Merging Galaxy Cluster Research Articles

Suzaku Observations of the Cold Front in A3667: A Nonequilibrium Ionization State of the Intracluster Medium

Abstract We present a systematic study of the thermodynamic properties of the intracluster medium (ICM), including the plasma conditions in the ICM, in the merging galaxy cluster A3667 using Suzaku. We analyze the X-ray spectra of the ICM between the northwestern and the southeastern radio relics across a prominent cold front in A3667 to search for the ICM in a nonequilibrium ionization (NEI) state. We find that the ICM inside the cold front exhibits an NEI state with an ionization parameter of (2.2 ± 0.4) × 1011 s cm−3 at the 90% confidence level, which is lower than that expected from a collisional ionization equilibrium state of the ICM (i.e., n e t > 1012 s cm−3). The timescale calculated from the ionization parameter of the ICM inside the cold front is 3.9 − 0.8 + 0.7 Myr , which is much shorter than the thermal equilibration timescale. A weak transonic sloshing motion might explain the possibility that the ICM inside the cold front of A3667 is in the NEI state. In addition, the NEI state of the ICM in A3667 seems to be associated with the elongated radio halo bridging the region between the northwestern radio relic and the prominent cold front in A3667 across the cluster center.

X-ray and optical analysis of the distant merging double cluster SPT-CLJ2228-5828, its gas bridge, and its shock front

Galaxy cluster mergers are excellent laboratories for studying a wide variety of different physical phenomena. An example of such a cluster system is the distant SPT-CLJ2228-5828 merger located at z≈ 0.77. Previous analyses via the thermal Sunyaev-Zeldovich effect and weak lensing (WL) data suggested that the system was potentially a dissociative cluster post-merger, similar to the Bullet cluster. In this work, we perform an X-ray and optical follow-up analysis of this rare system. We used new deep XMM-Newton data to study the hot gas in X-rays in great detail, spectroscopic Gemini data to precisely determine the redshift of the two mass concentrations, and new Hubble Space Telescope data to improve the total mass estimates of the two components. We find that SPT-CLJ2228-5828 constitutes a pre-merging double cluster system instead of a post-merger as previously thought. The merging process of the two clusters has started, with their gas on the outskirts colliding with a ∼ 22^ ∘ ∘ on the plane of the sky. Both clusters have a similar radius of R_ ∼ 700 kpc, with the two X-ray emission peaks separated by ≈ 1 Mpc (2.1^ We fully characterized the surface brightness, gas density, temperature, pressure, and entropy profiles of the two merging clusters for their undisturbed non-interacting side. The two systems have very similar X-ray properties, with a moderate cluster mass of tot ∼ (2.1-2.4) ⊙ according to X-ray mass proxies. Both clusters show good agreement with known X-ray scaling relations when their merging side is ignored. The WL mass estimate of the western cluster agrees well with the X-ray-based mass, whereas the eastern cluster is surprisingly only marginally detected from its WL signal. A gas bridge with ≈ 333 kpc length connecting the two merging halos is detected at a $5.8σ$ level. The baryon overdensity of the excess gas (not associated with the cluster gas) is δ_ b ∼ (75-320) across the length of the bridge, and its gas mass is M_ gas ∼ 1.4 ⊙ . The gas density and temperature jumps at ∼ 10^ cm^-3 and ∼ 5.5 keV, respectively, are also found across the gas bridge, revealing the existence of a weak shock front with a Mach number mathcal M ∼ 1.1. The gas pressure and entropy also increase at the position of the shock front. We estimate the age of the shock front to be lesssim 100 Myr and its kinetic energy ∼ 2.4 erg s^-1. SPT-CLJ2228-5828 is the first such high-z pre-merger with a gas bridge and a shock front, consisting of similarly sized clusters, to be studied in X-rays.

