arXiv subjects

M. Brüggen

Publications and source records attributed to M. Brüggen.

At least 19 recordsLinked to original sources

arXiv2026

The LOFAR Decametre Sky Survey (LoDeSS) I. Survey description and first data release

The decametre radio sky ($\sim 10$-$30$ MHz) remains largely unexplored due to strong ionospheric distortions and severe radio-frequency interference. Despite its scientific potential, including access to steep-spectrum synchrotron emission from aged electron populations and coherent radio emission from planetary and stellar magnetospheres, sub-arcminute imaging at these frequencies has only recently become feasible with instruments such as the Low Frequency Array (LOFAR). In this work, we present the first data release (DR1) of the LOFAR decametre Sky Survey (LoDeSS), which maps the northern sky above a declination of $20^\circ$ between 15 and 30 MHz. LoDeSS reaches an angular resolution of $\sim45^{\prime\prime}$ and typical noise levels of $\sim11.5$ mJy beam$^{-1}$ at a central frequency of 23 MHz. Observations were obtained with the LOFAR Low Band Antenna (LBA) system using a multi-beam strategy, with typical integration times of $\sim5$ h per pointing. They were carried out at night to reduce the impact of ionospherically reflected radio-frequency interference. Data quality varies across fields as a result of the ionospheric conditions and the fraction of data flagged due to radio-frequency interference. This first release covers $\sim15\%$ of the full survey area and consists of fields originally selected around exoplanet candidates for radio auroral emission. The products released to the community include primary-beam-corrected images in Stokes I and V, and a source catalogue for fields with reliable flux density scales. This release demonstrates that LOFAR enables sensitive, wide-field imaging in the decametre regime with unprecedented resolution, opening new opportunities for studies of pulsars, exoplanetary systems, galaxy clusters, and other low-frequency phenomena.

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arXiv2026

ViCTORIA project: A pilot study of the M49 region in the Virgo cluster in polarisation

Large-scale magnetic fields permeate the Intracluster Medium (ICM) of galaxy clusters. Understanding these fields gives an insight into the origin of cosmic magnetic fields and the interaction of cluster galaxies with the ICM. Using MeerKAT observations of the Virgo cluster as part of the ViCTORIA project, we probe magnetic fields via Rotation Measure (RM) synthesis. The aim of this work is to characterise the magnetic fields in and around the infalling group M49 in the Virgo cluster. We will probe the interaction of M49 with its surrounding medium through its imprint on the polarised emission and Faraday rotation. We use L-band MeerKAT data in full polarisation calibrated with the ViCTORIA MeerKAT Survey (ViMS) pipeline, which is designed for the calibration, imaging, mosaicking and RM Synthesis of the fields in the Virgo cluster, to achieve high-fidelity reconstruction of, both, total intensity and polarised emission. We analyse the background sources of the cluster for potential local changes in the magnetic field, by calculating the RM distribution. In the final polarised image, consisting of four pointings of 45 min, we reach a noise level of $23.3 μ\textrm{Jy/beam}$. We detect 101 background sources in the $\sim 2.5\, \textrm{deg}^2$ region around M49, corresponding to a source density of 40 sources/$ \textrm{deg}^2$. Across the $112\, \textrm{deg}^2$ observations of the Virgo cluster, this implies a total number of 4000 polarised sources. In M49 we find structured, diffuse polarised emission extending in the direction of the total intensity radio tails. A higher polarisation fraction across the upper edge of the source indicates the effect of magnetic field draping. Around M49 we find a larger RM scatter (at $3.8σ$) in the wake of M49 compared to the region in front of it. This can inform the modelling of the turbulence created by galactic motions.

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arXiv2026

XMAGNET -- Stir before serving: a Lagrangian perspective on mixing-driven condensation in the intracluster medium

