Merging Clusters Research Articles

On the Polarisation of Radio Relics

Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The origin of the relic polarisation is still debated. It could be a result of tangentially stretching the magnetic field at the shock surface. This scenario would naturally explain the alignment of the polarisation (E-vectors) with the shock normal. We have implemented a toy model for the relic polarisation according to this scenario. We find that the magnetic field strength itself crucially affects the fractional polarisation. Moreover, we find that the shock strength has surprisingly little effect on the overall polarisation fraction. Finally, we find that the fractional polarisation may decrease downstream depending on the magnetic field strength. Our results demonstrates that the shock compression scenario provides a very plausible explanation for the radio relic polarisation which specific features permitting to test the origin of radio relic polarisation.

Parallel Structure From Motion for UAV Images via Weighted Connected Dominating Set

Incremental Structure from Motion (ISfM) has been widely used for UAV image orientation. Its efficiency, however, decreases dramatically due to the sequential constraint. Although the divide-and-conquer strategy has been utilized for efficiency improvement, cluster merging becomes difficult or depends on seriously designed overlap structures. This paper proposes an algorithm to extract the global model for cluster merging and designs a parallel SfM solution to achieve efficient and accurate UAV image orientation. First, based on vocabulary tree retrieval, match pairs are selected to construct an undirected weighted match graph, whose edge weights are calculated by considering both the number and distribution of feature matches. Second, an algorithm, termed weighted connected dominating set (WCDS), is designed to achieve the simplification of the match graph and build the global model, which incorporates the edge weight in the graph node selection and enables the successful reconstruction of the global model. Third, the match graph is simultaneously divided into compact and non-overlapped clusters. After the parallel reconstruction, cluster merging is conducted with the aid of common 3D points between the global and cluster models. Finally, by using three UAV datasets that are captured by classical oblique and recent optimized views photogrammetry, the validation of the proposed solution is verified through comprehensive analysis and comparison. The experimental results demonstrate that the proposed parallel SfM can achieve 17.4 times efficiency improvement and comparative orientation accuracy. In absolute BA, the geo-referencing accuracy is approximately 2.0 and 3.0 times the GSD (Ground Sampling Distance) value in the horizontal and vertical directions, respectively. For parallel SfM, the proposed solution is a more reliable alternative.

Measuring the α-abundance of Subsolar-metallicity Stars in the Milky Way’s Central Half-parsec: Testing Globular Cluster and Dwarf Galaxy Infall Scenarios

Abstract While the Milky Way nuclear star cluster (MW NSC) has been studied extensively, how it formed is uncertain. Studies have shown it contains a solar and supersolar metallicity population that may have formed in situ, along with a subsolar-metallicity population that may have formed via mergers of globular clusters and dwarf galaxies. Stellar abundance measurements are critical to differentiate between formation scenarios. We present new measurements of [M/H] and α-element abundances [α/Fe] of two subsolar-metallicity stars in the Galactic center. These observations were taken with the adaptive-optics-assisted high-resolution (R = 24,000) spectrograph NIRSPEC in the K band (1.8–2.6 micron). These are the first α-element abundance measurements of subsolar-metallicity stars in the MW NSC. We measure [M/H] = − 0.59 ± 0.11, [α/Fe] = 0.05 ± 0.15 and [M/H] = − 0.81 ± 0.12, [α/Fe] = 0.15 ± 0.16 for the two stars at the Galactic center; the uncertainties are dominated by systematic uncertainties in the spectral templates. The stars have an [α/Fe] in between the [α/Fe] of globular clusters and dwarf galaxies at similar [M/H] values. Their abundances are very different than the bulk of the stars in the nuclear star cluster. These results indicate that the subsolar-metallicity population in the MW NSC likely originated from infalling dwarf galaxies or globular clusters and are unlikely to have formed in situ.

Customer age as a determinant of preferences for information sources: Empirical analysis of a case from Central Europe

The issue of finding effective approaches to marketing communications has been the subject of interest for the academic community as well as marketing managers for several decades. This study presents the results of an empirical analysis concerning the preferences of more than 5,000 Central Europeans aimed at identifying the customers´ preferred way of obtaining information depending on their age. The results of the analysis suggest that younger customers prefer online media, while middle-aged and older customers tend to prefer traditional information sources. The scientific contribution of the study is mainly in the results of cluster analysis, where we discover interesting mergers of customer clusters with a combination of preferentially similar age groups of customers. These newly discovered clusters can serve as a guide for more effective targeting of marketing communication to optimize the use of corporate resources and increase the overall competitiveness of companies operating in the Central European market.