A uGMRT and MeerKAT Study of Radio Relics in the Low-mass Merging Cluster PSZ2 G200.95−28.16

Abstract Diffuse radio sources known as radio relics are direct tracers of shocks in the outskirts of merging galaxy clusters. PSZ2 G200.95–28.16, a low-mass merging cluster (M 500 = (2.7 ± 0.2) × 1014 M ⊙), features a prominent radio relic, first identified by R. Kale et al. We name this relic as the Seahorse. The MeerKAT Galaxy Cluster Legacy Survey has confirmed two additional radio relics, R2 and R3, in this cluster. We present new observations of this cluster with the upgraded Giant Metrewave Radio Telescope (uGMRT) at 400 and 650 MHz, paired with Chandra X-ray data. The largest linear sizes for the three relics are 1.53 Mpc, 1.12 kpc, and 340 kpc. All three radio relics are polarized at 1283 MHz. Assuming the diffusive shock acceleration model, the spectral indices of the relics imply shock Mach Numbers of 3.1 ± 0.8 and 2.8 ± 0.9 for the Seahorse and R2, respectively. The Chandra X-ray surface brightness map shows two prominent subclusters, but the relics are not perpendicular to the likely merger axis, as typically observed; no shocks are detected at the locations of the relics. We discuss possible merger scenarios in light of the low mass of the cluster and the radio and X-ray properties of the relics. The relic R2 follows the correlation known in the radio relic power and cluster-mass plane, but the Seahorse and R3 relics are outliers. We have also discovered a radio ring in our 650 MHz uGMRT image that could be an Odd Radio Circle candidate.

Low Injection Rate of Cosmic-Ray Protons in the Turbulent Reacceleration Model of Radio Halos in Galaxy Clusters

A giant radio halo (RH) is a diffuse synchrotron emission observed on the scale of megaparsecs, typically found in the central region of merging galaxy clusters. Its large size and steep spectrum suggest that it originates from the reenergization of an aged population of cosmic-ray electrons (CREs), while the secondary leptons produced in the pp hadronic collision of cosmic-ray protons (CRPs) may contribute to the emission. In this study, we investigate the reacceleration model including both primary and secondary CREs, assuming that the primary cosmic rays (CRs) originate from internal galaxies. In our new method, we follow the cosmological evolution of each cluster and calculate the energy spectra and 1D spatial distributions of CRs. The primary CRE model with a ∼3 Gyr duration of reacceleration successfully reproduces the statistical properties of the RHs observed in the recent LOFAR survey, as well as the spectrum and profile of the Coma cluster. The gamma-ray and neutrino emissions produced by reaccelerated CRPs are consistent with the upper limits. However, if the CRP injection rate is high and the secondary CREs become significant, the model with the required ∼3 Gyr reacceleration overproduces the number of RHs. The limit on the CRP injection rate, L p ≲ 1041 erg s−1, is significantly lower than that expected from the early starburst activity or jets from active galactic nuclei. This discrepancy requires a revision of either the model of CR supply from galaxies or the turbulent reacceleration model.

Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B r ). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma beta β i —the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects of β i on guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced by β i , we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large βi>(Br2+Bg2)/Br2 . We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as vout∝1/βi . These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters.

Measuring the intracluster medium velocity structure within the A3266 galaxy cluster

We present a detailed analysis of the velocity structure of the hot intracluster medium (ICM) within the A3266 galaxy cluster, including new observations taken between June and November 2023. Firstly, morphological structures within the galaxy cluster were examined using a Gaussian gradient magnitude (GGM) and an adaptively smoothed GGM filter applied to the EPIC-pn X-ray image. Then, we applied a novel XMM-Newton EPIC-pn energy scale calibration, which uses instrumental Cu Kα as a reference for the line emission, to measure the line-of-sight velocities of hot gas within the system. This approach enabled us to create two-dimensional projected maps for velocity, temperature, and metallicity, showing that the hot gas displays a redshifted systemic velocity relative to the cluster redshift across all fields of view. Further analysis of the velocity distribution through non-overlapping circular regions demonstrates consistent redshifted velocities extending up to 1125 kpc from the cluster core. Additionally, the velocity distribution was assessed along regions following surface brightness discontinuities, where we observed redshifted velocities in all regions, with the largest velocities reaching 768 ± 284 km/s. Moreover, we computed the velocity probability density function (PDF) from the velocity map. We applied a normality test, finding that the PDF adheres to an unimodal normal distribution consistent with theoretical predictions. Lastly, we computed a velocity structure function for this system using the measured line-of-sight velocities. These insights advance our understanding of the dynamic processes within the A3266 galaxy cluster and contribute to our broader knowledge of ICM behavior in merging galaxy clusters.