We aim to characterize the thermodynamic and dynamical conditions leading to condensation in cluster cores, and to assess the role of magnetic fields. We implement a Monte-Carlo tracer particle algorithm in the GPU-accelerated code AthenaPK, and run a purely hydrodynamical and a magnetohydrodynamical (MHD) simulations of an idealized cool-core cluster. We identify the subset of hot ICM tracers that undergo a transition to the cold phase and reconstruct their histories over a lookback time of $300\,\mathrm{Myr}$ prior to condensation. In both runs, the large majority of tracers transitioning to the cold phase follow a thermodynamic pathway driven by mixing, whereby hot ambient gas is entrained onto low-entropy seed clumps that subsequently grow into larger clouds and filaments. In the hydrodynamical run, these seeds form mainly via in-situ cooling at the edges of AGN cavities. In the MHD run, the cold gas cycle is more complex: AGN outflows occasionally shred portions of existing filaments into fragments which are then uplifted, seeding further condensation. In the MHD run, the properties of condensing tracers begin to diverge from the background ICM significantly earlier than in the hydrodynamical run (${\sim}150\,\rm Myr$ before the cooling transition versus ${\sim}30\,\rm Myr$), with vorticity and magnetic energy growing together. The turbulent Mach number at condensation is also systematically lower than in the hydrodynamical run. We examine the post-condensation evolution of individual cold structures in the MHD run, namely a massive core filament and two isolated clouds in quiescent regions. We find that magnetic tension dominates over ram pressure as the primary drag force, significantly reducing the clouds' terminal velocity. Our results demonstrate that magnetic fields substantially impact the assembly history and kinematic properties of the cold phase in cool-core clusters.

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arXiv2026

The plethora of diffuse emission in Abell 2034 as revealed by MeerKAT polarization observations

We present MeerKAT observations of the galaxy cluster Abell 2034, a massive (M_500=5.21 10^14 solar masses) nearby cluster in a merging state. Previous observations at 144 MHz have shown that the cluster exhibits a plethora of diffuse emission, with multiple diffuse sources of uncertain classification because of the lack of spectral and polarimetric observations. MeerKAT multi-frequency observations, centered at 816 MHz and 1.28 GHz, together with archival low-frequency LOFAR observations at 144 MHz have allowed us to shed light on the properties of these sources. The polarization properties and spectral index information let us conclude that the cluster hosts one radio relic, a source with a very steep spectrum, previously classified as candidate relic, and filaments of very steep emission around the tailed radio galaxies identified at low frequencies. The presence of a radio halo is confirmed, and its spectrum shows hints for curvature between 144 MHz ad 1.28 GHz. The polarimetric data in the L-band, together with the model of the gas density derived from X-ray observations are used to constrain the magnetic field in the intracluster medium. We assume a radially symmetric magnetic field model, whose strength declines with the cluster gas density as B(r) ~ n_e(r)^0.5, and normalize its strength within R_500. We find that B_500=1 muG best explains the Faraday depth properties of the cluster, though the detection of sources close to the cluster center would be crucial to discriminate among different values. We conclude that the cluster Abell 2034 shows diffuse emission with complex morphologies that do not follow the historical categories of halos and relics. Deep multi-frequency and polarimetric observations are fundamental to understand their origin.

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arXiv2026

Radio Halos in Galaxy Clusters as unveiled by the SKA telescope

Giant radio halos (RHs) are diffuse, Mpc-scale synchrotron sources observed in a growing fraction of galaxy clusters. They trace relativistic particles and magnetic fields in the intracluster medium (ICM), providing a unique window into non-thermal processes and their role in cluster evolution. RHs are primarily found in merging systems, supporting models in which turbulence generated during cluster collisions re-accelerates pre-existing electrons to the energies required for the observed radio emission. In this scenario, the occurrence, power, and spectral properties of RHs depend on the energetics of cluster mergers, with the most massive and dynamically disturbed clusters hosting the most powerful halos. Low-frequency observations are crucial to uncover ultra-steep-spectrum RHs, a key prediction of turbulent re-acceleration models, and are expected to arise from less energetic merger events. LOFAR has enabled statistical studies of large cluster samples, placing robust constraints on RH occurrence and spectral trends. In this Chapter, we model RH formation and evolution using Monte Carlo simulations calibrated on LoTSS-DR2 findings, and we present predictions for SKA-Low in the AA4 configuration. Our results show that SKA will probe an unprecedented region of cluster mass and redshift space, detecting at least $\sim 2500$ RHs up to $z \approx 0.6$, including $\gtrsim 1000$ ultra-steep-spectrum systems, and revealing halos in clusters down to $\sim 10^{14}\, M_\odot$ and out to $z \approx 1$. These surveys will provide stringent tests of turbulent re-acceleration models and significantly advance our understanding of non-thermal processes in galaxy clusters.

