Rossella Cassano

Context. Radio halos are diffuse megaparsec-scale nonthermal radio sources located at the center of galaxy clusters. They trace relativistic particles and magnetic fields in the intra-cluster medium. The source of energy for their formation is believed to be the merging of galaxy clusters, which generates turbulence and reaccelerates aged electrons. Aims. We studied the diffuse radio emission, spectral properties, and the connection between thermal and nonthermal emission in the massive ( M ₅₀₀ ∼ 7 × 10 ¹⁴ M _⊙ ), dynamically disturbed galaxy clusters Abell 773 and Abell 1351. Methods. We combined observations from the LOFAR Two-meter Sky Survey Data Release 2 at 144 MHz and the new upgraded Giant Meterwave Radio Telescope at 650 MHz for both clusters. Archival XMM-Newton X-ray images were utilized to supplement our analysis. Results. We confirm that both clusters host a radio halo, each of which has a largest linear size of ∼2 Mpc. We measure an integrated spectral index ($ \alpha_{144}^{650} $) of ∼ − 1.0 for both clusters. Via point-to-point analysis, we show that the radio halo in A773 resembles a classical radio halo that follows a sublinear relation between radio and X-ray surface brightness. Conversely, A1351 exhibits a more complex and asymmetric radio halo that is embedded with several radio sources, including the brightest cluster galaxy, a tail galaxy, and the ridge. We find a deviation from the sublinear relation in the point-to-point analysis that is due to the presence of these contaminating radio sources.

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.

Modern radio telescopes are revolutionising our understanding of non-thermal phenomena in galaxy clusters, collecting large samples of extended sources with unprecedented sensitivity and angular resolution. In this work, we present novel MeerKAT observations for a sample of 21 galaxy clusters that are part of the CHEX-MATE project. These systems were selected based on their high mass and displaying signs of dynamical activity. Thanks to the high-quality data at hand, we were able to detect extended radio emission in every target considered. We report two new halos, one new relic, and two new candidate relics. We also confirm a previous candidate halo and two candidate relics. After investigating the scaling relations with the cluster properties, we confirmed the presence of a radio halo power-mass correlation and relate it to a higher radio halo emissivity in more massive clusters. For radio relics, we highlight the MeerKAT capabilities to significantly extend the depth of radio observations to a new, unexplored field of low-radio power sources (≲10 ²³ W Hz ⁻¹ at 1.28 GHz). Thanks to such high-sensitivity data, we have found that the radio relic power can be characterised by a wide range of values for a given cluster mass and relic size. Ultimately, we discuss how current radio observations, in combination with large radio surveys, are increasingly capable of testing numerical simulation predictions and coming close to performing direct comparisons with their data, enabling new insights on the evolution of radio relics.

Context. Diffuse synchrotron radio sources that are associated with the intracluster medium of galaxy clusters are of special interest at high redshift for understanding the magnetization and particle acceleration mechanisms. Aims. El Gordo is the most massive galaxy cluster at high redshift (z = 0.87). It hosts a radio halo and a double radio relic system. We aim to understand the role of turbulence in the origin of its diffuse radio emission by combining radio and X-ray observations. Methods. We observed El Gordo with the Upgraded GMRT at 0.3–1.45 GHz and obtained the integrated spectra, the spatially resolved spectral map, and the scaling relations of the radio and X-ray surface brightness. We constructed a density fluctuation power spectrum for the central 1 Mpc region using Chandra data. Results. The radio halo and the double relics are detected at all the bands, and in addition, we detect an extension to the eastern relic. The radio halo has a spectral index of −1.0 ± 0.3 with a possible steepening beyond 1.45 GHz. All the relics have spectral indices of −1.4, except for the extension of the east relic, which has −2.1 ± 0.4. The radio and X-ray surface brightness point-to-point analysis at bands 3 and 4 show slopes of 0.60 ± 0.12 and 0.76 ± 0.12, respectively. The spectral index and X-ray surface brightness are anticorrelated. The density fluctuations peak at ∼700 kpc at an amplitude of (δρ/ρ) = 0.15 ± 0.02. We derived a 3D turbulent Mach number of ∼0.6 from the gas density fluctuation power spectrum under the assumption that all the fluctuations are attributable to turbulence. Conclusions. The derived properties of El Gordo agree with those of low-redshift clusters. This indicates that the fast magnetic amplification that was proposed for high-redshift clusters is at work in El Gordo as well. We discuss the consistency of our results with turbulent reacceleration, which might be representative of high-redshift merging clusters.

