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Radio relics are elongated sources related to shocks driven by galaxy cluster merger events. Although these objects are highly polarized at GHz frequencies (≳20%), high-resolution studies of their polarization properties are still lacking. We present the first high-resolution and high-sensitivity polarimetry study of the merging galaxy cluster CIZA J2242.8+5301 in the 1–4 GHz frequency band. We use the QU-fitting approach to model the Stokes I, Q, and U emission, obtaining best-fit intrinsic polarization fraction (p0), intrinsic polarization angle (χ0), rotation measure (RM), and wavelength-dependent depolarization (σRM) maps of the cluster. Our analysis focuses on the northern relic (RN). For the first time in a radio relic, we observe a decreasing polarization fraction in the downstream region. Our findings are possibly explained by geometrical projections and/or by decreasing of the magnetic field anisotropy toward the cluster center. From the amount of depolarization of the only detected background radio galaxy, we estimate a turbulent magnetic field strength of Bturb ∼ 5.6 μG in the relic. Finally, we observe RM fluctuations of about 30 rad m−2 around the median value of 140.8 rad m−2 at the relic position. [ABSTRACT FROM AUTHOR]

We present new deep, high-resolution, 1.5 GHz observations of the prototypical nearby Perseus galaxy cluster from the Karl G. Jansky Very Large Array. We isolate for the first time the complete tail of radio emission of the bent-jet radio galaxy NGC 1272, which had been previously mistaken to be part of the radio mini-halo. The possibility that diffuse radio galaxy emission contributes to mini-halo emission may be a general phenomenon in relaxed cool-core clusters, and should be explored. The collimated jets of NGC 1272 initially bend to the west, and then transition eastward into faint, 60 kpc long extensions with eddy-like structures and filaments. We suggest interpretations for these structures that involve bulk motions of intracluster gas, the galaxy's orbit in the cluster including projection effects, and the passage of the galaxy through a sloshing cold front. Instabilities and turbulence created at the surface of this cold front and in the turbulent wake of the infalling host galaxy most likely play a role in the formation of the observed structures. We also discover a series of faint rings, southeast of NGC 1272, which are a type of structure that has never been seen before in galaxy clusters. [ABSTRACT FROM AUTHOR]

Radio synchrotron emission from the bridges of low-density gas connecting galaxy clusters and groups is a challenge for particle acceleration processes. In this work, we analyze the Coma radio bridge using new LOw Frequency ARray (LOFAR) observations at 144 MHz. LOFAR detects the bridge and its substructures with unprecedented sensitivity and resolution. We found that the radio emission peaks on the NGC 4839 group. Toward the halo, in front of the NGC 4839 group, the radio brightness decreases and streams of radio emission connect the NGC 4839 group to the radio relic. Using X-ray observations, we found that thermal and non-thermal plasma are moderately correlated with a sublinear scaling. We use archival radio data at 326 MHz to constrain the spectral index in the bridge, and quantify the distribution of particles and magnetic field at different frequencies. We found that the spectrum is steeper than −1.4 ± 0.2, and that the emission is clumpier at 326 MHz than at 144 MHz. Using cosmological simulations and a simplified approach to compute particle acceleration, we derive under which conditions turbulent acceleration of mildly relativistic electrons generate the radio emission in the bridge. Assuming that the initial energy ratio of the seed electrons is 3 · 10−4 with respect to the thermal gas, we are able to reproduce the observed luminosity. Our results suggest that the seed electrons released by radio galaxies in the bridge and the turbulence generated by the motion of gas and galaxies are essential to producing the radio emission. [ABSTRACT FROM AUTHOR]

We present LOFAR observations of one of the most spectacular objects in the radio sky: Abell 2255. This is a nearby (z = 0.0806) merging galaxy cluster hosting one of the first radio halos ever detected in the intracluster medium (ICM). The deep LOFAR images at 144 MHz of the central ∼10 Mpc2 region show a plethora of emission on different scales, from tens of kiloparsecs to above megaparsec sizes. In this work, we focus on the innermost region of the cluster. Among the numerous interesting features observed, we discover remarkable bright and filamentary structures embedded in the radio halo. We incorporate archival WSRT 1.2 GHz data to study the spectral properties of the diffuse synchrotron emission and find a very complex spectral index distribution in the halo spanning a wide range of values. We combine the radio data with Chandra observations to investigate the connection between the thermal and nonthermal components by quantitatively comparing the radio and X-ray surface brightness and the spectral index of the radio emission with the thermodynamical quantities of the ICM. Despite the multitude of structures observed in the radio halo, we find that the X-ray and radio emission are overall well correlated. The fact that the steepest spectrum emission is located in the cluster center and traces regions with high entropy possibly suggests the presence of seed particles injected by radio galaxies that are spread in the ICM by the turbulence generating the extended radio halo. [ABSTRACT FROM AUTHOR]

