Showing papers on "Cluster (physics) published in 2011"

Conceptualizing Cluster Evolution: Beyond the Life Cycle Model?

Abstract: Martin R. and Sunley P. Conceptualizing cluster evolution: beyond the life cycle model?, Regional Studies. Although the literature on the evolution of industrial clusters is not vast, a preferred approach has already become evident based around the idea of a cluster ‘life cycle’. This approach has several limitations. This paper explores a different conception of cluster evolution, drawing on the ‘adaptive cycle’ model that has been developed in evolutionary ecology. Using this model, cluster evolution is viewed as an adaptive process with different possible outcomes based on episodic interactions of nested systems. Though not without limitations, this approach offers greater scope as a framework for shaping the research agenda into the evolution of clusters. Martin R. and Sunley P. 对集群演进进行概念化：生命周期模型之外的新解读？区域研究。尽管目前关于产业集群演进的相关研究有限，围绕集群生命周期这一概念分析方法展开研究已成为明显的趋势。然而这一方法存在诸多制约。本文探讨了关于集群演进的不同概念，着重于演进生态学中的适应性管理模式。采用这一模式，集群演进被看作是适应性的程序，其结果基于嵌套系统之间阶段性的互动。尽管没有局限，这一方法为集群演进的研究搭建了更为广阔的分析框架及研究议程。 集群 演进 生命周期模型 复杂系统 适应性周...

Baryons at the edge of the x-ray–brightest galaxy cluster

Abstract: Studies of the diffuse x-ray–emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and insights into plasma astrophysics. However, measurements of the faint cluster outskirts have become possible only recently. Using data from the Suzaku x-ray telescope, we determined an accurate, spatially resolved census of the gas, metals, and dark matter out to the edge of the Perseus Cluster. Contrary to previous results, our measurements of the cluster baryon fraction are consistent with the expected universal value at half of the virial radius. The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy distribution of the gas, which is important for understanding the ongoing growth of clusters from the surrounding cosmic web.

A 48‐Metal Cluster Exhibiting a Large Magnetocaloric Effect

Abstract: NNSFC[20825103, 20901064, 90922031, 2007CB815304, 21021061]; Fundamental Research Funds for the Central Universities[2010121016]

Cluster-based inorganic-organic hybrid materials.

Abstract: Clusters as building blocks have been used for two types of inorganic–organic hybrid materials. The first are hybrid polymers, with polymer-like properties and structures, where the cluster units crosslink the polymer chains. They are prepared by co-polymerization of organic monomers with functional ligands attached to the clusters. The second type is crystalline metal–organic framework structures which are obtained by coordination chemistry approaches, i.e. by coordinating multifunctional organic ligands to cluster units. This tutorial review shows that both types of cluster-based materials are limiting cases with many options for varying both the cluster units as well as the connecting organic entities.

On the Cluster Physics of Sunyaev-Zel'dovich Surveys I: The Influence of Feedback, Non-thermal Pressure and Cluster Shapes on Y-M Scaling Relations

Abstract: The utility of large Sunyaev Zel'dovich (SZ) surveys for determining cosmological parameters from cluster abundances is limited by the theoretical uncertainties in the integrated SZ-flux-to-mass relation, Y-M. We explore how non-thermal pressure and the anisotropic shape of the gas distribution of the intracluster medium (ICM) impacts Y-M scaling using a suite of SPH simulations of the cosmic web. We contrast results for models with different treatments of entropy injection and transport, varying radiative cooling, star formation and accompanying supernova feedback, cosmic rays, and energetic feedback from active galactic nuclei (AGN). We find that the gas kinetic-to-thermal pressure ratio from internal bulk motions depends on the cluster mass, and increases in the outer-cluster due to enhanced substructure, as does the asphericity of the ICM gas. With only a ~5-10% correction to projected (observable) ellipticities, we can infer the 3D ellipticities. Our simulated Y-M-slope roughly follows the self-similar prediction, except for a steepening due to a deficit of gas in lower mass clusters at low redshift in our AGN-feedback simulations. AGN feedback enhances slightly the overall Y-M-scatter, from ~11% to ~13%, a reflection of accretion history variations due to cluster merging. If we split the cluster system into lower, middle and upper bands of both P_kin/P_th and long-to-short axis ratio, we find a ~10% effect on Y-M. Identifying observable second parameters related to internal bulk flows and anisotropy for cluster-selection to minimize Y-M scatter in a "fundamental plane" would allow tighter cosmological parameter constraints.

