arXiv subjects

A. Botteon

Publications and source records attributed to A. Botteon.

At least 109 records · Page 6Linked to original sources

arXiv2021

A snapshot of the oldest AGN feedback phases

Active Galactic Nuclei (AGN) inject large amounts of energy into their host galaxies and surrounding environment, shaping their properties and evolution. In particular, AGN jets inflate cosmic-ray lobes, which can rise buoyantly as light `bubbles' in the surrounding medium, displacing and heating the encountered thermal gas and thus halting its spontaneous cooling. These bubbles have been identified in a wide range of systems. However, due to the short synchrotron lifetime of electrons, the most advanced phases of their evolution have remained observationally unconstrained, preventing us to fully understand their coupling with the external medium, and thus AGN feedback. Simple subsonic hydrodynamic models predict that the pressure gradients, naturally present around the buoyantly rising bubbles, transform them into toroidal structures, resembling mushroom clouds in a stratified atmosphere. The way and timescales on which these tori will eventually disrupt depend on various factors including magnetic fields and plasma viscosity. Here we report LOFAR observations below 200 MHz, sensitive to the oldest radio-emitting particles, showing the late evolution of multiple generations of cosmic-ray AGN bubbles in a galaxy group with unprecedented level of detail. The bubbles' buoyancy power can efficiently offset the radiative cooling of the intragroup medium. However, the bubbles have still not thoroughly mixed with the thermal gas, after hundreds of million years, likely under the action of magnetic fields.

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arXiv2021

Pushing subarcsecond resolution imaging down to 30 MHz with the trans-European International LOFAR Telescope

Relatively little information is available about the Universe at ultra-low radio frequencies, i.e. below 50 MHz (ULF), although the ULF spectral window contains a wealth of unique diagnostics for studying galactic and extragalactic phenomena. Sub-arcsecond resolution imaging at these frequencies is extremely difficult, due to the long baselines (>1000 km) required and large ionospheric perturbations. We have conducted a pilot project to investigate the ULF performance and potential of the International LOFAR Telescope (ILT), a trans-European interferometric array with baselines up to ~2000 km and observing frequencies down to 10 MHz. We have successfully produced images with sub-arcsecond resolution for 6 radio sources at frequencies down to 30 MHz. This is more than an order of magnitude better resolution than pre-ILT observations at similar frequencies. The six targets that we have imaged (3C 196, 3C 225, 3C 273, 3C 295, 3C 298 and 3C 380) are bright radio sources with compact structures. By comparing our data of 3C 196 and 3C 273 with observations at higher frequencies, we investigate their spatially resolved radio spectral properties. Our success shows that at frequencies down to 30 MHz, sub-arcsecond imaging with the ILT is possible. Further analysis is needed to determine the feasibility of observations of fainter sources or sources with less compact emission.

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arXiv2021

A 3.5 Mpc-long radio relic in the galaxy cluster ClG 0217+70

Mega-parsec scale radio sources in the form of halos and relics are often detected in dynamically disturbed galaxy clusters. Although they are associated with merger-induced turbulence and shocks, respectively, their formation is not fully understood. We aim to identify the mechanisms responsible for particle acceleration and magnetic field amplification in the halo and relics of the galaxy cluster ClG 0217+70. We observed ClG 0217+70 with LOFAR at 141 MHz and VLA at 1.5 GHz, and combine these observations with VLA 1.4 GHz archival data to study the morphological and spectral properties of the diffuse sources. We add Chandra archival data to examine the thermal and non-thermal properties of the halo. Our LOFAR and VLA data confirm the presence of a giant radio halo in the cluster centre and multiple relics in the outskirts. The radio and X-ray emission from the halo are correlated, implying a tight relation between the thermal and non-thermal components. The diffuse radio structure in the south east with a projected size of 3.5 Mpc is the most extended radio relic detected to date. The spectral index across the relic width steepens towards the cluster centre, suggesting the electron ageing in the post-shock regions. The shock Mach numbers for the relics derived from the spectral index map range between 2.0 and 3.2. However, the integrated spectral indices lead to increasingly high Mach numbers for the relics farther from the cluster centre. This discrepancy could be because the relation between injection and integrated spectra does not hold for distant shocks, suggesting that the cooling time for the radio-emitting electrons is longer than the crossing time of the shocks. The variations in the surface brightness of the relics and the low Mach numbers imply that the radio-emitting electrons are re-accelerated from fossil plasma that could originate in active galactic nuclei.