The Unusually Weak and Exceptionally Steep Radio Relic in A2108

Abstract Mergers between galaxy clusters often drive shocks into the intracluster medium, the effects of which are sometimes visible via temperature and density jumps in the X-ray and via radio emission from relativistic particles energized by the shock’s passage. A2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev–Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow-up observations confirmed it to be a merging low-mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. Giant Metrewave Radio Telescope (GMRT) data in bands 2, 3, & 4 (covering 120–750 MHz) show an extended radio feature resembling a radio relic near the location of a temperature discontinuity in the X-rays. We measure a Mach number from the X-ray temperature jump (  X = 1.6 ± 0.2 ). Several characteristics of radio relics (location and morphology of extended radio emission) are found in A2108, making this cluster one of the few low-mass mergers (M L−M = 1.8 ± 0.4 × 1014 M ⊙) likely hosting a radio relic. The radio spectrum is exceptionally steep (α = −2 at the lowest frequencies), and the radio power is very weak (P 1.4 GHz = 1 × 1022 W Hz−1). To account for the shock/relic offset, we propose a scenario in which the shock created the relic by re-accelerating a cloud of pre-existing relativistic electrons and then moved away, leaving behind a fading relic. The electron-aging timescale derived from the high-frequency steepening in the relic spectrum is consistent with the shock travel time to the observed X-ray discontinuity. However, the lower flux in GMRT band 4 data causing the steepening could be due to instrumental limitations and deeper radio data are needed to constrain the spectral slope of the relic at high frequencies. A background cluster, 4′ from the merger, may have contributed to the ROSAT and Planck signals, but SZ modeling shows that the merger system is still X-ray underluminous, supporting the use of this approach to identifying merger-disrupted clusters.

Discovery of a Double Radio Relic in ZwCl1447.2+2619: A Rare Testbed for Shock-acceleration Models with a Peculiar Surface-brightness Ratio

Abstract We report a discovery of a double radio relic in the cluster merger ZwCl1447.2+2619 (z = 0.376) with uGMRT observations at 420 MHz and 700 MHz. The linear sizes of the northern and southern relics are ∼0.3 Mpc and ∼1.2 Mpc, respectively, which is consistent with the theoretical expectation that a larger relic is produced in the less massive subcluster side. However, ZwCl1447.2+2619 is unlike other known double radio relic systems, where the larger relics are much more luminous by several factors. In this merger, the higher surface brightness of the smaller northern relic makes its total radio luminosity comparable to that of the much larger southern relic. The surface brightness ratio ∼0.1 between the two radio relics differs significantly from the relation observed in other double radio relic systems. From our radio spectral analysis, we find that both relics signify similar weak shocks with Mach numbers of 2.9 ± 0.8 and 2.0 ± 0.7 for the northern and southern relics, respectively. Moreover, the northern relic is connected to a discrete radio source with an optical counterpart, which indicates the possible presence of cosmic-ray injection and reacceleration. Therefore, we propose that this atypical surface brightness ratio can be explained with the particle acceleration efficiency precipitously dropping in the weak shock regime and/or with reacceleration of fossil cosmic rays. Our multi-wavelength analysis and numerical simulation suggest that ZwCl1447.2+2619 is a postmerger, which has experienced a near head-on collision ∼0.7 Gyr ago.

Dynamic Cluster-Based Flow Management for Software Defined Networks

Software-defined network (SDN) allows traffic engineering and flow management by decoupling the control and data planes. SDN has attracted a lot of attention recently because of its centralized control, flow programmability and flexible resource management. However, to support centralized control, flow programmability and flexible resource management, most of the advanced SDN designs require per-flow management, which would easily make the flow table of a software switch overflow and incur undesired processing overhead when a network scales up. To overcome such huge traffic demands, we propose a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cluster-based macro-flow management</i> framework to balance the trade-off between customized services and management costs. The key idea of our design is to group similar flows into a cluster and replace per-flow management with per-cluster management. To achieve this goal, we cluster flows with consideration of the similarity of their traffic patterns. To adapt to varying traffic load, we further propose a hierarchical cluster merging scheme to dynamically merge clusters and make the best tradeoff between flow entry saving and routing performance guarantee. Our evaluation demonstrates that the proposed cluster-based flow management framework can significantly reduce the flow table usage, while producing minimum negative impact on routing performance.

