arXiv subjects

R. J. van Weeren

Publications and source records attributed to R. J. van Weeren.

At least 145 records · Page 8Linked to original sources

arXiv2021

Radio relics in PSZ2 G096.88+24.18: A connection with pre-existing plasma

Giant radio relics are arc-like structures of diffuse, non-thermal synchrotron radiation that trace shock waves induced by galaxy cluster mergers. The particle (re-)acceleration mechanism producing such radio relics is unclear. One major open question is whether relics can be formed directly from a population of thermal seed electrons, or if pre-existing relativistic seed electrons are required. In some cases AGN can provide such a population of sub-GeV electrons. However, it is unclear how common this connection is. In this paper we present LOFAR 140 MHz and VLA L-band radio observations, as well as Chandra data of PSZ2 G096.88+24.18, a merging galaxy cluster system hosting a pair of radio relics. A large patch of diffuse emission connects a bright radio galaxy with one of the relics, likely affecting the properties of the relic. We find that the most plausible explanation for the connection is that the merger shock wave has passed over an AGN lobe. The shock passing over this seed population of electrons has led to an increased brightness in the relic only in the region filled with seed electrons.

astro-ph.HE↗

arXiv2021

Nonthermal phenomena in the center of Abell 1775: An 800 kpc head-tail, revived fossil plasma and slingshot radio halo

Thermal gas in the center of galaxy clusters can show substantial motions that generate surface-brightness and temperature discontinuities known as cold fronts. The motions may be triggered by minor or off-axis mergers that preserve the cool core of the system. The dynamics of the thermal gas can also generate radio emission from the intra-cluster medium (ICM) and impact the evolution of clusters' radio sources. We aim to study the central region of Abell 1775, a system in an ambiguous dynamical state at $z=0.072$ which is known to host an extended head-tail radio galaxy, with the goal of investigating the connection between thermal and nonthermal components in its center. We made use of a deep (100 ks) Chandra observation accompanied by LOFAR 144 MHz, GMRT 235 MHz and 610 MHz, and VLA 1.4 GHz radio data. We find a spiral-like pattern in the X-ray surface brightness that is mirrored in the temperature and pseudo-entropy maps. Additionally, we characterize an arc-shaped cold front in the ICM. We interpret these features in the context of a slingshot gas tail scenario. The structure of the head-tail radio galaxy "breaks" at the position of the cold front, showing an extension that is detected only at low frequencies, likely due to its steep and curved spectrum. We speculate that particle reacceleration is occurring in the outer region of this tail, which in total covers a projected size of $\sim800$ kpc. We also report the discovery of revived fossil plasma with ultra-steep spectrum radio emission in the cluster core together with a central diffuse radio source that is bounded by the arc-shaped cold front. The results reported in this work demonstrate the interplay between thermal and nonthermal components in the cluster center and the presence of ongoing particle reacceleration in the ICM on different scales.

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arXiv2021

Raining in MKW 3s: a Chandra-MUSE analysis of X-ray cold filaments around 3CR 318.1

We present the analysis of X-ray and optical observations of gas filaments observed in the radio source 3CR 318.1, associated with NGC 5920, the Brightest Cluster Galaxy (BCG) of MKW 3s, a nearby cool core galaxy cluster. This work is one of the first X-ray and optical analyses of filaments in cool core clusters carried out using MUSE observations. We aim at identifying the main excitation processes responsible for the emission arising from these filaments. We complemented the optical VLT/MUSE observations, tracing the colder gas phase, with X-ray $\textit{Chandra}$ observations of the hotter highly ionized gas phase. Using the MUSE observations, we studied the emission line intensity ratios along the filaments to constrain the physical processes driving the excitation, and, using the $\textit{Chandra}$ observations, we carried out a spectral analysis of the gas along these filaments. We found a spatial association between the X-ray and optical morphology of these filaments, which are colder and have lower metal abundance than the surrounding intra-cluster medium (ICM), as already seen in other BCGs. Comparing with previous results from the literature for other BCGs, we propose that the excitation process that is most likely responsible for these filaments emission is a combination of star formation and shocks, with a likely contribution from self-ionizing, cooling ICM. Additionally, we conclude that the filaments most likely originated from AGN-driven outflows in the direction of the radio jet.