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arXiv2026

The Large-Scale Structure of the Universe through the SKA lenses

The large-scale distribution of galaxies in the Universe forms an intricate, interconnected network known as the cosmic web. Cosmological simulations within the standard Lambda-CDM framework successfully reproduce this filamentary structure and predict that the nodes and filaments are filled with tenuous plasma at temperatures ranging from 10^5-10^8 K. The hottest and luminous plasma in the nodes corresponds to the intra-cluster medium, while the cooler, more tenuous, gas extends along filaments and cluster outskirts. Galaxies and galaxy groups form and flow along these filaments before accreting onto galaxy clusters (the nodes), outlining the dynamical evolution of large-scale structures. During this process, an enormous amount of energy is dissipated through complex plasma processes that can be traced by radio emitting electrons. Despite strong theoretical support for this picture, observational validation remains limited. While massive clusters have been widely detected across various wavelengths, cluster outskirts and the diffuse intergalactic medium within filaments has remained elusive due to their extremely faint emission. The advent of highly sensitive radio facilities such as LOFAR, uGMRT, and MeerKAT has recently enabled a few successful detections of emission from comparatively denser regions of the cosmic-web. These include radio megahalos, permeating the entire cluster volume, as well as bridges of radio emission connecting cluster pairs. In this chapter, we summarize current theoretical insights into the cosmic web, discuss observational strategies and recent discoveries, and highlight how the forthcoming Square Kilometre Array (SKA) is expected to transform our understanding of the cosmic web and the distribution of baryons in the Universe.

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arXiv2026

Nearby galaxies in the LOFAR Two-metre Sky Survey IV. A fundamental plane of the radio-SFR relation

Radio continuum emission has the potential to be an extinction-free tracer of star formation. However, the relation between radio continuum luminosity and star formation rate, the radio-SFR relation, is affected by various effects such as cosmic-ray transport, free-free absorption, and cosmic-ray electron energy losses. 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 SFR range. We include, both, global (individual galaxies) and local (kiloparsec sized) measurements. We measured radio continuum luminosities at $144\,\rm MHz$ using observations with the LOw Frequency ARray (LOFAR) and measure radio spectral indices using ancillary $1.4\,\rm GHz$ data. Selecting 70 nearby (distance $d < 30\,\rm Mpc$) galaxies, 15 of which were used for local measurements, with rich ancillary data we present a study of the radio-SFR relation using total infrared, mid-infrared, H$α$, and far-ultraviolet as complementary SFR tracers. About one third of our sample are at least moderately star-forming edge-on galaxies with the remaining ones chosen to be a representative sample of a wide range of morphological types and SFR values. For the first time, we show that the radio luminosity ($L_{144}$), the star-formation rate (SFR), and the radio spectral index ($α$) define a 'fundamental plane' in the [$\log(L_{144})$, $\log(\rm SFR)$, $α$] space. This allows us to define a unified radio-SFR relation that works both for global and local data when using the radio spectral index as a second parameter. A unified radio-SFR relation for, both, global and local data may 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.

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arXiv2026

Magnetised CGM Gas at z~1 revealed by SPICE-RACS

Magnetic fields are expected to permeate the circumgalactic medium (CGM) of galaxies, yet direct constraints at high redshift remain limited by the lack of high-quality Faraday rotation measure (RM) data. Using the RMs from SPICE-RACS DR2 combined with the DESI DR1 quasar catalogue, we compile the largest sample to date of 2483 quasar sightlines with associated RMs, including 612 with intervening Mg II absorbers tracing foreground galaxies and 1871 control sightlines without Mg II absorbers. After subtracting the Galactic RM contribution and restricting the analysis to sightlines with low Milky Way HI column density and H$α$ intensity, we obtain a foreground-cleaned sample of 757 quasars (191 Mg II / 566 control) spanning redshifts $0.13<z<3.45$. In this foreground-cleaned sample, Mg II sightlines exhibit a $4.5σ$ excess in the residual RM dispersion of $4.13 \pm 0.91~\mathrm{rad\,m^{-2}}$ relative to the control sample, at a median absorber redshift of $z\sim1.14$. This implies model-dependent CGM magnetic field strengths of $\sim0.4 - 0.8\, μ$G over projected radii of $20 - 150$ kpc. This indicates that substantial CGM magnetisation was already established by $z\sim1$, enabling new constraints on the growth and amplification of magnetic fields in galaxy halos over cosmic time.