Massive and refined - II. The statistical properties of turbulent motions in massive galaxy clusters with high spatial resolution

Abstract: We study the properties of chaotic motions in the intra cluster medium using a set of 20 galaxy clusters simulated with large dynamical range, using the adaptive mesh refinement code ENZO. The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achieving an maximum available Reynolds number of the order of Re ∼ 500−1000 for the largest eddies. We investigated the correlations between the energy of these motions in the intra cluster medium and the dynamical state of the host systems. We find that the statistical properties of turbulent motions and their evolution with time imply that major merger events are responsible for the injection of the bulk of turbulent kinetic energy into the cluster. Turbulence is found to account for ∼20−30 per cent of the thermal energy in merging clusters, and ∼5 per cent in relaxed clusters. We compare the energies of turbulence and motions in our simulated clusters with upper-limits for real nearby clusters derived from XMM-Newton data. When turbulent motions are compared on the same spatial scales, the data from simulations are well within the range presently allowed by observations. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formation of giant radio halos in turbulent (merging) clusters. On the basis of our simulations, we confirm the conclusions of previous semi-analytical studies that the fraction of turbulent clusters appears to be consistent with that of clusters hosting radio halos.

The life cycle of star clusters in a tidal field

Abstract: The evolution of globular clusters due to 2-body relaxation results in an outward flow of energy and at some stage all clusters need a central energy source to sustain their evolution. Henon provided the insight that we do not need to know the details of the energy production in order to understand the relaxation-driven evolution of the cluster, at least outside the core. He provided two self-similar solutions for the evolution of clusters based on the view that the cluster as a whole determines the amount of energy that is produced in the core: steady expansion for isolated clusters, and homologous contraction for clusters evaporating in a tidal field. We combine these models: the half-mass radius increases during the first half of the evolution, and decreases in the second half; while the escape rate approaches a constant value set by the tidal field. We refer to these phases as `expansion dominated' and `evaporation dominated'. These simple analytical solutions immediately allow us to construct evolutionary tracks and isochrones in terms of cluster half-mass density, cluster mass and galacto-centric radius. From a comparison to the Milky Way globular clusters we find that roughly 1/3 of them are in the second, evaporation-dominated phase and for these clusters the density inside the half-mass radius varies with the galactocentric distance R as rho_h ~ 1/R^2. The remaining 2/3 are still in the first, expansion-dominated phase and their isochrones follow the environment-independent scaling rho_h ~ M^2; that is, a constant relaxation time-scale. We find substantial agreement between Milky Way globular cluster parameters and the isochrones, which suggests that there is, as Henon suggested, a balance between the flow of energy and the central energy production for almost all globular clusters.

Catalytic Activities of Subnanometer Gold Clusters (Au16–Au18, Au20, and Au27–Au35) for CO Oxidation

Abstract: Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer gold clusters. Particular attention is placed on 12 different clusters in the size range of Au16–Au35, whose atomic structures in the anionic state have been resolved from previous experiments. Adsorption energies of a single CO or O2 molecule as well as coadsorption energies of both CO and O2 molecules on various distinctive surface sites of each anionic cluster and their neutral counterpart are computed. In general, the anionic clusters can adsorb CO and O2 more strongly than their neutral counterparts. The coadsorption energies of both CO and O2 molecules decrease as the size of gold clusters increases with the exception of Au34 (an electronic “magic-number” cluster). Besides the known factor of low coordination site, we find that a relatively small cone angle (<110°) associated with each surface site is another key geometric factor that can enhance the binding strength of CO a...

Pilot applications of internally contracted multireference coupled cluster theory, and how to choose the cluster operator properly.

Abstract: The internally contracted multireference coupled cluster (icMRCC) method allows a highly accurate description of both static and dynamic correlation with a computational scaling similar to single reference coupled cluster theory. The authors show that the method can lose its orbital invariance and size consistency when no special care is taken in the elimination of redundant excitations. Using the BeH2 model system, four schemes are compared which differ in their treatment of linear dependencies between excitations of different rank (such as between singles and doubles). While the energy curves agree within tens of μEh when truncating the cluster operator at double excitations (icMRCCSD), inclusion of triple excitations (icMRCCSDT) leads to significant differences of more than 1 mEh. One scheme clearly yields the best results, while the others even turn out to be not size consistent. The former procedure uses genuine single and double excitations and discards those linear combinations of (spectator) doubl...