Jet Interaction with Galaxy Cluster Mergers

Active galactic nucleus (AGN) bubbles in cool-core galaxy clusters are believed to facilitate the transport of cosmic-ray electrons (CRe) throughout the cluster. Recent radio observations reveal the complex morphologies of cluster diffuse emission, potentially linked to interactions between AGN bursts and the cluster environment. We perform 3D magnetohydrodynamical simulations of binary cluster mergers and inject a bidirectional jet at the center of the main cluster. Kinetic, thermal, magnetic, and cosmic ray (CR) energy are included in the jet and we use the two-fluid formalism to model the CR component. We explore a wide range of cluster merger and jet parameters. We discuss the formation of various wide-angle-tail and X-shaped sources in the early evolution of the jet and merger. During the last phase of the evolution, we find that the CR material efficiently permeates the central region of the cluster reaching radii of ∼1–2 Mpc within ∼5–6 Gyr, depending on the merger mass ratio. We find that solenoidal turbulence dominates during the binary merger and we explore the possibility for the CR jet material to be reaccelerated by super-Alfvènic turbulence and contribute to cluster scale radio emission. We find high volume fractions, ≳70%, at which the turbulent acceleration time is shorter than the electron cooling time. Finally, we study the merger shock interaction with the CRe material and show that it is unlikely that this material significantly contributes to the radio relic emission associated with the shocks. We suggest that multiple jet outbursts and/or off-center radio galaxies would increase the likelihood of detecting these merger shocks in the radio due to shock reacceleration.

Magnetic fields in the outskirts of PSZ2 G096.88+24.18 from a depolarization analysis of radio relics

Context. Radio relics are diffuse, non-thermal radio sources present in a number of merging galaxy clusters. They are characterized by elongated arc-like shapes and highly polarized emission (up to ∼60%) at gigahertz frequencies, and are expected to trace shock waves in the cluster outskirts induced by galaxy cluster mergers. Their polarized emission can be used to study the magnetic field properties of the host cluster. Aims. In this paper, we investigate the polarization properties of the double radio relics in PSZ2 G096.88+24.18 using the rotation measure (RM) synthesis, and try to constrain the characteristics of the magnetic field that reproduce the observed depolarization as a function of resolution (beam depolarization). Our aim is to understand the nature of the low polarization fraction that characterizes the southern relic with respect to the northern relic. Methods. We present new 1–2 GHz Karl G. Jansky Very Large Array (VLA) observations in multiple configurations. We derived the RM and polarization of the two relics by applying the RM synthesis technique, and thus solved for bandwidth depolarization in the wide observing bandwidth. To study the effect of beam depolarization, we degraded the image resolution and studied the decreasing trend of polarization fraction with increasing beam size. Finally, we performed 3D magnetic field simulations using multiple models for the magnetic field power spectrum over a wide range of scales, in order to constrain the characteristics of the cluster magnetic field that can reproduce the observed beam depolarization trend. Results. Using RM synthesis, we obtained a polarization fraction of (18.6 ± 0.3)% for the northern relic and (14.6 ± 0.1)% for the southern one. Having corrected for bandwidth depolarization, and after noticing the absence of relevant complex Faraday spectrum, we inferred that the nature of the depolarization for the southern relic is external, and possibly related to the turbulent gas distribution within the cluster, or to the complex spatial structure of the relic. The best-fit magnetic field power spectrum, which reproduces the observed depolarization trend for the southern relic, was obtained for a turbulent magnetic field model, described by a power spectrum derived from cosmological simulations, and defined within the scales of Λmin = 35 kpc and Λmax = 400 kpc. This yields an average magnetic field of the cluster within 1 Mpc3 volume of ∼2 μG.