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arXiv2026

Galaxy clusters in the LoTSS-DR3: Catalogues and detection pipeline for diffuse radio emission

The third data release of the LOFAR Two-metre Sky Survey provides an unprecedented view of the northern sky at 144 MHz. While compact sources can be efficiently identified with automated software packages, the detection of diffuse radio emission associated with galaxy clusters still requires dedicated processing and visual inspection. Given the scale of current and forthcoming radio surveys, automated approaches based on artificial intelligence are becoming essential to the identification of the most interesting targets. We aim to develop an automated pipeline to construct a catalogue of galaxy clusters hosting diffuse radio emission from LoTSS-DR3 20arcsec images. The pipeline is designed to provide both the probability that a cluster hosts diffuse radio emission and an interpretable image of its shape and morphology. We employed Radio U-Net, a convolutional neural network optimised for image segmentation (i.e. pixel-level identification) of diffuse radio emission. To associate detected emission with individual clusters, we combined the network output with positional, mass, and redshift information from four X-ray- and Sunyaev-Zeldovich-selected cluster catalogues, resulting in a merged sample of 3822 clusters covered by the LoTSS-DR3. We produced a pixel-level segmentation map of the full LoTSS-DR3 and a quantitative indicator for the presence of diffuse emission in each cluster. This enables the selection of sub-samples with specific properties for targeted follow-up or statistical studies. As a demonstration of the first application, we identified a sub-sample of 357 clusters selected at the highest network accuracy (76%), and we showed some examples of newly detected systems. For the second, using a larger statistical sample, we verified that the detection fraction of diffuse radio sources in the four catalogues increases with the mass and redshift of the clusters. [Abridged]

astro-ph.CO↗

arXiv2026

Magnetic properties of the Abell 3391-3395 system revealed using wide-field MeerKAT polarimetry

Magnetic fields in cluster outskirts and the intercluster medium are poorly constrained because diffuse synchrotron emission is hard to detect at low surface brightness. Faraday rotation measures (RMs) of polarized background sources can probe foreground large-scale structure. The nearby interacting Abell 3391-3395 system hosts a well-established X-ray bridge, making it an excellent target for studying magnetization in the intercluster environment. We characterize the magnetized environment of Abell 3391/95 and its surroundings by constructing a dense RM grid from wide-field polarimetry. We observed Abell 3391/95 with MeerKAT in full polarization using a three-pointing mosaic. The data were calibrated with direction-independent and direction-dependent techniques and imaged using visibility-plane mosaicing for a large field of view at high sensitivity. Using Faraday synthesis, we formed Faraday cubes and measured RMs for polarized background sources. We defined on- and off-target regions using contours from a wavelet-filtered eROSITA image. We identified 434 polarized sources within the field, with a polarized source density ranging from about 30 sources per square degree in the outer regions to about 110 sources per square degree in the central field, and a field-averaged density of 73 sources per square degree. The clusters show a statistically significant enhancement of RM scatter relative to the off-target region. In contrast, the bridge shows comparatively low RM scatter, while an RM structure-function analysis on matched angular scales yields a tentative indication of larger RM differences in the bridge than off-target. Combined with low per-source depolarization, this suggests a bridge magnetic field relatively ordered on ~10 kpc scales, but less ordered on larger scales. The non-detection of diffuse synchrotron emission in the bridge yields improved upper limits on the emissivity.

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arXiv2026

The eROSITA Final Equatorial-Depth Survey (eFEDS): X-ray stacking analysis of Subaru's optically selected clusters spanning low richness regime

This is the second paper in a series exploring the X-ray properties of galaxy clusters optically selected by the Subaru Hyper Suprime-Cam (HSC) survey, using data from the SRG/eROSITA Final Equatorial-Depth Survey (eFEDS). We aim to investigate scaling relations between observable cluster properties and mass, and to study the radial X-ray profiles of a large sample of optically selected clusters. We analyze a sample of 997 CAMIRA clusters with richness $N > 15$ and redshifts of $0.1 < z < 1.3$. Using bolometric luminosities derived from count rates and a weak-lensing mass calibration, we study the $L-M$ and $N-M$ scaling relations through stacking analysis, while accounting for selection effects and redshift evolution. We also compare clusters with and without X-ray counterparts in the eFEDS catalog in terms of their scaling relations and surface brightness profiles. The best-fit $L-M$ slope ($1.56^{+0.14}_{-0.12}$) is slightly steeper than the self-similar prediction, yet remains consistent with our previous findings. The $N-M$ slope ($0.766^{+0.070}_{-0.060}$) broadly agrees with theoretical expectations and other optical samples. The data do not require any additional redshift evolution beyond the standard self-similar scaling, although current constraints on evolution remain weak. X-ray detected clusters exhibit a marginally steeper $L-M$ slope, higher central surface brightness, and more centrally concentrated X-ray profiles than undetected systems. Our results highlight systematic differences in the X-ray properties between optically and X-ray selected cluster samples. This study extends scaling relation analyses into lower mass and luminosity regimes, demonstrating the value of combining deep X-ray and optical surveys like eROSITA and Subaru HSC.

