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Register a new userExemplary Merging Clusters: Weak-lensing and X-ray Analysis of the Double Radio Relic Merging Galaxy Clusters MACS 1752.0+4440 and ZWCL 1856.8+6616
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The investigation of radio relic merging galaxy clusters is strengthening our understanding of the formation and evolution of galaxy clusters, the nature of dark matter, the intracluster medium, and astrophysical particle acceleration. Each merging cluster provides only a single view of the cluster formation process and the variety of merging clusters is vast. Clusters hosting double radio relics are rare and extremely important because they allow tight constraints on the merger scenario. We present a weak-lensing and X-ray analysis of MACSJ1752.0+4440 (z=0.365) and ZWCL1856.8+6616 (z=0.304), two double radio relic merging galaxy clusters. Our weak-lensing mass estimates show that each cluster is a major merger with approximately 1:1 mass ratio. The total mass of MACSJ1752.0+4440 (ZWCL1856.8+6616) is $M_{200}=14.7^{+3.8}_{-3.3}times10^{14} $M$_odot$ ($M_{200}=2.4^{+0.9}_{-0.7}times10^{14} $M$_odot$). We find that these two clusters have comparable features in their weak-lensing and gas distributions, even though the systems have vastly different total masses. From the likeness of the X-ray morphologies and the remarkable symmetry of the radio relics, we propose that both systems underwent nearly head-on collisions. However, revelations from the hot-gas features and our multiwavelength data analysis suggest that ZWCL1856.8+6618 is likely at a later merger phase than MACSJ1752.0+4440. We postulate that the SW radio relic in MACSJ1752.0+4440 is a result of particle re-acceleration.
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Galaxy cluster mergers are a powerful laboratory for testing cosmological and astrophysical models. However, interpreting individual merging clusters depends crucially on their merger configuration, defined by the masses, velocities, impact parameters, and orientation of the merger axis with respect to the plane of the sky. In this work, we investigate the impact of merger parameters on the X-ray emitting intracluster medium and gravitational lensing maps using a suite of idealised simulations of binary cluster mergers performed using the GAMER-2 code. As a test case, we focus on modeling the Bullet Cluster-like merging system Abell 2146, in which deep textit{Chandra} X-ray and lensing observations revealed prominent merger shocks as well as the mass distribution and substructures associated with this merging cluster. We identify the most interesting parameter combinations, and evaluate the effects of various parameters on the properties of merger shocks observed by deep textit{Chandra} and lensing observations. We show that due gravitational compression of the cluster halos during the merger, previous mass estimates from weak lensing are too high. The plane of the merger is tilted further from the plane of the sky than estimated previously, up to $30^circ$ from the plane of the sky. We discuss the applicability of our results to multi-wavelength observations of merging galaxy clusters and their use as probes of cosmology and plasma physics.
The galaxy cluster ZwCl 2341.1+0000 is a merging system at z=0.27, which hosts two radio relics and a central, faint, filamentary radio structure. The two radio relics have unusually flat integrated spectral indices of -0.49 +/- 0.18 and -0.76 +/- 0.17, values that cannot be easily reconciled with the theory of standard diffusive shock acceleration of thermal particles at weak merger shocks. We present imaging results from XMM-Newton and Chandra observations of the cluster, aimed to detect and characterise density discontinuities in the ICM. As expected, we detect a density discontinuity near each of the radio relics. However, if these discontinuities are the shock fronts that fuelled the radio emission, then their Mach numbers are surprisingly low, both <=2. We studied the aperture of the density discontinuities, and found that while the NW discontinuity spans the whole length of the NW radio relic, the arc spanned by the SE discontinuity is shorter than the arc spanned by the SE relic. This startling result is in apparent contradiction with our current understanding of the origin of radio relics. Deeper X-ray data are required to confirm our results and to determine the nature of the density discontinuities.