We present Very Large Array (VLA) radio and Chandra X-ray observations of the merging galaxy cluster A3411.For the cluster, we find an overall temperature of 6.4+0.6−1.0 keV and an X-ray luminosity of 2.8 ± 0.1 × 1044 erg s−1 between 0.5 and 2.0 keV. The Chandra observation reveals the cluster to be undergoing a merger event. The VLA observations show the presence of large-scale diffuse emission in the central region of the cluster, which we classifyas a 0.9 Mpc size radio halo. In addition, a complex region of diffuse, polarized emission is found in the southeasternoutskirts of the cluster along the projected merger axis of the system. We classify this region of diffuse emissionas a radio relic. The total extent of this radio relic is 1.9 Mpc. For the combined emission in the cluster region,we find a radio spectral index of −1.0 ± 0.1 between 74 MHz and 1.4 GHz. The morphology of the radio relicis peculiar, as the relic is broken up into five fragments. This suggests that the shock responsible for the relic hasbeen broken up due to interaction with a large-scale galaxy filament connected to the cluster or other substructuresin the intracluster medium. Alternatively, the complex morphology reflects the presence of electrons in fossil radiobubbles that are re-accelerated by a shock. [ABSTRACT FROM AUTHOR]

We present a multi-wavelength study of the emission-line nebulae located ~38" (3 kpc in projection) southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [Cii] line emission at 158μm from the nebulae using observations made with the Herschel Photodetector Array Camera and Spectrometer (PACS). The infrared line emission is extended and co-spatial with optical Hα+ [Nii], far-ultraviolet C iv lines, and soft X-ray emission. The filamentary nebulae evidently containmulti-phase material spanning a temperature range of at least five orders of magnitude, from ~100 K to ~107 K. This material has most likely been uplifted by the active galactic nucleus from the center of M87. The thermal pressure of the 104 K phase appears to be significantly lower than that of the surrounding hot intraclustermedium (ICM), indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then themagnetic field strength required to balance the pressure of the surrounding ICM is B ~ 30-70 μG. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT ~ 0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with κT < 0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an integrated hydrogen column density of NH ~ 1.6 ×1021 cm-2, providing a natural explanation for the apparent temperature floor to the X-ray emission at κT ~ 0.5 keV. The FIR through ultraviolet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process. An additional source of energy may, in principle, be provided by X-ray photoionization from cooling X-ray emitting plasma. The relatively small line ratio of [O i]/[C ii] < 7.2 indicates a large optical depth in the FIR lines. The large optical depth in the FIR lines and the intrinsic absorption inferred from the X-ray and optical data imply significant reservoirs of cold atomic and molecular gas distributed in filaments with small volume filling fraction, but large area covering factor. [ABSTRACT FROM AUTHOR]

We present Low Frequency Array (LOFAR) Low Band observations of the Boötes and 3C 295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam–1, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17-52 deg2. From the observations, we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and Very Large Array 74 MHz differential source counts, scaling with a spectral index of –0.7. We find that a spectral index scaling of –0.5 is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens toward lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum (α < –1.1) radio sources that could be associated with massive high-redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz, and 1.4 GHz for sources in the Boötes field. We cross-correlate these radio sources with optical and infrared catalogs and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the 0.7 ≲ z ≲ 2.5 range. [ABSTRACT FROM AUTHOR]

Radio relics are diffuse radio sources observed in galaxy clusters, probably produced by shock acceleration during cluster-cluster mergers. Their large size, of the order of 1 Mpc, indicates that the emitting electrons need to be (re)accelerated locally. The usually invoked diffusive shock acceleration models have been challenged by recent observations and theory. We report the discovery of complex radio emission in the Galaxy cluster PLCKG287.0+32.9, which hosts two relics, a radio halo, and several radio filamentary emission. Optical observations suggest that the cluster is elongated, likely along an intergalactic filament, and displays a significant amount of substructure. The peculiar features of this radio relic are that (1) it appears to be connected to the lobes of a radio galaxy and (2) the radio spectrum steepens on either side of the radio relic. We discuss the origins of these features in the context of particle re-acceleration. [ABSTRACT FROM AUTHOR]