On the Structure of the Perseus Cluster of Galaxies

The investigation of the distribution of number densities and radial velocities in the Perseus cluster led us to the conclusion that this cluster possesses dominant substructures. This result is in agreement with a corresponding result for the Hydra-I cluster and with former conclusions of different authors that substructures are a typical phenomenon for a high percentage of all clusters of galaxies. The complex of the Perseus cluster has a diameter of 60–80 minutes of arc.     

Density and Radial Velocity Strucutre of the Hydra I Cluster of Galaxies

The investigation of the distributions of number densities, radial velocities and morphological types in the Hydra I cluster led us to the conclusion that this cluster possesses dominant substructures. This result is in agreement with former conclusions of different authors that substructures are a typical phenomenon for a high percentage of all clusters of galaxies. The galaxies inside the radius 60 from the cluster centre giv ea mean cluster redshift 3332±102 km/s and a velocity dispersion 679 ± 87 km/s.     

Chemical Enrichment of the Intra-Cluster Medium

We investigate the metal enrichment of the intra-cluster medium by using a method that combines N-Body simulations and a semi-analytic model (SAM) of galaxy formation. The cluster of galaxies is simulated in a flat, low density universe, with a numerical resolution that allows the detection of substructures in the dark matter background of the cluster. The phenomenological approach used to model the physical processes involved in the galaxy formation and metal production is applied to the substructures found in the dark matter halos detected at different redshifts. Details of the chemical implementation in the SAM and first results related to the mean properties of the baryonic matter components are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

A substructure analysis of the A3558 cluster complex

The 'algorithm driven by the density estimate for the identification of clusters' ( DEDICA ) is applied to the A3558 cluster complex in order to find substructures. This complex, located at the centre of the Shapley Concentration supercluster, is a chain formed by the ACO clusters A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 1329-313. We find a large number of clumps, indicating that strong dynamical processes are active. In particular, it is necessary to use a fully three-dimensional sample (i.e. using the galaxy velocity as third coordinate) in order also to recover the clumps superimposed along the line of sight. Even though a large number of detected substructures was already found in a previous analysis, this method is more efficient and faster when compared with a wide battery of tests, and permits the direct estimate of the detection significance. Almost all subclusters previously detected by the wavelet analyses found in the literature are recognized by DEDICA . On the basis of the substructure analysis, we also briefly discuss the origin of the A3558 complex by comparing two hypotheses: (i) the structure is a cluster–cluster collision seen just after the first core–core encounter; or (ii) this complex is the result of a series of incoherent group–group and cluster–group mergings, focused in that region by the presence of the surrounding supercluster. We studied the fraction of blue galaxies in the detected substructures and found that the bluest groups reside between A3562 and A3558, i.e. in the expected position for the scenario of cluster–cluster collision.     

Clumpy Substructures in Globular Cluster Tidal Tails

In this paper, we present some preliminary results of numerical simulations of a globular cluster orbiting in the inner region of a triaxial galaxy. The formation and evolution of tidal tails around the cluster is discussed, together with their morphology, which shows clumpy substructures as observed, for example, in the galactic globular cluster Palomar 5.     

Substructure recovery by three-dimensional discrete wavelet transforms

We present and discuss a method to identify substructures in combined angular-redshift samples of galaxies within clusters. The method relies on the use of the discrete wavelet transform (hereafter DWT) and has already been applied to the analysis of the Coma cluster. The main new ingredient of our method with respect to previous studies lies in the fact that we make use of a 3D data set rather than a 2D one. We test the method on mock cluster catalogues with spatially localized substructures and on a N -body simulation. Our main conclusion is that our method is able to identify the existing substructures provided that: (a) the subclumps are detached in part or all of the phase space, (b) one has a statistically significant number of redshifts, increasing as the distance decreases due to redshift distortions; (c) one knows a priori the scale on which substructures are to be expected. We have found that to allow an accurate recovery we must have both a significant number of galaxies (≈200 for clusters at z ≥0.4 or about 800 at z ≤0.4) and a limiting magnitude for completeness m _B =16.
The only true limitation to our method seems to be the necessity of knowing a priori the scale on which the substructure is to be found. This is an intrinsic drawback of the method and no improvement in numerical codes based on this technique could make up for it.     

Coma星系团的研究进展

Ｃｏｍａ闭是离我们最近的典型富星系闭之一。对此，人们积累了丰富的观测资料，并且进行了广泛深入的研究。在本文中着重介绍了有关Ｃｏｍａ闭的成员判定、结构、光度函数、动力学和次结构等方面的研究情况，并简单介绍了目前的最新研究动态。     

Resolving the Mass Discrepancy between Strong and Weak Lensing Methods

We readdress the outstanding cluster mass discrepancy between strong and weak gravitational lensing techniques utilizing updated data of both giant arcs and weak lensing measurements from the literature.We find that the systematically higher values of cluster masses revealed by strong lensing can be attributed to the oversimplification of the lensing model when estimating the cluster mass enclosed within the giant arcs.This arises because inhomogeneities and substructures in the central cores of clusters may invalidate the spherical symmetry assumption used widely in previous applications.When a more realistic modeling of the arcs is used,then the masses by strong lensing agree fairly well with those given by weak lensing when both are extrapolated to the same cluster regions.We conclude that as of now no significant discrepancy has been found among different cluster mass estimators including optical galaxies,X-ray gas and lensing.     