A Multiwavelength Approach to Constraining the Merger Properties of ACT-CL J0034.4+0225

ACT-CL J0034.4+0225 is a previously unrecognized merging galaxy cluster at z = 0.38588 ± 0.00068. Our primary evidence is provided by a 21 ks Chandra image that shows two surface brightness peaks separated by ∼49″ (259 kpc) surrounded by an extended cluster gas distribution. Each gas peak contains a brightest cluster galaxy, offset from the gas peak. We collect new South African Large Telescope optical spectra that, when augmented by archival data, yield redshifts for the two BGCs and 58 other cluster members. Archival Giant Metrewave Radio Telescope and MeerKAT data reveal a radio halo that encompasses the X-ray peaks. We provide and compare three X-ray-based mass estimates (5.0 × 1014 M ⊙, 6.4 × 1014 M ⊙, and 8.6 × 1014 M ⊙). The Planck and ACT Sunyaev–Zel’dovich masses are ≈5.8 × 1014 M ⊙. We constrain the merger state and properties by comparing them to an existing suite of N-body/hydrodynamical models using the measured gas peak separation (259 kpc, projected) and radial velocity difference (0–1000 km s−1). This constrains the epoch of the merger to be within ∼100 Myr of first pericenter passage. A strong lensing analysis constrains the mass ratio to be in the range 1:1–1:20, while the cluster morphology prefers values near the equal-mass range.

Probing particle acceleration in Abell 2256: From 16 MHz to gamma rays

Merging galaxy clusters often host spectacular diffuse radio synchrotron sources. These sources can be explained by a non-thermal pool of relativistic electrons that are accelerated by shocks and turbulence in the intracluster medium. The origin of the pool and details of the cosmic ray transport and acceleration mechanisms in clusters are still open questions. Due to the often extremely steep spectral indices of diffuse radio emission, it is best studied at low frequencies. However, the lowest frequency window available to ground-based telescopes (10−30 MHz) has remained largely unexplored as radio frequency interference and calibration problems related to the ionosphere become severe. Here, we present LOFAR observations from 16 to 168 MHz targeting the famous cluster Abell 2256. In the deepest-ever images at decametre wavelengths, we detected and resolved the radio halo, radio shock, and various steep spectrum sources. We measured standard single power-law behaviour for the radio halo and radio shock spectra, with spectral indices of α = −1.56 ± 0.02 from 24 to 1500 MHz and α = −1.00 ± 0.02 from 24 to 3000 MHz, respectively. Additionally, we found significant spectral index and curvature fluctuations across the radio halo, indicating an inhomogeneous emitting volume. In contrast to the straight power-law spectra of the large-scale diffuse sources, the various AGN-related sources showed extreme steepening towards higher frequencies and flattening towards low frequencies. We also discovered a new fossil plasma source with a steep spectrum between 23 and 144 MHz, with α = −1.9 ± 0.1. Finally, by comparing radio and gamma-ray observations, we ruled out purely hadronic models for the radio halo origin in Abell 2256, unless the magnetic field strength in the cluster is exceptionally high, which is unsupportable by energetic arguments and inconsistent with the knowledge of other cluster magnetic fields.