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arXiv2026

The SRG/eROSITA all-sky survey: Constraints on Ultra-light Axion Dark Matter through Galaxy Cluster Number Counts

Ultralight axions are hypothetical scalar particles that influence the evolution of large-scale structures of the Universe. Depending on their mass, they can potentially be part of the dark matter component of the Universe as candidates commonly referred to as fuzzy dark matter. While strong constraints have been established for pure fuzzy dark matter models, the more general scenario where ultralight axions constitute only a fraction of the dark matter has been limited to only a few observational probes. In this work, we use the galaxy cluster number counts obtained from the first All-Sky Survey (eRASS1) of the SRG/eROSITA mission together with gravitational weak lensing data from the Dark Energy Survey, the Kilo-Degree Survey, and the Hyper Suprime-Cam to constrain the fraction of ultralight axions in the mass range $10^{-32}$ eV to $10^{-24}$ eV. We put upper bounds on the ultralight axion relic density $Ω_\mathrm{a}$ in independent logarithmic axion mass bins by performing a full cosmological parameter inference. We find an exclusion region in the intermediate ultralight axion mass regime with the tightest bounds reported so far in the mass bins around $m_\text{a} = 10^{-27}$ eV with $Ω_\text{a} < 0.0035$ and $m_\text{a} = 10^{-26}$ eV with $Ω_\text{a} < 0.0079$ (95% C.L.). When combined with cosmic microwave background probes, these bounds are tightened to $Ω_\text{a} < 0.0030$ in the $m_\text{a} = 10^{-27}$ eV mass bin and $Ω_\text{a} < 0.0058$ in the $m_\mathrm{a} = 10^{-26}$ eV mass bin (95% C.L.). This is the first time that constraints on ultralight axions have been obtained using the growth of structure measured by galaxy cluster number counts. These results pave the way for large surveys, which can be utilized to obtain tight constraints on the mass and relic density of ultralight axions with better theoretical modeling of the abundance of halos.

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arXiv2026

Single- and double-headed odd radio circles in the LOFAR surveys

Deep radio surveys are now producing catalogs with millions of radio sources. Radio sources can have complex morphologies that depend on both the production mechanisms and the local environment. Recently, an unusual class of circular radio sources named "odd radio circles" (ORCs) were identified. They have sizes of about 1 arcmin, a circular/elliptical shape and appear edge-brightened. Subsequent observations have suggested that this class may comprise a variety of sources. Despite various attempts to explain them, their origin remains unclear. The main goal of this work is to increase the number of known ORCs and derive common characteristics that can help identify the origin of these sources. We searched the LOFAR Two Metre Sky Survey (LoTSS) Data Release 3 (DR3) at 144 MHz for ORCs using a combination of parameter filtering on catalog entries and visual inspection. We then identified possible optical counterparts and derived information such as redshift, physical size, and spectral index using further radio data at 54 and 1400 MHz. We isolated 18 sources with ORC structures. Four of these are double-headed ORCs (ORCs with two rings), and two are new discoveries. We also found five new single-headed ORCs and nine candidate ORCs. With this work we significantly expand the population of known ORCs. Our findings confirm that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases, and we note a relation between the ORCs' physical sizes and their integrated spectral indexes, with small ORCs avoiding steep spectra.

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arXiv2026

The LOFAR sub-arcsecond view of the high-redshift radio relic in PSZ2G091.83+26.11