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arXiv2021

The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOFAR Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultra-steep spectrum with $α_{54}^{144}\sim3.2$. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models.

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arXiv2021

A LOFAR-uGMRT spectral index study of distant radio halos

Context. Radio halos are megaparsec-scale diffuse radio sources{ mostly} located at the centres of merging galaxy clusters. The common mechanism invoked to explain their origin is the re-acceleration of relativistic particles caused by large-scale turbulence. Aims. Current re-acceleration models predict that a significant number of halos at high redshift should be characterised by very steep spectra ($α<-1.5$) because of increasing inverse Compton energy losses. In this paper, we investigate the spectral index properties of a sample of nine clusters selected from the second Planck Sunyaev-Zel'dovich catalogue showing diffuse radio emission with the Low Frequency Array (LOFAR) in the 120-168 MHz band. This is the first time that radio halos discovered at low frequencies are followed up at higher frequencies. Methods. We analysed upgraded Giant Metrewave Radio Telescope (uGMRT) observations in Bands 3 and 4, that is, 250-500 and 550-900 MHz respectively. These observations were combined with existing LOFAR data to obtain information on the spectral properties of the diffuse radio emission. Results. We find diffuse radio emission in the uGMRT observations for five of the nine high-$z$ radio halos previously discovered with LOFAR. For those, we measure spectral indices in the range of $-1$ to $-1.4$. For the uGMRT non-detections, we estimated that the halos should have a spectral index steeper than $-1.5$. We also confirm the presence of one candidate relic. Conclusions. Despite the small number of clusters, we find evidence that about half of the massive and merging clusters at high redshift host radio halos with a very steep spectrum. This is in line with theoretical predictions, although larger statistical samples are necessary to test models.

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arXiv2021

Origin of the ring structures in Hercules A -- Sub-arcsecond 144 MHz to 7 GHz observations

The prominent radio source Hercules A features complex structures in its radio lobes. Although it is one of the most comprehensively studied sources in the radio sky, the origin of the ring structures in the Hercules A radio lobes remains an open question. We present the first sub-arcsecond angular resolution images at low frequencies (<300 MHz) of Hercules A, made with the International LOFAR Telescope. With the addition of data from the Karl G. Jansky Very Large Array, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explore the origin of the rings within the lobes of Hercules A, and test whether their properties are best described by a shock model, where shock waves are produced by the jet propagating in the radio lobe, or by an inner-lobe model, where the rings are formed by decelerated jetted plasma. From spectral index mapping our large frequency coverage reveals that the curvature of the different ring spectra increases with distance away from the central active galactic nucleus. We demonstrate that the spectral shape of the rings is consistent with synchrotron aging, which speaks in favor of an inner-lobe model where the rings are formed from the deposition of material from past periods of intermittent core activity.

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arXiv2021

The eROSITA Final Equatorial-Depth Survey (eFEDS): LOFAR view of brightest cluster galaxies and AGN feedback