A novel approach to select cluster head and optimising the data transmission in VANET

Communication between the vehicles has a major role in intelligent transportation system (ITS). Vehicular ad hoc network (VANET) is developed to improve the road safety. Data transmission between the vehicles has the major challenge in VANET, because VANET has high mobility and dynamic topology. In VANET, the cluster based topology used to send the data from one vehicle to many vehicles without any data loss. Cluster head (CH) plays a vital role during the data transmission in cluster based routing. CH manages the cluster formation and data transmission in each cluster. In this paper, we proposed a cluster based approach to improve the data transmission. In this system a novel approach is proposed for choosing the CH. The CH selection algorithm is done in three levels, first level normal CH selection, second level is cluster leaving and finally cluster merging. After choosing the CH, the routing path to be established based on the link quality which is a measure using the link life time. The proposed system is implemented using the SUMO and NS3 tools.

The hybrid ergodic lattice gas model for critical fluids and the molecular nature of the critical point

This paper introduces the hybrid ergodic lattice gas model as a conceptually simple, computationally cheap, yet accurate model that predicts macroscopic fluid pressure along the critical isotherm of a fluid accurately. A key concept is the hybrid evaluation of the properties: the kinetic and repulsive pressure components are calculated from equations of state, representing the ‘global’ nature of these values, whereas the attractive pressure component is calculated from the ‘local’ particle arrangement, in which particle clusters are identified using unsupervised machine learning. The proposed method captures the fluid pressure of the ideal gas regime and near the critical point, and is much more accurate then the Peng Robinson equation of state for liquid states. The predicted molecular structure is then used to identify three distinct stages of molecular behavior along the isotherm: I. cluster formation, II. cluster consolidation, and III. supercluster growth. Specifically, the critical point is shown to correspond to the state in II. where clusters merge together to form a single supercluster, corresponding to the percolation threshold of a cubic site lattice. The foundation of the model in a lattice gas opens the door for the analytical analysis of near-critical fluids using percolation theory and probabilistic analysis. As an example, a lattice-based derivation of a van der Waals attractive term is given. Finally, the lattice fluid is shown to be related to random networks, but deviates from the classical Erdős and Rényi’s G(N, L) model in that the critical point is found at a degree of 〈k〉 = 2 rather than the classical result of 〈k〉 = 1.

Star cluster formation from giant molecular clouds in the Small Magellanic Cloud about 2 Gyr ago: their origin, structures, and kinematics

ABSTRACT Recent observations have found that the age distribution of star clusters (SCs) in the Small Magellanic Cloud (SMC) shows a sharp peak around 2 Gyr ago. However, it is theoretically unclear what physical processes are responsible for such sudden formation of SCs in the SMC. Here, we investigate whether massive SCs with initial masses more than $10^5\, \text{M}_\odot$ can be formed during tidal interaction of the SMC with the Large Magellanic Cloud (LMC) about 2 Gyr ago, based on our new simulations, which include molecular hydrogen formation on dust grains and SC formation within giant molecular clouds (GMCs). We find that the formation of GMCs with masses more than $10^5\, \text{M}_\odot$ can be dramatically enhanced due to the tidal force of the LMC–SMC interaction. We also find that gravitationally bound massive SCs can be formed within these GMCs, though their mean stellar densities ($10^4\, \text{M}_\odot \,\text{pc}^{-3}$) are systematically lower than those of the genuine globular clusters (GCs). All simulated SCs have diffuse extended stellar envelopes that were formed from multiple merging of sub clusters within their natal GMCs. Furthermore, we find that some of the simulated SCs can have considerable global internal rotation and sub-structures surrounding them. Based on these simulation results, we discuss the origin of the observed diverse properties of SCs in the SMC and the physical roles of galaxy interaction in the formation of massive SCs from GMCs.