The life cycle of star cluster in a tidal field

Abstract: The evolution of globular clusters due to 2-body relaxation results in an outward flow of energy and at some stage all clusters need a central energy source to sustain their evolution. Henon provided the insight that we do not need to know the details of the energy production in order to understand the relaxation-driven evolution of the cluster, at least outside the core. He provided two self-similar solutions for the evolution of clusters based on the view that the cluster as a whole determines the amount of energy that is produced in the core: steady expansion for isolated clusters, and homologous contraction for clusters evaporating in a tidal field. We combine these models: the half-mass radius increases during the first half of the evolution, and decreases in the second half; while the escape rate approaches a constant value set by the tidal field. We refer to these phases as `expansion dominated' and `evaporation dominated'. These simple analytical solutions immediately allow us to construct evolutionary tracks and isochrones in terms of cluster half-mass density, cluster mass and galacto-centric radius. From a comparison to the Milky Way globular clusters we find that roughly 1/3 of them are in the second, evaporation-dominated phase and for these clusters the density inside the half-mass radius varies with the galactocentric distance R as rho_h ~ 1/R^2. The remaining 2/3 are still in the first, expansion-dominated phase and their isochrones follow the environment-independent scaling rho_h ~ M^2; that is, a constant relaxation time-scale. We find substantial agreement between Milky Way globular cluster parameters and the isochrones, which suggests that there is, as Henon suggested, a balance between the flow of energy and the central energy production for almost all globular clusters.

The XMM Cluster Survey: Optical analysis methodology and the first data release

Abstract: The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all publicly available data in the XMM-Newton Science Archive. Its main aims are to measure cosmological parameters and trace the evolution of X-ray scaling relations. In this paper we present the first data release from the XMM Cluster Survey (XCS-DR1). This consists of 503 optically confirmed, serendipitously detected, X-ray clusters. Of these clusters, 255 are new to the literature and 356 are new X-ray discoveries. We present 464 clusters with a redshift estimate (0.06 1.0, including a new spectroscopically-confirmed cluster at z = 1.01); (ii) 67 clusters with high Tx (> 5 keV); (iii) 131 clusters/groups with low Tx (< 2 keV); (iv) 27 clusters with measured Tx values in the SDSS `Stripe 82' co-add region; (v) 78 clusters with measured Tx values in the Dark Energy Survey region; (vi) 40 clusters detected with sufficient counts to permit mass measurements (under the assumption of hydrostatic equilibrium); (vii) 105 clusters that can be used for applications such as the derivation of cosmological parameters and the measurement of cluster scaling relations. The X-ray analysis methodology used to construct and analyse the XCS-DR1 cluster sample has been presented in a companion paper, Lloyd-Davies et al. (2010).

A fifteen atom silver cluster confined in bovine serum albumin

Abstract: Luminescent Ag15 clusters confined in bovine serum albumin (BSA) have been prepared by a simple wet chemical route. The luminescence, exhibiting a maximum at 685 nm, is observable to the naked eye. The chemical composition of these clusters was analyzed using matrix assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and energy dispersive analysis of X-rays (EDAX). Intact Ag15@BSA is observed by MALDI MS. Multiple charge states of the cluster are observed confirming the mass assignment. The clusters showed a quantum yield of 10.71% in water and the luminescence was stable in a pH range of 1–12. Stability of the clusters was enhanced by the addition of polyvinylpyrrolidone (PVP). The clusters showed luminescence in the solid state as well. Evolution of clusters with variation in the amount of reducing agent added shows that the cluster formation goes through an intermediate state of bound silver, formed instantaneously after the addition of Ag+, which transforms to the cluster. High yield synthesis and exciting photophysical properties make our new material interesting for various applications such as biolabeling and imaging.

Mass segregation and fractal substructure in young massive clusters – I. The McLuster code and method calibration

Abstract: By analysing models of the young massive cluster R136 in 30 Doradus, set-up using the herewith introduced and publicly made available code MCLUSTER, we investigate and compare different methods for detecting and quantifying mass segregation and substructure in non-seeing limited N-body data. For this purpose we generate star cluster models with different degrees of mass segregation and fractal substructure and analyse them.