astro-ph.GA↗

arXiv2021

LoTSS jellyfish galaxies: I. Radio tails in low redshift clusters

In this paper we present a large sample of jellyfish galaxies in low redshift clusters (z<0.05), identified through 120-168 MHz radio continuum from the LOFAR Two-metre Sky Survey (LoTSS). From a parent sample of 29 X-ray-detected SDSS galaxy clusters and their spectroscopic members, we visually identify 95 star-forming, LoTSS jellyfish galaxies with 144 MHz radio tails. Star formation rates (SFRs) and stellar masses are obtained for all galaxies from SED fits. For each jellyfish galaxy we determine the tail orientation with respect to the cluster centre and quantify the prominence of the radio tails with the 144 MHz shape asymmetry. After carefully accounting for redshift-dependent selection effects, we find that the frequency of jellyfish galaxies is relatively constant from cluster to cluster. LoTSS jellyfish galaxies are preferentially found at small clustercentric radius and large velocity offsets within their host clusters and have radio tails that are oriented away from the cluster centre. These galaxies also show enhanced star formation, relative to both 'normal' cluster galaxies and isolated field galaxies, but generally fall within the scatter of the L144MHz - SFR relation. The properties of the LoTSS jellyfish galaxies identified in this work are fully consistent with expectations from ram pressure stripping. This large sample of jellyfish galaxies will be valuable for further constraining ram pressure stripping and star formation quenching in nearby galaxy clusters. We show that LOFAR is a powerful instrument for identifying ram pressure stripped galaxies across extremely wide fields. Moving forward we will push the search for jellyfish galaxies beyond this initial cluster sample, including a comprehensive survey of the galaxy group regime.

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arXiv2021

LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cut-off in the electron energy distribution, as has been previously suggested. Thermal (free-free) absorption or synchrotron self absorption models are able to describe the low-frequency spectral shape of the hotspots, however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic.

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arXiv2021

The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223

Radio halos and relics are Mpc-scale diffuse radio sources in galaxy clusters, with a steep spectral index $α>1$ ($S\propto ν^{-α}$). It has been proposed that they arise from particle acceleration induced by turbulence and weak shocks, injected in the intracluster medium (ICM) during mergers. MACS J1149.5+2223 (MACS J1149) is a high redshift ($z=0.544$) galaxy cluster possibly hosting a radio halo and a relic. We analysed LOFAR, GMRT, and JVLA radio data at 144, 323, 1500 MHz, and Chandra X-ray data to characterise the thermal and non-thermal properties of the cluster. We obtained radio images at different frequencies to investigate the spectral properties of the radio halo. We used Chandra X-ray images to constrain the thermal properties of the cluster. We measured a steep spectrum of the halo, with $α=1.49\pm 0.12$ between 144 and 1500 MHz. The radio surface brightness distribution across the halo is found to correlate with the X-ray brightness of the ICM, with a sub-linear slope in the range 0.4 to 0.6. We also report two possible cold fronts in north-east and north-west, but deeper X-ray observations are required to firmly constrain the properties of the upstream emission. We show that the combination of high redshift, steep radio spectrum, and sub-linear radio-X scaling of the halo rules out hadronic models. An old ($\sim 1 $ Gyr ago) major merger likely induced the formation of the halo through stochastic re-acceleration of relativistic electrons. We suggest that the two possible X-ray discontinuities may actually be part of the same cold front. In this case, the coolest gas pushed towards the north-west might be associated with the cool core of a sub-cluster involved in the major merger. The peculiar orientation of the south-east relic might indicate a different nature of this source and requires further investigation.