Optical View of a Merging Galaxy Cluster: Abell 2254

We present here the optical observations of a merging galaxy cluster (z=0.178) Abell 2254 (A2254) observed by the TÜBİTAK- RTT150 Telescope in Antalya, Turkey. Its integrated CCD imaging results are reported. B, V, and R brightness values are revealed for each member of the cluster, for the first time. We present the Color Magnitude Diagram of A2254 for all galaxies. We found that the number of red galaxies is more than blue galaxies. We have obtained the galaxy luminosity function (GLF) using the B band data. Here, we aim to analyze the effects of major cluster mergers on galaxy properties for better understanding. We have seen that the GLF of Abell 2254 can be fitted with a single Schechter function in the B band. The best-fitting parameters of the Schechter function are the characteristic absolute magnitude in the B band M_B^*=〖19,67〗_(-0,96)^(+0,84) mag, and the slope at the faint end of the GLF α=〖-1,29〗_(-0,34)^(+0,23) for galaxies.

Spectral Index of the Filaments in the Abell 523 Radio Halo

The galaxy cluster Abell 523 hosts a radio halo characterized by the presence of two filaments transversely located with respect to the cluster merger axis. In this paper, we present a spectral index image of these filaments between 1.410 and 1.782 GHz obtained with Jansky Very Large Array observations. We find a steepening of the spectral index of the filaments at frequencies ≳1.4 GHz and an indication that bright patches are characterized by flat spectral indices. Our results are consistent with a scenario of highly-efficient turbulence induced by merger phenomena.

RELICS: ICL Analysis of the z = 0.566 Merging Cluster WHL J013719.8–08284

Abstract We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the z = 0.566 cluster of galaxies WHL J013719.8–08284 observed by the RELICS project with the Hubble Space Telescope. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free-of-a-priori-assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between ∼6% and 19%, in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 Å and 4800 Å, previously identified as a signature of merging clusters at 0.18 &lt; z &lt; 0.55. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8–08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are ∼50% higher than optical ones, which could be signatures of an older and/or higher-metallicity ICL population that can be associated with the buildup of the brightest cluster galaxy, passive evolution of previously injected young stars, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8–08284 fulfills the expected conditions for a fossil system progenitor.

NGC 1605a and NGC 1605b: An Old Binary Open Cluster in the Galaxy

Abstract This work communicates the discovery of a binary open cluster within the Galaxy. NGC 1605 presents an unusual morphology with a sparse stellar distribution and a double core in close angular proximity. The 2MASS and Gaia-EDR3 field-star decontaminated color–magnitude diagrams (CMDs) show two distinct stellar populations located at the same heliocentric distance of ∼2.6 kpc, suggesting that there are two clusters in the region, NGC 1605a and NGC 1605b, with ages of 2 Gyr and 600 Myr, respectively. Both Gaia parallax and PM distributions are compact and very similar indicating that they are open clusters (OCs) and share the same kinematics. The large age difference, 1.4 Gyr, point to a formation by tidal capture during a close encounter and the close spatial proximity and similar kinematics suggest an ongoing merger event. There is some prominent tidal debris that appears to trace the cluster's orbits during the close encounter and, unexpectedly, some of them appear to be bound structures; this may suggest that in addition to the evaporation, the merging clusters are being broken apart into smaller structures by the combination of the Galactic disk, the Perseus arm, and mutual tidal interactions. In this sense, the newly found binary cluster may be a key object in the observational validation of theoretical studies on binary cluster pairs formation by tidal capture as well as in the formation of massive clusters by merging, and tidal disruption of stellar systems.

Unveiling the internal structure of the Hercules supercluster

ABSTRACT We have investigated the structure of the Hercules supercluster (SCL160) based on data originally extracted from the Sloan Digital Sky Survey (SDSS-DR7). We have traced the mass distribution in the field through the numerical density, weighted by the r′ luminosity of the galaxies, and classified objects based on their spatial position and redshift. This has allowed us to address not only the kinematics of the supercluster as a whole, but also the internal kinematics of each cluster, which had not been explored further before. We have confirmed that the Hercules supercluster is composed of the galaxy clusters A2147, A2151, and A2152. A2151 consists of five subclusters, A2147 two, and A2152 at least two. They form the heart of the Hercules supercluster. We also have found two other gravitationally bound clusters, therefore increasing the known members of the supercluster. We have estimated a total mass of 2.1 ± 0.2 × 1015 M⊙ for the Hercules supercluster. To determine the dynamical masses in this work, we have resorted to the M200−σ scaling relation and the caustic technique. Comparing both methods with simulated data of bimodal merging clusters, we found that the caustic, as well as the σ-based masses, is biased through the merger age, showing a boost just after pericentric passage. This is not in line with the principle of the caustic method, which affirms it does not depend on the cluster dynamical state.