Modeling the formation and evolution of star cluster populations in galaxy simulations

Abstract: (Abridged) The formation and evolution of star cluster populations are related to the galactic environment. Cluster formation is governed by processes acting on galactic scales, and star cluster disruption is driven by the tidal field. In this paper, we present a self-consistent model for the formation and evolution of star cluster populations, for which we combine an N-body/SPH galaxy evolution code with semi-analytic models for star cluster evolution. The model includes star formation, feedback, stellar evolution, and star cluster disruption by two-body relaxation and tidal shocks. We apply the model by simulating a suite of 9 isolated disc galaxies and 24 galaxy mergers. The evolutionary histories of individual clusters in these simulations are discussed to illustrate how the environment of clusters changes in time and space. The resulting variability of the disruption rate with time and space affects the properties of star cluster populations. The combined effect of clusters escaping their dense formation sites (`cluster migration') and the preferential disruption of clusters residing in dense environments (`natural selection') implies that the mean disruption rate of the population decreases with cluster age. This affects the slope of the cluster age distribution, which becomes a function of the star formation rate density. The evolutionary histories of clusters in a galaxy merger vary widely and determine which clusters survive the merger. This impacts the age distributions and the locations of the surviving clusters at all times during a merger. We conclude that accounting for the interplay between the formation, disruption, and orbital histories of clusters enables a more sophisticated interpretation of observed properties of cluster populations, thereby extending the role of cluster populations as tracers of galaxy evolution.

Interaction energies of large clusters from many-body expansion

Abstract: In the canonical supermolecular approach, calculations of interaction energies for molecular clusters involve a calculation of the whole cluster, which becomes expensive as the cluster size increases. We propose a novel approach to this task by demonstrating that interaction energies of such clusters can be constructed from those of small subclusters with a much lower computational cost by applying progressively lower-level methods for subsequent terms in the many-body expansion. The efficiency of such “stratified approximation” many-body approach (SAMBA) is due to the rapid convergence of the many-body expansion for typical molecular clusters. The method has been applied to water clusters (H2O)n, n = 6, 16, 24. For the hexamer, the best results that can be obtained with current computational resources in the canonical supermolecular method were reproduced to within about one tenth of the uncertainty of the canonical approach while using 24 times less computer time in the many-body expansion calculations....

Discovery of a Dissociative Galaxy Cluster Merger with Large Physical Separation

Abstract: We present DLSCL J0916.2+2951 (z=0.53), a newly discovered major cluster merger in which the collisional cluster gas has become dissociated from the collisionless galaxies and dark matter. We identified the cluster using optical and weak lensing observations as part of the Deep Lens Survey. Our follow-up observations with Keck, Subaru, Hubble Space Telescope, and Chandra show that the cluster is a dissociative merger and constrain the dark matter self-interaction cross-section {\sigma}_{DM}m_{DM}^{-1}\leq7 cm^2g^{-1}. The system is observed at least 0.7\pm0.2 Gyr since first pass-through, thus providing a picture of cluster mergers 2-5 times further progressed than similar systems observed to date. This improved temporal leverage has implications for our understanding of merging clusters and their impact on galaxy evolution.

Evolution of Shocks and Turbulence in Major Cluster Mergers

Abstract: We performed a set of cosmological simulations of major mergers in galaxy clusters, in order to study the evolution of merger shocks and the subsequent injection of turbulence in the post-shock region and in the intra-cluster medium (ICM). The computations have been performed with the grid-based, adaptive mesh refinement hydrodynamical code Enzo, using a refinement criterion especially designed for refining turbulent flows in the vicinity of shocks. When a major merger event occurs, a substantial amount of turbulence energy is injected in the ICM of the newly formed cluster. Our simulations show that the shock launched after a major merger develops an ellipsoidal shape and gets broken by the interaction with the filamentary cosmic web around the merging cluster. The size of the post-shock region along the direction of shock propagation is of the order of 300 kpc h{sup -1}, and the turbulent velocity dispersion in this region is larger than 100 km s{sup -1}. We performed a scaling analysis of the turbulence energy within our cluster sample. The best fit for the scaling of the turbulence energy with the cluster mass is consistent with M{sup 5/3}, which is also the scaling law for the thermal energy in the self-similarmore » cluster model. This clearly indicates the close relation between virialization and injection of turbulence in the cluster evolution. As for the turbulence in the cluster core, we found that within 2 Gyr after the major merger (the timescale for the shock propagation in the ICM), the ratio of the turbulent to total pressure is larger than 10%, and after about 4 Gyr it is still larger than 5%, a typical value for nearly relaxed clusters. Turbulence at the cluster center is thus sustained for several gigayears, which is substantially longer than typically assumed in the turbulent re-acceleration models, invoked to explain the statistics of observed radio halos. Striking similarities in the morphology and other physical parameters between our simulations and the 'symmetrical radio relics' found at the periphery of the merging cluster A3376 are finally discussed. In particular, the interaction between the merger shock and the filaments surrounding the cluster could explain the presence of 'notch-like' features at the edges of the double relics.« less