During the performance verification phase of the SRG/eROSITA telescope, the eROSITA Final Equatorial-Depth Survey (eFEDS) has been carried out. It covers a 140 deg$^2$ field located at 126$^\circ <$ R.A. $< 146^\circ$ and -3$^\circ <$ Dec. $< +6^\circ$ with a nominal exposure over the field of 2.2 ks. 542 candidate clusters were detected in this field, down to a flux limit $F_X \sim 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5-2 keV band. In order to understand radio-mode feedback in galaxy clusters, we study the radio emission of brightest cluster galaxies of eFEDS clusters, and we relate it to the X-ray properties of the host cluster. Using LOFAR we identify 227 radio galaxies hosted in the BCGs of the 542 galaxy clusters and groups detected in eFEDS. We treat non-detections as radio upper limits. We analyse the properties of radio galaxies, such as redshift and luminosity distribution, offset from the cluster centre, largest linear size and radio power. We study their relation to the intracluster medium of the host cluster. We perform statistical tests to deal with upper limits on the radio luminosities. BCGs with radio-loud AGN are more likely to lie close to the cluster centre than radio-quiet BCGs. There is a clear relation between the cluster's X-ray luminosity and the radio power of the BCG. Statistical tests indicate that this correlation is not produced by selection effects in the radio band. We see no apparent link between largest linear size of the radio galaxy and central density of the host cluster. Converting the radio luminosity to kinetic luminosity, we find that radiative losses of the intracluster medium are in an overall balance with the heating provided by the central AGN. Finally, we tentatively classify our objects into disturbed and relaxed, and we show that the link between the AGN and the ICM apparently holds regardless of the dynamical state of the cluster.

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arXiv2021

Abell 1430: A merging cluster with exceptional diffuse radio emission

Diffuse radio emission has been found in many galaxy clusters, predominantly in massive systems which are in the state of merging. The radio emission can usually be classified as relic or halo emission, which are believed to be related to merger shocks or volume-filling turbulence, respectively. Recent observations have revealed radio bridges for some pairs of very closeby galaxy clusters. The mechanisms that may allow to explain the high specific density of relativistic electrons, necessary to explain the radio luminosity of these bridge regions, are poorly explored. We analyse the galaxy cluster Abell 1430 with LoTSS data in detail and complement it with recent JVLA L-band observations, XMM-Newton, Chandra, and SDSS data. Moreover, we compare our results to clusters extracted from the "The Three Hundred Project" cosmological simulation. We find that Abell 1430 consists of two components, namely A1430-A and A1430-B. We speculate that the two components undergo an off-axis merger. The more massive component shows diffuse radio emission which can be classified as radio halo showing a low radio power given the mass of the cluster. Most interestingly, there is extended diffuse radio emission, dubbed as the `Pillow', which is apparently related to A1430-B and thus related to low density intracluster or intergalactic medium. To date, a only few examples for emission originating from such regions are known. These discoveries are crucial to constrain possible acceleration mechanisms, which may allow to explain the presence of relativistic electrons in these regions. Our results indicate a spectral index of $α_{144\,\text{MHz}}^{1.5\,\text{GHz}}=-1.4\pm0.5$ for the Pillow. If future observations confirm a slope as flat as the central value of -1.4 or even flatter, this would pose a severe challenge for the electron acceleration scenarios.

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arXiv2021

LoTSS jellyfish galaxies: II. Ram pressure stripping in groups versus clusters

Numerous examples of ram pressure stripping in galaxy clusters are present in literature; however, substantially less work has been focused on ram pressure stripping in lower mass groups. In this work we use the LOFAR Two-metre Sky Survey (LoTSS) to search for jellyfish galaxies in ~500 SDSS groups (z<0.05), making this the most comprehensive search for ram pressure stripping in groups to date. We identify 60 jellyfish galaxies in groups with extended, asymmetric radio continuum tails, which are found across the entire range of group mass from $10^{12.5} < M_\mathrm{group} < 10^{14}\,h^{-1}\,\mathrm{M_\odot}$. We compare the group jellyfish galaxies identified in this work with the LoTSS jellyfish galaxies in clusters presented in Roberts et al. (2021), allowing us to compare the effects of ram pressure stripping across three decades in group/cluster mass. We find that jellyfish galaxies are most commonly found in clusters, with the frequency decreasing towards the lowest mass groups. Both the orientation of observed radio continuum tails, and the positions of group jellyfish galaxies in phase space, suggest that galaxies are stripped more slowly in groups relative to clusters. Finally, we find that the star formation rates of jellyfish galaxies in groups are consistent with `normal' star-forming group galaxies, which is in contrast to cluster jellyfish galaxies that have clearly enhanced star formation rates. On the whole, there is clear evidence for ongoing ram pressure stripping in galaxy groups (down to very low group masses), though the frequency of jellyfish galaxies and the strength of ram pressure stripping appears smaller in groups than clusters. Differences in the efficiency of ram pressure stripping in groups versus clusters likely contributes to the positive trend between quenched fraction and host halo mass observed in the local Universe.