A Rare, Strong Shock Front in the Merging Cluster SPT-CLJ 2031-4037

We present our findings from the new deep Chandra observations (256 ks) of the merging galaxy cluster SPT-CLJ 2031-4037 at z = 0.34. Our observations reveal intricate structures seen in a major merger akin to the Bullet Cluster. The X-ray data confirm the existence of two shock fronts, one to the northwest and one to the southeast, by directly measuring the temperature jump of gas across the surface brightness edges. The stronger shock front in the northwest has a density jump of 3.16 ± 0.34 across the sharp surface brightness edge and Mach number M=3.36−0.48+0.87 , which makes this cluster one of the rare merging systems with a Mach number M > 2. We use the northwestern shock to compare two models for shock heating—the instant heating model and the Coulomb collisional heating model—and we determine that the temperatures across the shock front agree with the Coulomb collisional model of heating. For the shock front in the southeastern region, we find a density jump of 1.53 ± 0.14 and a Mach number of M=1.36−0.08+0.09 .

ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger

Galaxy cluster mergers are rich sources of information to test cluster astrophysics and cosmology. However, cluster mergers produce complex projected signals that are difficult to interpret physically from individual observational probes. Multi-probe constraints on the gas and dark matter (DM) cluster components are necessary to infer merger parameters that are otherwise degenerate. We present Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX), a systematic framework to jointly infer multiple merger parameters quantitatively via a pipeline that directly compares a novel combination of multi-probe observables to mock observables derived from hydrodynamical simulations. We report a first application of the ICM-SHOX pipeline to MACS J0018.5+1626, wherein we systematically examine simulated snapshots characterized by a wide range of initial parameters to constrain the MACS J0018.5+1626 merger geometry. We constrain the epoch of MACS J0018.5+1626 to the range 0–60 Myr post-pericenter passage, and the viewing angle is inclined ≈27°–40° from the merger axis. We obtain constraints for the impact parameter (≲250 kpc), mass ratio (≈1.5–3.0), and initial relative velocity when the clusters are separated by 3 Mpc (≈1700–3000 km s−1). The primary and secondary clusters initially (at 3 Mpc) have gas distributions that are moderately and strongly disturbed, respectively. We discover a velocity space decoupling of the DM and gas distributions in MACS J0018.5+1626, traced by cluster-member galaxy velocities and the kinematic Sunyaev–Zel'dovich effect, respectively. Our simulations indicate this decoupling is dependent on the different collisional properties of the two distributions for particular merger epochs, geometries, and viewing angles.

Radio relics in massive galaxy cluster mergers in the TNG-Cluster simulation

Radio relics are diffuse synchrotron sources in the outskirts of merging galaxy clusters energized by the merger shocks. In this paper, we present an overview of the radio relics in massive cluster mergers identified in the new TNG-Cluster simulation. This is a suite of magnetohydrodynamical cosmological zoom-in simulations of 352 massive galaxy clusters with M500c = 1014.0 − 15.3 M⊙ sampled from a 1 Gpc-sized cosmological box. The simulations were performed using the moving-mesh code AREPO with the galaxy formation model and high numerical resolution consistent with the TNG300 run of the IllustrisTNG series. We post-processed the shock properties obtained from the on-the-fly shock finder to estimate the diffuse radio emission generated by cosmological shockwaves for a total of ∼300 radio relics at redshift z = 0 − 1. TNG-Cluster returned a variety of radio relics with diverse morphologies, encompassing classical examples of double radio relics, single relics, and “inverted” radio relics that are convex to the cluster center. Moreover, the simulated radio relics reproduced both the abundance and statistical relations of observed relics. We find that extremely large radio relics (> 2 Mpc) are predominantly produced in massive cluster mergers with M500c ≳ 8 × 1014 M⊙. This underscores the significance of simulating massive mergers to study giant radio relics similar to those found in observations. We released a library of radio relics from the TNG-Cluster simulation, which will serve as a crucial reference for upcoming next-generation surveys.