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arXiv2021

A Robust Model for Flux Density Calculations of Radio Halos in Galaxy Clusters: Halo-FDCA

Here we present Halo-FDCA, a robust open source Python package for modeling and estimating total flux densities of radio (mini) halos in galaxy clusters. Radio halos are extended ( ~200 - 1500 kpc in size) synchrotron emitting sources found in galaxy clusters that trace the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). These sources are centrally located and have a low surface brightness. Their exact origin is still unknown but they are likely related to cosmic rays being re-accelerated in-situ by merger or sloshing driven ICM turbulence. The presented algorithm combines the numerical power of the Markov Chain Monte Carlo routine and multiple theoretical models to estimate the total radio flux density of a radio halo from a radio image and its associated uncertainty. This method introduces a flexible analytic fitting procedure to replace existing simplistic manual measurements prone to biases and inaccuracies. It allows to easily determine the properties of the emission and is particularly suitable for future studies of large samples of clusters.

astro-ph.CO↗

arXiv2021

Hard X-ray selected giant radio galaxies -- III. The LOFAR view

Giant radio galaxies (GRGs), with extended structures reaching hundreds of kpc, are among the most spectacular examples of ejection of relativistic plasma from super-massive black holes. In this work, third of a series, we present LOw Frequency ARray (LOFAR) images at 144 MHz, collected in the framework of the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources extracted from our sample of hard X-ray selected GRGs (HXGRG, i.e. from INTEGRAL/IBIS and Swift/BAT catalogues at >20 keV). Thanks to the resolution and sensitivity of LoTSS, we could probe the complex morphology of these GRGs, unveiling cases with diffuse (Mpc-scale) remnant emission, presence of faint off-axis wings, or a misaligned inner jet. In particular, for one source (B21144+35B), we could clearly detect a $\sim$300 kpc wide off-axis emission, in addition to an inner jet which orientation is not aligned with the lobes axis. For another source (J1153.9+5848) a structure consistent with jet precession was revealed, appearing as an X-shaped morphology with relic lobes having an extension larger than the present ones, and with a different axis orientation. From an environment analysis, we found 2 sources showing an overdensity of cosmological neighbours, and a correspondent association with a galaxy cluster from catalogues. Finally, a comparison with radio-selected GRGs from LoTSS DR1 suggested that, on average, HXGRG can grow to larger extents. These results highlight the importance of deep low-frequency observations to probe the evolution of radio galaxies, and ultimately estimate the duty cycle of their jets.

astro-ph.HE↗

arXiv2021

Downstream depolarization in the Sausage relic: a 1-4 GHz Very Large Array study

Radio relics are elongated sources related to shocks driven by galaxy cluster merger events. Although these objects are highly polarized at GHz frequencies ($\gtrsim 20\%$), 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 ($p_0$), intrinsic polarization angle ($χ_0$), Rotation Measure (RM) and wavelength-dependent depolarization ($σ_{\rm RM}$) 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 towards the cluster center. From the amount of depolarization of the only detected background radio galaxy, we estimate a turbulent magnetic field strength of $B_{\rm turb}\sim5.6~μ$Gauss in the relic. Finally, we observe Rotation Measure fluctuations of about 30 rad m$^{-2}$ around at the median value of 140.8 rad m$^{-2}$ at the relic position.

astro-ph.GA↗

arXiv2021

Low-frequency monitoring of flare star binary CR Draconis: Long-term electron-cyclotron maser emission

Recently detected coherent low-frequency radio emission from M dwarf systems shares phenomenological similarities with emission produced by magnetospheric processes from the gas giant planets of our Solar System. Such beamed electron-cyclotron maser emission can be driven by a star-planet interaction or a breakdown in co-rotation between a rotating plasma disk and a stellar magnetosphere. Both models suggest that the radio emission could be periodic. Here we present the longest low-frequency interferometric monitoring campaign of an M dwarf system, composed of twenty-one $\approx$8 hour epochs taken in two series of observing blocks separated by a year. We achieved a total on-source time of 6.5 days. We show that the M dwarf binary CR Draconis has a low-frequency 3$σ$ detection rate of 90$^{+5}_{-8}$% when a noise floor of $\approx$0.1 mJy is reached, with a median flux density of 0.92 mJy, consistent circularly polarised handedness, and a median circularly polarised fraction of 66%. We resolve three bright radio bursts in dynamic spectra, revealing the brightest is elliptically polarised, confined to 4 MHz of bandwidth centred on 170 MHz, and reaches a flux density of 205 mJy. The burst structure is mottled, indicating it consists of unresolved sub-bursts. Such a structure shares a striking resemblance with the low-frequency emission from Jupiter. We suggest the near-constant detection of high brightness temperature, highly-circularly-polarised radiation that has a consistent circular polarisation handedness implies the emission is produced via the electron-cyclotron maser instability. Optical photometric data reveal the system has a rotation period of 1.984$\pm$0.003 days. We observe no periodicity in the radio data, but the sampling of our radio observations produces a window function that would hide the near two-day signal.