Generation of small gold clusters with unique geometries through cluster-to-cluster transformations: octanuclear clusters with edge-sharing gold tetrahedron motifs

Abstract: Molecular gold clusters with defined nuclearity and geometrical structures have attracted continuing interest, due not only to the fundamental aspects of their unique nuclearityand structure-dependent optical and electronic properties, but also to their potential in the development of novel nanomaterials and catalysts. Phosphine-coordinated small gold clusters (Aun) with core nuclearities (n) ranging from 6 to 13, which form one of the major cluster families, are typical examples. X-ray crystallographic studies have shown that these clusters generally adopt toroidal or sphere-like structures due to prominent aurophilic interactions involving the central gold atom. Examples of linear edge-shared arrays of simple polyhedra have been limited to date, but recent theoretical studies have predicted that such core structures can be formed when coupled with appropriate surrounding shell (ligand) modules. In this relation, current developments in cluster synthetic methods have shown the utility of multidentate ligands for selective formation of particular geometrical structures. Herein, we report that the use of a diphosphine ligand in a cluster-to-cluster transformation through a growth/etching process leads to facile generation of two novel cluster cations [Au8(dppp)4Cl2] 2+ (2) and [Au8(dppp)4] 2+ (4 ; dppp = 1,3-bis(diphenylphosphino)propane). Structural studies of these clusters revealed their unprecedented Au8 core geometries [18] containing edge-fused gold tetrahedron motifs, which are isomeric with each other. We also highlight their geometry-dependent visible absorption and emission properties, and the selective optical response of 2 towards mercury ions. The starting material for the growth-based strategy was [Au6(dppp)4](NO3)2 (1 (NO3)2), [16] whose core contains a tetrahedral Au4 unit plus two gold atoms bridged at opposite edges of the tetrahedron. Each of the gold atoms forming the central tetrahedron is coordinated by a single phosphine ligand, while the exo gold atoms accommodate two phosphine ligands (Figure 1). We found that growth of 1 took place readily through a reaction with the mononuclear chlorogold(I) complex [Au(PPh3)Cl], affording [Au8(dppp)4Cl2] 2+

How can SMEs in a cluster respond to global demands for corporate responsibility

Abstract: This article argues why and how a participatory approach to implement corporate social responsibility (CSR) in a cluster would be beneficial for small- and medium-sized enterprises (SMEs) who are members of the NCE – Subsea cluster in Bergen, Norway. The political and strategic reasons as well as internal motivation for SMEs to incorporate CSR into their business strategies are discussed with support from relevant literature. Furthermore, we offer a discussion on the characteristics of different approaches to incorporating CSR as part of business strategy and provide examples from the Norwegian context. As part of this discussion, we look into some of the clusters in Europe which have adopted a systematic approach to implement CSR as part of their agenda. We propose a means to undertake CSR as part of the cluster through a network model to address the limitations faced by the SMEs when they try to implement CSR individually. A network model (cluster approach) to CSR will motivate the uptake of CSR among SMEs when the network (cluster) is characterized by close geographical proximity and operates in the same sector. The uptake of CSR as part of the network (cluster) agenda can also lead to innovation through cooperation and competition. The particular challenges faced by the SMEs in implementing and sustaining CSR can be also minimized by being part of a network (cluster).

Quantum Clusters in Cavities: Trapped Au15 in Cyclodextrins

Abstract: We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime c...

A parameter space exploration of galaxy cluster mergers. i. gas mixing and the generation of cluster entropy

Abstract: We present a high-resolution set of adiabatic binary galaxy cluster merger simulations using FLASH. These are the highest resolution simulations to date of such mergers using an adaptive mesh refinement grid-based code with Eulerian hydrodynamics. In this first paper in a series, we investigate the effects of merging on the entropy of the hot intracluster gas, specifically with regard to the ability of merging to heat and disrupt cluster "cool cores." We find, in line with recent works, that the effect of fluid instabilities that are well resolved in grid-based codes is to significantly mix the gases of the two clusters and to significantly increase the entropy of the gas of the final merger remnant. This result is characteristic of mergers over a range of initial mass ratio and impact parameter. In line with this, we find that the kinetic energy associated with random motions is higher in our merger remnants which have high-entropy floors, indicating that the motions have efficiently mixed the gas and heated the cluster core with gas of initially high entropy. We examine the implications of this result for the maintenance of high-entropy floors in the centers of galaxy clusters and the derivation of the properties of dark matter from the thermal properties of the X-ray-emitting gas.