A New Dissociative Galaxy Cluster Merger: RM J150822.0+575515.2

Galaxy cluster mergers that exhibit clear dissociation between their dark matter, intracluster gas, and stellar components are great laboratories for probing dark matter properties. Mergers that are binary and in the plane of the sky have the additional advantage of being simpler to model, allowing for a better understanding of the merger dynamics. We report the discovery of a galaxy cluster merger with all these characteristics and present a multiwavelength analysis of the system, which was found via a search in the redMaPPer optical cluster catalog. We perform a galaxy redshift survey to confirm the two subclusters are at the same redshift (0.541, with 368 ± 519 km s−1 line-of-sight velocity difference between them). The X-ray morphology shows two surface brightness peaks between the brightest cluster galaxies (BCGs). We construct weak-lensing mass maps that reveal a mass peak associated with each subcluster. Fitting Navarro–Frenk–White profiles to the lensing data, we find masses of M 200c = 36 ± 11 × 1013 and 38 ± 11 × 1013 M ⊙ h −1 for the southern and northern subclusters, respectively. From the mass maps, we infer that the two mass peaks are separated by 520−125+162 kpc along the merger axis, whereas the two BCGs are separated by 697 kpc. We also present deep GMRT 650 MHz data to search for a radio relic or halo and find none. Using the observed merger parameters, we find analog systems in cosmological n-body simulations and infer that this system is observed between 96 and 236 Myr after pericenter, with the merger axis within 28° of the plane of the sky.

Hydrodynamical simulations of merging galaxy clusters: giant dark matter particle colliders, powered by gravity

Hydrodynamical simulations of merging galaxy clusters: giant dark matter particle colliders, powered by gravity

Simulations of galaxy cluster mergers with velocity-dependent, rare, and frequent self-interactions

ABSTRACT Self-interacting dark matter (SIDM) has been proposed to solve small-scale problems in $\rm {\Lambda CDM}$ cosmology. In previous work, constraints on the self-interaction cross-section of dark matter have been derived assuming that the self-interaction cross-section is independent of velocity. However, a velocity-dependent cross-section is more natural in most theories of SIDM. Using idealized N-body simulations without baryons, we study merging clusters with velocity-dependent SIDM. In addition to the usual rare scattering in the isotropic limit, we also simulate these systems with anisotropic, small-angle (frequent) scatterings. We find that the collisionless brightest cluster galaxy (BCG) has an offset from the DM peak that grows at later stages. Finally, we also extend the existing upper bounds on the velocity-independent, isotropic self-interaction cross-section to the parameter space of rare and frequent velocity-dependent self-interactions by studying the central densities of dark matter-only isolated haloes. For these upper-bound parameters, the DM-BCG offsets just after the first pericentre in the dark matter-only simulations are found to be ≲10 kpc. On the other hand, because of BCG oscillations, we speculate that the distribution of BCG offsets in a relaxed cluster is a statistically viable probe. Therefore, this motivates further studies of BCG off-centring in hydrodynamic cosmological simulations.

Discovery of diffuse radio source in Abell 1060

Abstract Non-thermal components in the intra-cluster medium (ICM), such as turbulence, magnetic field, and cosmic rays, imprint the past and current energetic activities of jets from active galactic nuclei (AGN) of member galaxies as well as disturbance caused by galaxy cluster mergers. Meter- and centimeter-radio observations of synchrotron radiation allow us to diagnose the nonthermal component. Here we report on our discovery of an unidentified diffuse radio source, named the Flying Fox, near the center of the Abell 1060 field. The Flying Fox has an elongated ring-like structure and a central bar shape, but there is no obvious host galaxy. The average spectral index of the Flying Fox is −1.4, which is steeper than that for radio sources seen at meter wavelengths. We discussed the possibilities of radio lobes, phoenixes, radio halos and relics, and an odd radio circle. In conclusion, the Flying Fox is not clearly explained by known radio sources.