astro-ph.SR↗

arXiv2021

Extremely deep 150 MHz source counts from the LoTSS Deep Fields

With the advent of new generation low-frequency telescopes, such as the LOw Frequency ARray (LOFAR), and improved calibration techniques, we have now started to unveil the sub GHz radio sky with unprecedented depth and sensitivity. The LOFAR Two Meter Sky Survey (LoTSS) is an ongoing project in which the whole northern radio sky will be observed at 150 MHz with a sensitivity better than 100 $μ$Jy beam$^{-1}$ at a resolution of \asec{6}. Additionally, deeper observations are planned to cover smaller areas with higher sensitivity. The Lockman Hole, the Boötes and the Elais-N1 regions are among the most well known northern extra-galactic fields, and the deepest of the LoTSS Deep Fields so far. We exploit these deep observations to derive the deepest radio source counts at 150~MHz to date. Our counts are in broad agreement with those from the literature, and show the well known upturn at $\leq$ few mJy, mainly associated with the emergence of the star-forming galaxy population. More interestingly, our counts show for the first time a very pronounced drop around S$\sim$2 mJy, which results in a prominent `bump' at sub-mJy flux densities. Such a feature was not observed in previous counts' determinations (neither at 150 MHz nor at higher frequency). While sample variance can play a role in explaining the observed discrepancies, we believe this is mostly the result of a careful analysis aimed at deblending confused sources and removing spurious sources and artifacts from the radio catalogues. This `drop and bump' feature cannot be reproduced by any of the existing state-of-the-art evolutionary models, and appears to be associated with a deficiency of AGN at intermediate redshift ($1<z<2$) and an excess of low-redshift ($z<1$) galaxies and/or AGN.

astro-ph.GA↗

arXiv2021

The intra-cluster magnetic field in the double relic galaxy cluster Abell 2345

Magnetic fields are ubiquitous in galaxy clusters, yet their radial profile, power spectrum, and connection to host cluster properties are poorly known. Merging galaxy clusters hosting diffuse polarized emission in the form of radio relics offer a unique possibility to study the magnetic fields in these complex systems. In this paper, we investigate the intra-cluster magnetic field in Abell 2345. This cluster hosts two radio relics that we detected in polarization with 1-2 GHz JVLA observations. X-ray XMM-Newton images show a very disturbed morphology. We derived the Rotation Measure (RM) of five polarized sources within $\sim$ 1 Mpc from the cluster center applying the RM synthesis. Both, the average RM and the RM dispersion radial profiles probe the presence of intra-cluster magnetic fields. Using the thermal electron density profile derived from X-ray analysis and simulating a 3D magnetic field with fluctuations following a power spectrum derived from magneto-hydrodynamical cosmological simulations, we build mock RM images of the cluster. We constrained the magnetic field profile in the eastern radio relic sector by comparing simulated and observed RM images. We find that, within the framework of our model, the data require a magnetic field scaling with thermal electron density as $B(r)\propto n_e(r)$. The best model has a central magnetic field (within a 200 kpc radius) of $2.8\pm0.1$ $μ$G. The average magnetic field at the position of the eastern relic is $\sim$0.3 $μ$G, a factor 2.7 lower than the equipartition estimate.

astro-ph.GA↗

arXiv2021

Radio halos in a mass-selected sample of 75 galaxy clusters. I. Sample selection and data analysis

