spiral galaxy NGC 4565, a target from the Continuum Halos in Nearby Galaxies—an EVLA Survey. We conduct rotation measure (RM) synthesis to probe the magnetic field structure and analyze the vertical radio continuum intensity profiles using the 1D cosmic-ray (CR) transportation models. The radio continuum emission of NGC 4565 is vertically compact, with a vertical-to-radial extent ratio of ∼1/6. Its vertical profile is optimally described by a two-component Gaussian distribution, yielding a mean Gaussian halo scale height of ∼3.0 kpc. The magnetic field is weak, predominantly disk parallel, with an equipartition strength of ≲5 μ G and an RM profile indicative of an axisymmetric spiral structure. Nevertheless, we identify a localized, faint vertical magnetic field component in the northeastern region, hinting at an X-shaped structure that spatially coincides with extraplanar structures detected in H i and soft X-ray emission. The CR transport modeling favors a flux-tube advection scenario, with a slow initial velocity of v ₀ ≈ 60 km s ⁻¹ , consistent with a limited energy input from star formation. Therefore, the absence of an extended radio halo can be explained by the low star formation rate, the weak magnetic field, and the inefficient CR transport. The localized X-shaped field may trace a weak, magnetically guided outflow or a tidal perturbation induced by the nearby companion. NGC 4565 is thus a key quiescent benchmark for understanding the physical conditions required to drive large-scale outflows and generate extended radio halos.

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.

Context. Cosmic rays and magnetic fields play an important role in the formation and evolution of galaxies. Radio continuum observations enable their study in the halos of edge-on galaxies. Aims. We explore the frequency dependence of the radio scale height which depends on cosmic ray transport and electron cooling. We test the influence of fundamental galaxy properties, such as the star formation rate (SFR), mass and size. Methods. We used radio continuum data of 16 edge-on galaxies from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES). We included maps from the LOw Frequency ARray (LOFAR) at 144 MHz and from the Jansky Very Large Array (VLA) at 3 GHz with 7″ angular resolution. We extracted vertical intensity profiles within the effective radio radius and fitted beam-convolved double-exponential models to separate the thin and thick discs. For the thick radio discs, we computed their mean spectral indices and scale-height ratios between 144 MHz and 3 GHz. Results. We find a mean scale-height ratio of 1.26 ± 0.16. This is much lower than what we would expect for either cosmic ray diffusion or advection if synchrotron and inverse Compton losses dominate for the electrons. There is a moderate positive correlation between the ratio and spectral index of the thick disc: galaxies with high ratios have flat radio spectra. The ratio does not depend on any other galaxy parameter. The radio spectrum of the thick disc, as indicated by the radio spectral index, steepens with total mass (strong correlation) and flattens with SFR-to-mass surface density (moderate correlation). Conclusions. Galaxies with galactic winds have flat radio continuum spectra and large scale heights at low frequencies. This shows effective transport of cosmic rays in such systems.

Context. Cosmic rays and magnetic fields play an important role for the formation and evolution of galaxies. Radio continuum observations allows us to study them in the haloes of edge-on galaxies. Aims. We explore the frequency dependence of the radio scale height which depends on cosmic ray transport and electron cooling. We test the influence of fundamental galaxy properties, such as star-formation rate (SFR), mass and size. Methods. We used radio continuum data of 16 edge-on galaxies from the Continuum HAloes in Nearby Galaxies -- an EVLA Survey (CHANG-ES). We included maps from the LOw Frequency ARray at 144 MHz and from the Jansky Very Large Array at 3 GHz with 7" angular resolution. We extracted vertical intensity profiles within the effective radio radius and fitted beam-convolved double-exponential models to separate thin and thick discs. For the thick radio discs, we computed mean spectral indices and scale-height ratios between 144 MHz and 3 GHz. Results. We find a mean scale-height ratio of 1.26 \pm 0.16. This is much lower than what we would expect for either cosmic ray diffusion or advection if synchrotron and inverse Compton losses dominate for the electrons. There is a moderate positive correlation between the ratio and spectral index of the thick disc: galaxies with high ratios have flat radio spectra. The ratio does not depend on any other galaxy parameter. The radio spectrum of the thick disc, as indicated by the radio spectral index, steepens with total mass (strong correlation) and flattens with SFR-to-mass surface density (moderate correlation). Conclusions. Galaxies with galactic winds have flat radio continuum spectra and large scale heights at low frequencies. This shows effective transport of cosmic rays in such systems.