The mass and anisotropy profiles of galaxy clusters from the projected phase-space density: testing the method on simulated data

We present a new method of constraining the mass and velocity anisotropy profiles of galaxy clusters from kinematic data. The method is based on a model of the phase-space density, which allows the anisotropy to vary with radius between two asymptotic values. The characteristic scale of transition between these asymptotes is fixed and tuned to a typical anisotropy profile resulting from cosmological simulations. The model is parametrized by two values of anisotropy, at the centre of the cluster and at infinity, and two parameters of the NFW density profile, the scale radius and the scale mass. In order to test the performance of the method in reconstructing the true cluster parameters, we analyse mock kinematic data for 20 relaxed galaxy clusters generated from a cosmological simulation of the standard Λ cold dark matter model. We use Bayesian methods of inference and the analysis is carried out following the Markov Chain Monte Carlo approach. The parameters of the mass profile are reproduced quite well, but we note that the mass is typically underestimated by 15 per cent, probably due to the presence of small velocity substructures. The constraints on the anisotropy profile for a single cluster are in general barely conclusive. Although the central asymptotic value is determined accurately, the outer one is subject to significant systematic errors caused by substructures at large clustercentric distance. The anisotropy profile is much better constrained if one performs joint analysis of at least a few clusters. In this case, it is possible to reproduce the radial variation of the anisotropy over two decades in radius inside the virial sphere.     

Dynamical Substructures of Galaxy Cluster Abell 85

Abell 85 is a cD cluster of galaxies with the redshift of 0.055 in the southern hemisphere. Based on the spectroscopic data obtained by predecessors and the data of the SDSS (Sloan Digital Sky Survey), 370 member galaxies of the cluster are sieved by means of the 3σ method and their dynamical states are analyzed. From the spatial distribution and the local line-of-sight velocity distribution of these member galaxies it is found that this cluster of galaxies contains 4 clear substructures and they are just in the process of pairwise coalescence. This shows that the cluster Abell 85 is in the actively dynamical state, far from reaching the dynamical equilibrium.     

The cluster substructure–alignment connection

Using a sample of 903 APM clusters we investigate whether their dynamical status, as evidenced by the presence of significant substructures, is related to the large-scale structure of the Universe. We find that the cluster dynamical activity is strongly correlated with the tendency of clusters to be aligned with their nearest neighbour and in general with the nearby clusters that belong to the same supercluster. Furthermore, dynamically active clusters are more clustered than the overall cluster population. These are strong indications that clusters develop in a hierarchical fashion by anisotropic merging along the large-scale filaments within which they are embedded.     

The importance of interactions for mass loss from satellite galaxies in cold dark matter haloes

We investigate the importance of interactions between dark matter substructures for the mass loss they suffer whilst orbiting within a sample of high-resolution galaxy cluster mass cold dark matter (CDM) haloes formed in cosmological N -body simulations. We have defined a quantitative measure that gauges the degree to which interactions are responsible for mass loss from substructures. This measure indicates that interactions are more prominent in younger systems when compared to older more relaxed systems. We show that this is due to the increased number of encounters a satellite experiences and a higher mass fraction in satellites. This is in spite of the uniformity in the distributions of relative distances and velocities of encounters between substructures within the different host systems in our sample.
Using a simple model to relate the net force felt by a single satellite to the mass loss it suffers, we show that interactions with other satellites account for ∼30 per cent of the total mass loss experienced over its lifetime. The relation between the age of the host and the importance of interactions increases the scatter about this mean value from ∼25 per cent for the oldest to ∼45 per cent for the youngest system we have studied. We conclude that satellite interactions play a vital role in the evolution of substructure in dark matter haloes and that a significant fraction of the tidally stripped material can be attributed to these interactions.     

The dark matter environment of the Abell 901/902 supercluster: a weak lensing analysis of the HST STAGES survey

We present a high-resolution dark matter reconstruction of the   z = 0.165  Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901α. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'.     