Radio halos are synchrotron diffuse sources at the centre of a fraction of galaxy clusters. The study of large samples of clusters with adequate radio and X-ray data is necessary to investigate the origin of radio halos and their connection with the cluster dynamics and formation history. The aim of this paper is to compile a well-selected sample of galaxy clusters with deep radio observations to perform an unbiased statistical study of the properties of radio halos. We selected 75 clusters with M > = 6e14 Msun at z=0.08-0.33 from the Planck Sunyaev-Zel'dovich catalogue. Clusters without suitable radio data were observed with the Giant Metrewave Radio Telescope (GMRT) and/or the Jansky Very Large Array (JVLA) to complete the information about the possible presence of diffuse emission. We used archival Chandra X-ray data to derive information on the clusters' dynamical states. This observational campaign led to the detection of several cluster-scale diffuse radio sources and candidates that deserve future follow-up observations. Here we summarise their properties and add information resulting from our new observations. For the clusters where we did not detect any hint of diffuse emission, we derived new upper limits to their diffuse flux. We have built the largest mass-selected (> 80 per cent complete in mass) sample of galaxy clusters with deep radio observations available to date. The statistical analysis of the sample, which includes the connection between radio halos and cluster mergers, the radio power-mass correlation, and the occurrence of radio halos as a function of the cluster mass, will be presented in paper II.

astro-ph.CO↗

arXiv2021

Radio halos in a mass-selected sample of 75 galaxy clusters. II. Statistical analysis

Many galaxy clusters host Mpc scale diffuse radio sources called radio halos. Their origin is connected to the processes that lead to the formation of clusters themselves. In order to unveil this connection, statistical studies of radio halos are necessary. We selected a sample of galaxy clusters with M500>6e14Msun and z=0.08-0.33 from the Planck SZ catalogue. In paper I, we presented the radio and X-ray data analysis that we carried out on these clusters. Here, we study the radio properties of the sample, in connection to the mass and dynamical state of clusters. We used the dynamical information derived from the X-ray data to assess the role of mergers in the origin of radio halos. We studied the distribution of clusters in the radio power-mass diagram and the role of dynamics on the radio luminosity and emissivity of radio halos. We measured the occurrence of radio halos as a function of the cluster mass and we compared it with the expectations of turbulent acceleration models. We found that more than the 90% of radio halos are in merging clusters and that their radio power correlates with the mass of the host clusters. The correlation shows a large dispersion. Interestingly, we showed that cluster dynamics contributes significantly to this dispersion with more disturbed clusters being more radio luminous. Clusters without radio halos are generally relaxed and the upper limits to their diffuse emission lie below the correlation. We showed that the radio emissivity of clusters exhibits an apparent bimodality, with the emissivity of radio halos being at least 5 times larger than the non-emission associated with more relaxed clusters. We found that the fraction of radio halos drops from ~70% in high mass clusters to ~35% in the lower mass systems of the sample and we showed that this result is in good agreement with the expectations from turbulent re-acceleration models.

astro-ph.CO↗

arXiv2020

Physical insights from the spectrum of the radio halo in MACS J0717.5+3745

We present new LOFAR observations of the massive merging galaxy cluster MACS J0717.5+3745. The cluster hosts the most powerful radio halo known to date. These new observations, in combination with published uGMRT (300$-$850 MHz) and VLA (1$-$6.5 GHz) data, reveal that the halo is more extended than previously thought, with a largest linear size of $\sim2.2 \rm Mpc$. The halo shows a steep spectrum ($α_{144\,\text{MHz}}^{1.5\,\text{GHz}}\sim-1.4$) and a steepening ($α_{1.5 \text{GHz}}^{5.5 \text{GHz}}\sim-1.9$) above 1.5 GHz. We find a strong scattering in spectral index maps on scales of 50$-$100 kpc. We suggest that such a strong scattering may be a consequence of the regime where inverse Compton dominate the energy losses of electrons. The spectral index becomes steeper and shows an increased curvature in the outermost regions of the halo. We combined the radio data with \textit{Chandra} observations to investigate the connection between the thermal and non-thermal components of the intracluster medium (ICM). Despite a significant substructure in the halo emission, the radio brightness correlates strongly with the X-ray brightness at all observed frequencies. The radio-versus-X-ray brightness correlation slope steepens at a higher radio frequency (from $b_{144 \text{MHz}}=0.67\pm0.05$ to $b_{3.0 \text{GHz}}=0.98\pm0.09$) and the spectral index shows a significant anti correlation with the X-ray brightness. Both pieces of evidence further support a spectral steepening in the external regions. The compelling evidence for a steep spectral index, the existence of a spectral break above 1.5 GHz, and the dependence of radio and X-ray surface brightness correlation on frequency are interpreted in the context of turbulent reacceleration models. Under this scenario, our results allowed us to constrain that the turbulent kinetic pressure of the ICM is up to 10%.