Enhanced inverse Compton (IC) losses at high redshift steepen diffuse radio spectra in galaxy clusters, making low-frequency (~100 MHz) observations favorable. However, low-frequency studies often lack the resolution needed to locate particle acceleration sites or separate diffuse emission from radio galaxies. In this paper, we unveil the properties of the radio relic in the distant cluster PSZ2G091.83+26.11 (z=0.822) by resolving the acceleration site and inspecting the downstream region. Using the European LOFAR (ILT) at 145 MHz, we study a radio relic at (sub-)arcsecond resolution for the first time below 1 GHz, complemented by arcsecond-resolution VLA data at higher frequencies. We confirm the diffuse emission is not a radio galaxy. A spectral index gradient toward the cluster center matches previous 5'' maps. High-resolution 0.4'' and 1.9'' images reveal emission ahead of the shock, connecting the relic to a radio galaxy. 1.9'' profiles across the downstream at 145 MHz and 3.0 GHz follow a log-normal magnetic field distribution. The 145 MHz shock surface shows a sharp discontinuity at the same location of a change in electron density, Rotation Measure, and fractional polarization, likely tied to magnetic field changes. Finally, we find hints of redshift evolution of the radio power versus cluster mass correlation. The impressive angular resolution achievable by the LOFAR long baselines is opening an unprecedented view of the low energetic plasma in galaxy clusters. This is extremely significant in the case of high-redshift clusters, where radio emission at low frequencies is less affected by energy losses but its detection is strongly limited by poor resolution.

astro-ph.CO↗

arXiv2026

The LOFAR Two-metre Sky Survey: VII. Third Data Release

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 hrs of data (18.6 PB) accumulated over 10.5 years. The images were produced 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 arcsec (9 arcsec below declination 10$^\circ$) 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 arcsec 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$μ$Jy beam$^{-1}$. 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.

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arXiv2026

MeerKAT observations of Abell 1775 and Abell 1795: the discovery of a hadronic radio halo?

Giant radio haloes are diffuse synchrotron sources typically found in merging galaxy clusters, while smaller mini-haloes occur in cool-core clusters. Both trace cosmic-ray electrons in the intracluster medium, though recent observations suggest their distinction is not always clear. We present new 903-1655 MHz MeerKAT observations of Abell 1775 and Abell 1795, both hosting cool cores and cold fronts. Combined with reprocessed 120-168 MHz LOFAR Two-metre Sky Survey data, we perform imaging and spectral analyses of their radio emission. In both clusters, we detect radio haloes with distinct inner and outer components. In Abell 1775, the halo appears diffuse at 1.3 GHz, while LOFAR images reveal steep-spectrum filaments. In Abell 1795, the inner component corresponds to a previously reported mini-halo candidate, but the full structure extends to $\sim$1 Mpc with a spectral index of $α=-1.08\pm0.06$. The presence of such a large, flat-spectrum halo in a dynamically relaxed cluster makes Abell 1795 an outlier relative to typical merging systems. This suggests that some relaxed clusters may still retain sufficient turbulence to sustain particle re-acceleration, or that hadronic interactions producing secondary electrons play a significant role. Together with other recent discoveries in cool-core systems, our results indicate that some large radio haloes may have been overlooked in past studies due to limited dynamic range near bright central AGN. Finally, we detect steep-spectrum emission south of Abell 1795's central AGN, tracing a 45 kpc X-ray and optical filament that terminates in an X-ray cavity, likely linked to a past AGN outburst.

astro-ph.CO↗

arXiv2026

Combined LOFAR-uGMRT analysis of the diffuse radio emission in the massive clusters Abell 773 and Abell 1351

Radio halos are megaparsec-scale diffuse, non-thermal radio sources located at the centers of galaxy clusters, tracing relativistic particles and magnetic fields in the intra-cluster medium. Their origin is generally attributed to cluster mergers that generate turbulence and re-accelerate aged electrons. We study the diffuse radio emission, spectral properties, and the connection between thermal and non-thermal components in the massive galaxy clusters Abell 773 and Abell 1351 ($M_{500} \sim 7 \times 10^{14}\,M_{\odot}$), both of which are dynamically disturbed. We combine LOFAR LoTSS-DR2 observations at 144 MHz with uGMRT observations at 650 MHz, supplemented by archival XMM-Newton X-ray imaging. We confirm that both clusters host radio halos extending up to a largest linear size of $\sim 2$ Mpc. We measure an integrated spectral index $α_{144}^{650} \sim -1.0$ for both clusters. The radio halo in Abell 773 resembles a classical halo and follows a sublinear radio--X-ray surface brightness relation. In contrast, Abell 1351 shows a more complex and asymmetric morphology, influenced by embedded radio sources including the brightest cluster galaxy, a tailed radio galaxy, and a ridge-like feature. These contaminating sources lead to deviations from the sublinear trend in the point-to-point radio--X-ray analysis of Abell 1351.

astro-ph.CO↗

arXiv2026

CHANG-ES XXXVII. Effects of spectral aging on radio scale heights

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.

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