A Multiwavelength Dynamical State Analysis of ACT-CL J0019.6+0336

In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts a radio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination of XMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopic information, and 1.16 GHz MeerKAT data to study the cluster properties. Various X-ray and optical morphology parameters are calculated to investigate the level of disturbance. We find disturbances in two X-ray parameters and the optical density map shows elongated and axisymmetric structures with the main cluster component southeast of the cluster centre and another component northwest of the cluster centre. We also find a BCG offset of ∼950 km/s from the mean velocity of the cluster, and a discrepancy between the SZ mass, X-ray mass, and dynamical mass (MX,500 and MSZ,500 lies &gt;3σ away from Mdyn,500), showing that J0019 is a merging cluster and probably in a post-merging phase.

SMART (splitting-merging assisted reliable) Independent Component Analysis for Brain Functional Networks.

Independent component analysis (ICA) has been widely applied to estimate brain functional networks from functional magnetic resonance imaging (fMRI) data. ICA is a data-driven approach, however, the number of components must be prespecified. Indeed, it is difficult to estimate or determine an optimal number of components in fMRI analysis. In this paper, we propose a SMART (splitting-merging assisted reliable) ICA to overcome the problem. Our method first estimates group-level components using different settings and then yields reliable components by using a splitting and merging clustering approach. Subject-specific components are obtained using our previously proposed group information guided ICA (GIG-ICA) based on reliable group-level components to estimate individual-subject independent components. Simulations with unique components for subjects showed our method extracted components with high similarity to the ground truth spatial maps (SMs). For real fMRI data, the functional networks extracted by our method showed both similarity and specificity across subjects. To sum up, our method can effectively and accurately identify subject-specific brain functional networks without a need of parameter setting.Clinical Relevance- SMART ICA automatically extracts reliable subject-specific brain functional networks that can be used for biomarker identification.

Constraining merging galaxy clusters with X-ray and lensing simulations and observations: the case of Abell 2146

ABSTRACT 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 idealized simulations of binary cluster mergers performed using the gamer-2 code. As a test case, we focus on modelling the Bullet Cluster-like merging system Abell 2146, in which deep 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 Chandra and lensing observations. We show that due to gravitational compression of the cluster haloes 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° from the plane of the sky. We discuss the applicability of our results to multiwavelength observations of merging galaxy clusters and their use as probes of cosmology and plasma physics.

Pinning effects in a two-dimensional cluster glass

We study numerically the glass formation and depinning transition of a system of two-dimensional cluster-forming monodisperse particles in presence of pinning disorder. The pairwise interaction potential is nonmonotonic, and is motivated by the intervortex forces in type-$1.5$ superconductors. Such systems can form cluster glasses due to the intervortex interactions following a thermal quench, without underlying disorder. We study the effects of vortex pinning in these systems. We find that a small density of pinning centers of moderate depth has limited effect on vortex glass formation, i.e., formation of vortex glasses is dominated by intervortex interactions. At higher densities pinning can significantly affect glass formation. The cluster glass depinning, under a constant driving force, is found to be plastic, with features distinct from non-cluster-forming systems such as clusters merging and breaking. We find that in general vortices with cluster-forming interaction forces can exhibit stronger pinning effects than regular vortices.

1321+045: A compact steep‐spectrum radio source in a cool‐core galaxy cluster

AbstractCluster‐central gigahertz peak and compact steep‐spectrum (CSS) sources offer an opportunity to study the earliest phases of active galactic nucleus (AGN) feedback, but few have yet been examined in detail. We present results from radio and X‐ray observations of 1321+045, a CSS source in a 4.4 keV cluster at . The cluster has a strongly cooling core, and disturbances from a minor cluster merger may have triggered a period of jet activity which formed the 16 kpc radio lobes ago. However, new very long baseline array (VLBA) imaging shows a ∼20 pc jet on a different projected axis, which is probably only a few hundred years old. We consider possible histories for the system, with either one or two periods of jet activity. While this single system is informative, a broader study of the youngest cluster‐central radio sources is desirable.