astro-ph.GA↗

arXiv2020

Understanding the radio relic emission in the galaxy cluster MACS J0717.5+3745: spectral analysis

Radio relics are diffuse, extended synchrotron sources that originate from shock fronts generated during cluster mergers. The massive merging galaxy cluster MACS J0717.5+3745 hosts one of the more complex relics known to date. We present upgraded Giant Metrewave Radio Telescope band 3 (300-500 MHz) and band 4 (550-850 MHz) observations. These new observations, combined with published VLA and the new LOFAR HBA data, allow us to carry out a detailed, high spatial resolution spectral analysis of the relic over a broad range of frequencies. The integrated spectrum of the relic closely follows a power-law between 144 MHz and 5.5 GHz with a mean spectral slope $α=-1.16\pm0.03$. Despite its complex morphology, the subregions of the relic and the other isolated filaments also follow power-law behaviors, and show similar spectral slopes. Assuming Diffusive Shock Acceleration, we estimate a dominant Mach number of $\sim 3.7$ for the shocks that make up the relic. Comparison with recent numerical simulations suggests that in the case of radio relics, the slopes of the integrated radio spectra are determined by the Mach number of the accelerating shock, with $α$ nearly constant, namely between $-1.13$ and $-1.17$, for Mach numbers $3.5 - 4.0$. The spectral shapes inferred from spatially resolved regions show curvature, we speculate that the relic is inclined along the line-of-sight. The locus of points in the simulated color-color plots changes significantly with the relic viewing angle. We conclude that projection effects and inhomogeneities in the shock Mach number dominate the observed spectral properties of the relic in this complex system. Based on the new observations we raise the possibility that the relic and a narrow-angle-tailed radio galaxy are two different structures projected along the same line-of-sight.

astro-ph.GA↗

arXiv2020

The Coma cluster at LOFAR frequencies I: insights into particle acceleration mechanisms in the radio bridge

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 analyse 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 find that the radio emission peaks on the NGC 4839 group. Towards 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 find that thermal and non-thermal plasma are moderately correlated with a sub-linear 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 find that the spectrum is steeper than $-1.4 \pm 0.2$, and that the emission could be 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 could generate the radio emission in the bridge. Assuming that the initial energy ratio of the seed electrons is $3 \cdot 10^{-4}$ with respect to the thermal gas, we are able to reproduce the observed luminosity. Our results suggest that the seed electrons released by radiogalaxies in the bridge and the turbulence generated by the motion of gas and galaxies are essential to produce the radio emission.

astro-ph.CO↗

arXiv2020

Diffuse Radio Emission from Galaxy Clusters in the LOFAR Two-metre Sky Survey Deep Fields

Low-frequency radio observations are revealing an increasing number of diffuse synchrotron sources from galaxy clusters, dominantly in the form of radio halos or radio relics. The existence of this diffuse synchrotron emission indicates the presence of relativistic particles and magnetic fields. It is still an open question what mechanisms exactly are responsible for the population of relativistic electrons driving this synchrotron emission. The LOFAR Two-metre Sky Survey Deep Fields offer a unique view of this problem. Reaching noise levels below 30 $μ$Jy/beam, these are the deepest images made at the low frequency of 144 MHz. This paper presents a search for diffuse emission in galaxy clusters in the first data release of the LOFAR Deep Fields. We detect a new high-redshift radio halo with a flux density of $8.9 \pm 1.0$ mJy and corresponding luminosity of $P_{144\mathrm{MHz}}=(3.6 \pm 0.6)\times10^{25}$ W Hz$^{-1}$ in an X-ray detected cluster at $z=0.77$ with a mass estimate of $M_{500} = 3.3_{-1.7}^{+1.1} \times 10^{14} M_\odot.$ Deep upper limits are placed on clusters with non-detections. We compare the results to the correlation between halo luminosity and cluster mass derived for radio halos found in the literature. This study is one of few to find diffuse emission in low mass ($M_{500} < 5\times10^{14} M_\odot$) systems and shows that deep low-frequency observations of galaxy clusters are fundamental for opening up a new part of parameter space in the study of non-thermal phenomena in galaxy clusters.

astro-ph.HE↗