The thermal and non-thermal components in galaxy clusters have properties that, although shaped from different physical phenomena, can share some similarities, mainly driven by their halo mass and the accretion processes. Scaling relations have been proven to exist for both components and studied in X-ray (thermal) and radio (non-thermal) bands. On the X-ray side, both integrated and spatially resolved profiles have shown a predictable and correlated behaviour. At the radio wavelength, such investigations are so far limited to the integrated quantities (e.g. total power and mass). We aimed to investigate the scaling relations between the mass of a galaxy cluster and its radio emission at low frequencies, treating both the integrated and the spatially resolved quantities for a sample of well-selected objects in a self-consistent analysis. We crossmatched LoTSS DR2 and CHEX-MATE datasets in order to get the deepest and most homogeneous radio data of a representative sample of objects. Among the 40 CHEX-MATE objects in the LOFAR DR2 area, we investigated the 18 objects showing radio halo emission, which span a broad mass range, by extracting and analysing their radio emission profiles. We analytically derived the expected relation between the radio power (P_ν) and radio surface brightness profile (I_R(r)), and performed a comparison with observational results. We obtained that properly accounting for the mass and redshift dependence in the radio profile can reduce the overall scatter by a factor of ∼4, with an evident residual dependence on the cluster dynamical status. We show that assuming the halo size R_H ∼ R₅₀₀ did not allow us to reconcile the expected (from our analytical derivations) and observed mass profile scaling. Instead, accounting for no R_H − M relation, allowed us to reconcile the observed radio profile mass scaling and the one predicted starting from the P_ν − M relation. We discuss the implications of a lack of R_H − M relation, assessing possible systematics and biases in the analyses, and interpreting it as a natural consequence of the structure formation process, where the halo size depends on both the cluster dynamical status, related to the strength of the merger, and mass. Finally, we also considered the role of the magnetic field in the P_ν − M relation, putting constraints on its dependence upon the cluster mass and finding consistent results with expectations from our radio power mass scaling.

We present the first multifrequency analysis of the candidate ultrasteep spectrum radio halo in the galaxy cluster PLCKESZ G171.94−40.65, using the upgraded Giant Metrewave Radio telescope (400 MHz), and Karl G. Jansky Very Large Array (1–2 GHz) observations. Our radio data have been complemented with archival Chandra X-ray observations to provide a crucial insight into the complex intracluster medium physics, happening at large scales. We detect the radio halo emission to the extent of ∼1.5 Mpc at 400 MHz, significantly larger than previously reported, along with five tailed galaxies in the central region. We also report the discovery of an unknown diffuse source “U,” at the cluster periphery, with an extent of 300 kpc. Using the available observations, we have found that the radio spectrum of the halo is well-fitted with a single power law, having a spectral index of −1.36 ± 0.05, indicating that it is not an ultrasteep spectrum radio halo. Our low-resolution (25″) resolved spectral map shows an overall uniform spectral index, with some patches of fluctuations. The X-ray and radio surface brightness are morphologically cospatial, with a slight extension along the northwest–southeast direction, seen in both maps. The radio and X-ray surface brightness indicates strong positive correlations, with sublinear correlation slopes (∼0.71). Multiple tailed galaxies and the radio halo indicate a high dynamical activity at the cluster central region.

We present the first detailed analysis of the ultra-steep spectrum radio halo in the merging galaxy cluster A521, based on upgraded Giant Metrewave Radio telescope observations. The combination of radio observations (300–850 MHz) and archival X-ray data provides a new window into the complex physics occurring in this system. When compared to all previous analyses, our sensitive radio images detected the centrally located radio halo emission to a greater extent of ∼1.3 Mpc. A faint extension of the southeastern radio relic has been discovered. We detected another relic, recently discovered by MeerKAT, and coincident with a possible shock front in the X-rays, at the northwest position of the center. We find that the integrated spectrum of the radio halo is well-fitted with a spectral index of −1.86 ± 0.12. A spatially resolved spectral index map revealed the spectral index fluctuations, as well as an outward radial steepening of the average spectral index. The radio and X-ray surface brightness is well correlated for the entire and different subparts of the halo, with sublinear correlation slopes (0.50–0.65). We also found a mild anticorrelation between the spectral index and X-ray surface brightness. Newly detected extensions of the SE relic and the counter relic are consistent with the merger in the plane of the sky.