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arXiv2021

Discovery of a radio halo (and relic) in a $M_{500} < 2 \times 10^{14}$ M$_\odot$ cluster

Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed galaxy clusters. Current observations and models suggest that halos trace turbulent regions in the intra-cluster medium where mildly relativistic particles are re-accelerated during cluster mergers. Due to the higher luminosities and detection rates with increasing cluster mass, radio halos have been mainly observed in massive systems ($M_{500} \gtrsim 5 \times10^{14}$ M$_\odot$). Here, we report the discovery of a radio halo with a largest linear scale of $\simeq$750 kpc in PSZ2G145.92-12.53 ($z=0.03$) using LOFAR observations at 120$-$168 MHz. With a mass of $M_{500} = (1.9\pm0.2) \times 10^{14}$ M$_\odot$ and a radio power at 150 MHz of $P_{150} = (3.5 \pm 0.7) \times 10^{23}$ W/Hz, this is the least powerful radio halo in the least massive cluster discovered to date. Additionally, we discover a radio relic with a mildly convex morphology at $\sim$1.7 Mpc from the cluster center. Our results demonstrate that LOFAR has the potential to detect radio halos even in low-mass clusters, where the expectation to form them is very low ($\sim$5%) based on turbulent re-acceleration models. Together with the observation of large samples of clusters, this opens the possibility to constrain the low end of the power-mass relation of radio halos.

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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.

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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

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

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.

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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.

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arXiv2021

The LOFAR LBA Sky Survey I. survey description and preliminary data release

LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (<1 mJy/b and <15") observations at ultra-low frequencies (<100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66 MHz, at a resolution of 15 arcsec and at a sensitivity of 1 mJy/b. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740 sqdeg around the HETDEX spring field region at a resolution of 47" with a median noise level of 5 mJy/b. The images and the catalogue with 25,247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1 mJy/b and the resolution of 15" once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity.

astro-ph.IM↗

arXiv2021

Discovery of a Supercluster in the eROSITA Final Equatorial Depth Survey: X-ray Properties, Radio Halo, and Double Relics

We examine the X-ray, optical, and radio properties for the members clusters of a new supercluster discovered during the SRG/eROSITA Performance Verification phase. In the 140 deg2 eROSITA Final Equatorial Depth Survey (eFEDS) field we detect a previously unknown supercluster consisting of a chain of eight galaxy clusters at z=0.36. The redshifts of these members are determined through HSC photometric measurements. We examine the X-ray morphological and dynamical properties, gas and total mass out to R500 of the members and compare them with the general population of clusters detected in the eFEDS field. We further investigate the gas in the bridge region between the cluster members for a potential WHIM detection. Radio follow-up observations with LOFAR and uGMRT are used to search for diffuse emission and constrain the dynamic state of the system. We do not find significant differences in the morphological parameters and properties of the intra-cluster medium of the clusters embedded in this large-scale filament compared to eFEDS clusters. We also provide upper limits on the electron number density and mass of the warm-hot intergalactic medium as provided by the eROSITA data. These limits are consistent with previously reported values for the detections in the vicinity of clusters of galaxies. In LOFAR and uGMRT follow-up observations of the northern part of this supercluster we find two new radio relics that are the result of major merger activity in the system. These early results show the potential of eROSITA to probe large-scale structures such as superclusters and the properties of their members. Our forecasts show that we will be able to detect 450 superclusters with 3000 member clusters located in the eROSITA_DE region at the final eROSITA all-sky survey depth, enabling statistical studies of the properties of superclusters and their constituents embedded in the cosmic web.

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