Density Functional Study on the Morphology and Photoabsorption of CdSe Nanoclusters

Abstract: The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)(x), x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of Uganda. This suggests that our structures are realistic models of the actual quantum dots.

G0.253+0.016: a molecular cloud progenitor of an Arches-like cluster

Abstract: Young massive clusters (YMCs) with stellar masses of 10^4 - 10^5 Msun and core stellar densities of 10^4 - 10^5 stars per cubic pc are thought to be the `missing link' between open clusters and extreme extragalactic super star clusters and globular clusters. As such, studying the initial conditions of YMCs offers an opportunity to test cluster formation models across the full cluster mass range. G0.253+0.016 is an excellent candidate YMC progenitor. We make use of existing multi-wavelength data including recently available far-IR continuum (Herschel/Hi-GAL) and mm spectral line (HOPS and MALT90) data and present new, deep, multiple-filter, near-IR (VLT/NACO) observations to study G0.253+0.016. These data show G0.253+0.016 is a high mass (1.3x10^5 Msun), low temperature (T_dust~20K), high volume and column density (n ~ 8x10^4 cm^-3; N_{H_2} ~ 4x10^23 cm^-2) molecular clump which is close to virial equilibrium (M_dust ~ M_virial) so is likely to be gravitationally-bound. It is almost devoid of star formation and, thus, has exactly the properties expected for the initial conditions of a clump that may form an Arches-like massive cluster. We compare the properties of G0.253+0.016 to typical Galactic cluster-forming molecular clumps and find it is extreme, and possibly unique in the Galaxy. This uniqueness makes detailed studies of G0.253+0.016 extremely important for testing massive cluster formation models.

Cosmological Simulations of Galaxy Clusters

Abstract: We review recent progress in the description of the formation and evolution of galaxy clusters in a cosmological context by using state-of-art numerical simulations. We focus our presentation on the comparison between simulated and observed X-ray properties, while we will also discuss numerical predictions on properties of the galaxy population in clusters, as observed in the optical band. Many of the salient observed properties of clusters, such as scaling relations between X-ray observables and total mass, radial profiles of entropy and density of the intracluster gas, and radial distribution of galaxies are reproduced quite well. In particular, the outer regions of cluster at radii beyond about 10 per cent of the virial radius are quite regular and exhibit scaling with mass remarkably close to that expected in the simplest case in which only the action of gravity determines the evolution of the intra-cluster gas. However, simulations generally fail at reproducing the observed “cool core” structure of clusters: simulated clusters generally exhibit a significant excess of gas cooling in their central regions, which causes both an overestimate of the star formation in the cluster centers and incorrect temperature and entropy profiles. The total baryon fraction in clusters is below the mean universal value, by an amount which depends on the cluster-centric distance and the physics included in the simulations, with interesting tensions between observed stellar and gas fractions in clusters and predictions of simulations. Besides their important implications for the cosmological application of clusters, these puzzles also point towards the important role played by additional physical processes, beyond those already included in the simulations. We review the role played by these processes, along with the difficulty for their implementation, and discuss the outlook for the future progress in numerical modeling of clusters.

SMBH Formation via Gas Accretion in Nuclear Stellar Clusters

Abstract: Black holes exceeding a billion solar masses have been detected at redshifts greater than six. The rapid formation of these objects may suggest a massive early seed or a period of growth faster than Eddington. Here we suggest a new mechanism along these lines. We propose that in the process of hierarchical structure assembly, dense star clusters can be contracted on dynamical time scales due to the nearly free-fall inflow of self-gravitating gas with a mass comparable to or larger than that of the clusters. This increases the velocity dispersion to the point that the few remaining hard binaries can no longer effectively heat the cluster, and the cluster goes into a period of homologous core collapse. The cluster core can then reach a central density high enough for fast mergers of stellar-mass black holes and hence the rapid production of a black hole seed that could be $10^5 M_\odot$ or larger.