We present XMM-Newton/EPIC observations of six merging galaxy clusters and study the distributions of their temperature, iron (Fe) abundance and pseudo-entropy along the merging axis. For the first time, we focus simultaneously, and in a comprehensive way, on the chemical and thermodynamic properties of the freshly collided intracluster medium (ICM). The Fe distribution of these clusters along the merging axis is found to be in good agreement with the azimuthally-averaged Fe abundance profile in typical non-cool-core clusters out to $r_{500}$. In addition to showing a moderate central abundance peak, though less pronounced than in relaxed systems, the Fe abundance flattens at large radii towards $sim$0.2-0.3 $Z_odot$. Although this shallow metal distribution is in line with the idea that disturbed, non-cool-core clusters originate from the merging of relaxed, cool-core clusters, we find that in some cases, remnants of metal-rich and low entropy cool cores can persist after major mergers. While we obtain a mild anti-correlation between the Fe abundance and the pseudo-entropy in the (lower entropy, $K$ = 200-500 keV cm$^2$) inner regions, no clear correlation is found at (higher entropy, $K$ = 500-2300 keV cm$^2$) outer radii. The apparent spatial abundance uniformity that we find at large radii is difficult to explain through an efficient mixing of freshly injected metals, particularly in systems for which the time since the merger is short. Instead, our results provide important additional evidence in favour of the early enrichment scenario - in which the bulk of the metals are released outside galaxies at $z$ > 2-3 - and extend it from cool-core and (moderate) non-cool-core clusters to a few of the most disturbed merging clusters as well. These results constitute a first step towards a deeper understanding of the chemical history of merging clusters.
Galaxy clusters, the most massive collapsed structures, have been routinely used to determine cosmological parameters. When using clusters for cosmology, the crucial assumption is that they are relaxed. However, subarcminute resolution Sunyaev-Zeldovich (SZ) effect images compared with high resolution X-ray images of some clusters show significant offsets between the two peaks. We have carried out self-consistent N-body/hydrodynamical simulations of merging galaxy clusters using FLASH to study these offsets quantitatively. We have found that significant displacements result between the SZ and X-ray peaks for large relative velocities for all masses used in our simulations as long as the impact parameters were about 100-250 kpc. Our results suggest that the SZ peak coincides with the peak in the pressure times the line-of-sight characteristic length and not the pressure maximum (as it would for clusters in equilibrium). The peak in the X-ray emission, as expected, coincides with the density maximum of the main cluster. As a consequence, the morphology of the SZ signal and therefore the offset between the SZ and X-ray peaks change with viewing angle. As an application, we compare the morphologies of our simulated images to observed SZ and X-ray images and mass surface densities derived from weak lensing observations of the merging galaxy cluster CL0152-1357. We find that a large relative velocity of 4800 km/s is necessary to explain these observations. We conclude that an analysis of the morphologies of multi-frequency observations of merging clusters can be used to put meaningful constraints on the initial parameters of the progenitors.
Multi-band photometric and multi-object spectroscopic surveys of merging galaxy clusters allow for the characterization of the distributions of constituent dark matter and galaxy populations, constraints on the dynamics of the merging subclusters, and an understanding of galaxy evolution of member galaxies. We present deep photometric observations from Subaru/SuprimeCam and a catalog of $sim$5400 spectroscopic cluster members from Keck/DEIMOS across 29 merging galaxy clusters ranging in redshift from $z=0.07$ to $0.55$. The ensemble is compiled based on the presence of radio relics, which highlight cluster scale collisionless shocks in the intra-cluster medium. Together with the spectroscopic and photometric information, the velocities, timescales, and geometries of the respective merging events may be tightly constrained. In this preliminary analysis, the velocity distributions of 28 of the 29 clusters are shown to be well fit by single Gaussians. This indicates that radio relic mergers largely occur transverse to the line of sight and/or near apocenter. In this paper, we present our optical and spectroscopic surveys, preliminary results, and a discussion of the value of radio relic mergers for developing accurate dynamical models of each system.
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