Orbital parameters of infalling dark matter substructures

We present distributions of the orbital parameters of dark matter substructures at the time of merging into their host halo. Accurate knowledge of the orbits of dark matter substructures is a crucial input to studies which aim to assess the effects of the cluster environment on galaxies, the heating of galaxy discs and many other topics. Orbits are measured for satellites in a large number of N -body simulations. We focus on the distribution of radial and tangential velocities, but consider also distributions of orbital eccentricity and semimajor axis. We show that the distribution of radial and tangential velocities has a simple form and provide a fitting formula for this distribution. We also search for possible correlations between the infall directions of pairs of satellites, finding evidence for positive correlation at small angular separations as expected if some infall occurs along filaments. We also find (weak) evidence for correlations between the direction of the infall and infall velocity and the spin of the host halo.     

Substructure of the core of the perseus supercluster of galaxies

The hierarchical substructure of the core of the Perseus cluster of galaxies (A426, A262, A347, N507, and N383) is investigated using the method of S-tree diagrams. In the main system M (100 galaxies), two substructures AM (35 galaxies) and BM (13 galaxies) are determined. The group AM can be identified with the cluster A262 and the group N507; BM can be identified with the group N383; A426 and A347 are in the main system. The axis of symmetry of the system M passes through the Seyfert galaxy N1275 and is parallel to the axis of symmetry of the group AM, which is the core of increased density of the cluster of galaxies. Radial segregation of galaxies by morphological types is observed in the systems M and AM. The distribution of galaxies by position angles is uniform. Translated from Astrofizika, Vol. 41, No. 1, pp. 65–72, January-March, 1998.     

Tidal tails of star clusters

Based on recent findings of a formation mechanism of substructure in tidal tails by Küpper et al., we investigate a more comprehensive set of N -body models of star clusters on orbits about a Milky Way like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up.
The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case.
We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disc shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark matter substructures in the haloes of galaxies.     

Zw 1718.10108: a highly disturbed galaxy cluster at low Galactic latitude

We report the discovery of highly distorted X-ray emission associated with the nearby cluster Zw 1718.10108, one of the dominant members of which is the powerful radio galaxy 3C353. This cluster has been missed by previous X-ray cluster surveys because of its low Galactic latitude ( b =19.5°), despite its brightness in the hard X-ray band (210 keV flux of 1.210¹¹ erg cm² s¹). Our optical charge-coupled device image of the central part of the cluster reveals many member galaxies which are dimmed substantially by heavy Galactic extinction. We have measured redshifts of three bright galaxies near the X-ray emission peak and they are all found to be around z =0.028. The ASCA gas imaging spectrometer and ROSAT high-resolution imager images show three aligned X-ray clumps embedded in low surface-brightness X-ray emission extended by 30 arcmin. The averaged temperature measured with ASCA is kT =4.3±0.2 keV, which appears to be hot for the bolometric luminosity when compared with the temperatureluminosity correlation of galaxy clusters. The irregular X-ray morphology and evidence for a non-uniform temperature distribution suggest that the system is undergoing a merger of substructures. Since the sizes and luminosities of the individual clumps are consistent with those of galaxy groups, Zw 1718.10108 is interpreted as an on-going merger of galaxy groups in a dark matter halo forming a cluster of galaxies and thus is in a transition phase of cluster formation.     

Virgo星系团内次结构的特征尺度

在小波分析的基础上,用峰值计数法考察了Ｖｉｒｇｏ团天区的结构与次结构．在Ｖｉｒｇｏ团典型的双峰结构下面,发现还有１８０’和１２０’两个明显的次结构的特征尺度．该结果有力地支持了等级式的成团模式．与之对应的波长下的小波强度分布清楚地显示了Ｖｉｒｇｏ团丰富的次结构及其分布情况,从中还发现了两个新的较小的次结构。     

Characteristic scales of subclusters in the Virgo area

In this paper, we present a “peak number count method”, based on wavelet transformation, and use it to explore the structure and substructures in the Virgo area. It is found that, besides the two-humped structure, there are also quite a lot of small subclusters with characteristic scales of 180′ and 120′. This result strongly supports the hierarchical clustering model of structures. The wavelet coefficient map clearly shows these substructures and their distributions. Additionally, two new small subclusters are discovered.     

Survival and disruption of subsystems during a cold collapse

Cold collapse of a cluster composed of small identical clumps, each of which is in virial equilibrium, is considered. Since the clumps have no relative motion with respect to each other initially, the cluster collapses under its own gravity. At the first collapse of the cluster, most of the clumps are destroyed, but some survive. In order to find the condition for the clumps to survive, we made a systematic study in two-parameter space: the number of the clumps N _c and the size of the clump r _v . We obtained the condition N _c ≫ 1 and n _k  ≥ 1, where n _k is related to r _v and the initial radius of the cluster R _ini through the relation R _ini/ r _v  = 2 N ^{( n k +5)/6}_c. A simple analytical argument supports the numerical result. This n _k corresponds to the index of the power spectrum of the density fluctuation in the cosmological hierarchical clustering, and thus our result may suggest that in the systems smaller than 2/Ω h ²)Mpc, the first violent collapse is strong enough to sweep away all the substructures that exist before the collapse.