Identifying galaxy cluster mergers with deep neural networks using idealized Compton-y and X-ray maps

ABSTRACT We present a novel approach to identify galaxy clusters that are undergoing a merger using a deep learning approach. This paper uses massive galaxy clusters spanning 0 ≤ z ≤ 2 from The Three Hundred project, a suite of hydrodynamic resimulations of 324 large galaxy clusters. Mock, idealized Compton-y and X-ray maps were constructed for the sample, capturing them out to a radius of 2R200. The idealized nature of these maps mean they do not consider observational effects such as foreground or background astrophysical objects, any spatial resolution limits or restriction on X-ray energy bands. Half of the maps belong to a merging population as defined by a mass increase ΔM/M ≥ 0.75, and the other half serves as a controlled, relaxed population. We employ a convolutional neural network architecture and train the model to classify clusters into one of the groups. A best-performing model was able to correctly distinguish between the two populations with a balanced accuracy (BA) and recall of 0.77, ROC-AUC of 0.85, PR-AUC of 0.55, and F1 score of 0.53. Using a multichannel model relative to a single-channel model, we obtain a 3 per cent improvement in BA score, and a 6 per cent improvement in F1 score. We use a saliency interpretation approach to discern the regions most important to each classification decision. By analysing radially binned saliency values we find a preference to utilize regions out to larger distances for mergers with respect to non-mergers, greater than ∼1.2R200 and ∼0.7R200 for SZ and X-ray, respectively.

The Optical Properties of Galaxy Cluster Abell 2319

The optical photometric and spectroscopic investigations of the massive and merging galaxy cluster Abell 2319 (A2319) are presented here. RTT150 telescope of TÜBİTAK, Antalya, Türkiye used CCD imaging and spectroscopic observations. In this paper, 110 galaxies were determined in A2319 and defined as the magnitudes of the Bessel B and R filters in each cluster member galaxy. Spectral observations were done of the brightest cluster galaxy (BCG) and six additional brilliant galaxies. We estimated the Luminosity Function (LF) of galaxies for each filter. The resulting LF of cluster galaxies for each filter is well-fitted by the Double Schechter function. The best-fit parameter values derived as the characteristic absolute magnitudes are -21.08 ± 0.03, and -20.84 ± 0.01, -21.43 ± 0.02, and -20.54 ± 0.02, and the slopes at the faint end of the LF were -1.34 ± 0.04 and -1.12 ± 0.03, -1.47 ± 0.05 and -1.18 ± 0.03 for B and R filters, respectively.

Probing diffuse radio emission in bridges between galaxy clusters with uGMRT

Aims. Recent X-ray and Sunyaev–Zeldovich (SZ) observations have detected thermal emission between early-stage merging galaxy clusters. The main purpose of this work is to investigate the properties of the non-thermal emission in the interacting cluster pairs Abell 0399-Abell 0401 and Abell 21-PSZ2 G114.9. Methods. These two unique cluster pairs have been found in an interacting state. In both cases, their connection along a filament is supported by an SZ effect detected by the Planck satellite and, in the special case of Abell 0399-Abell 0401, the presence of a radio bridge has been already confirmed by LOFAR observations at 140 MHz. Here, we analyse new high-sensitivity, wideband (250–500 MHz) uGMRT data of these two systems and describe an injection procedure to place limits on the spectrum of Abell 0399-Abell 0401 and on the radio emission between Abell 21-PSZ2 G114.9. Results. In both cases, the low-surface-brightness diffuse emission is not detected in Band 3 (250–500 MHz). For the A399-A401 pair, we are able to constrain the steep spectral index of the bridge emission to be α > 2.2 with a 95% confidence level between 140 MHz and 400 MHz. We also detect a small patch of the bridge with a flatter spectral index, which may suggest a variable spectral index distribution across the bridge area. For the A21-PSZ2 G114.9 pair, we are able to place an upper limit on the flux density of the bridge emission with two different methods, finding at the central frequency of 383 MHz a conservative value of fu1 < 260 mJy at a 95% confidence level, and a lower value of fu2 < 125 mJy at an 80% confidence level, based on visual inspection and a morphological criterion. Conclusions. Our work provides a constraint on the spectrum in the bridge A399-A401 that disfavours shock acceleration as the main mechanism for the radio emission. The methods that we propose for the limits on the radio emission in the A21-PSZ2 G114.9 system represent a first step towards a systematic study of these sources.