Galaxy cluster masses without non-baryonic dark matter

We apply the modified acceleration law obtained from Einstein gravity coupled to a massive skew symmetric field,   F _μνλ  , to the problem of explaining X-ray galaxy cluster masses without exotic dark matter. Utilizing X-ray observations to fit the gas mass profile and temperature profile of the hot intracluster medium (ICM) with King 'β-models', we show that the dynamical masses of the galaxy clusters resulting from our modified acceleration law fit the cluster gas masses for our sample of 106 clusters without the need of introducing a non-baryonic dark matter component. We are further able to show for our sample of 106 clusters that the distribution of gas in the ICM as a function of radial distance is well fitted by the dynamical mass distribution arising from our modified acceleration law without any additional dark matter component. In a previous work, we applied this theory to galaxy rotation curves and demonstrated good fits to our sample of 101 low surface brightness, high surface brightness and dwarf galaxies including 58 galaxies that were fitted photometrically with the single-parameter mass-to-light ratio ( M / L )_stars. The results obtained there were qualitatively similar to those obtained using Milgrom's phenomenological Modified Newtonian Dynamics (MOND) model, although the determined galaxy masses were quantitatively different, and MOND does not show a return to Keplerian behaviour at extragalactic distances. The results obtained here are compared to those obtained using Milgrom's phenomenological MOND model which does not fit the X-ray galaxy cluster masses unless an auxiliary dark matter component is included.     

Tracing the nature of dark energy with galaxy distribution

Dynamical dark energy (DE) is a viable alternative to the cosmological constant. Constructing tests to discriminate between Λ and dynamical DE models is difficult, however, because the differences are not large. In this paper we explore tests based on the galaxy mass function, the void probability function (VPF), and the number of galaxy clusters. At high z , the number density of clusters shows large differences between DE models, but geometrical factors reduce the differences substantially. We find that detecting a model dependence in the cluster redshift distribution is a significant challenge. We show that the galaxy redshift distribution is potentially a more sensitive characteristic. We do this by populating dark matter haloes in N -body simulations with galaxies using well-tested halo occupation distributions. We also estimate the VPF and find that samples with the same angular surface density of galaxies, in different models, exhibition almost model-independent VPF which therefore cannot be used as a test for DE. Once again, geometry and cosmic evolution compensate each other. By comparing VPFs for samples with fixed galaxy mass limits, we find measurable differences.     

Fluctuations of non-point mass galaxy distribution

We examine cosmic energy equation for extended galaxy structures on the basis of different models of universe. We also extend the power spectrum and density fluctuations for extended structure by introducing softening parameter both for linear and non-linear regimes. The results are compared with earlier results of point mass structures. It is found that softening parameters introduced in the theory influence the thermodynamic fluctuation theory. Results obtained with spectrum analysis are also compared with Riemannian geometric approach (Ruppeiner in Rev. Mod. Phys. 67:605, 1995) to the galaxy clustering. The singular solutions of thermodynamic fluctuation results can be interpreted on the basis of power spectrum analysis in terms of power index law of two point correlation function.     

The Connection between Radio Halos and Cluster Mergers and the Statistical Properties of the Radio Halo Population

We discuss the statistical properties of the radio halo population in galaxy clusters. Radio bi-modality is observed in galaxy clusters: a fraction of clusters host giant radio halos while a majority of clusters do not show evidence of diffuse cluster-scale radio emission. The radio bi-modality has a correspondence in terms of dynamical state of the hosting clusters showing that merging clusters host radio halos and follow the well-known radio-X-ray correlation, while more relaxed clusters do not host radio halos and populate a region well separated from that correlation. These evidences can be understood in the framework of a scenario where merger-driven turbulence re-accelerate the radio emitting electrons. We discuss the main statistical expectations of this scenario underlining the important role of the upcoming LOFAR surveys to test present models.     

Galaxies in clusters: the observational characteristics of bow shocks, wakes and tails

The dynamical signatures of the interaction between galaxies in clusters and the intracluster medium (ICM) can potentially yield significant information about the structure and dynamical history of clusters. To develop our understanding of this phenomenon we present results from numerical modelling of the galaxy–ICM interaction, as the galaxy moves through the cluster. The simulations have been performed for a broad range of ICM temperatures ( kT _cl=1, 4 and 8 keV), representative of poor clusters or groups through to rich clusters.
There are several dynamical features that can be identified in these simulations. For supersonic galaxy motion, a leading bow shock is present, and also a weak gravitationally focused wake or tail behind the galaxy (analogous to Bondi–Hoyle accretion). For galaxies with higher mass replenishment rates and a denser interstellar medium (ISM), the dominant feature is a dense ram-pressure stripped tail. In line with other simulations, we find that the ICM/galaxy–ISM interaction can result in complex time-dependent dynamics, with ram-pressure stripping occurring in an episodic manner.
In order to facilitate this comparison between the observational consequences of dynamical studies and X-ray observations we have calculated synthetic X-ray flux and hardness maps from these simulations. These calculations predict that the ram-pressure stripped tail will usually be the most visible feature, though in nearby galaxies the bow shock preceding the galaxy should also be apparent in deeper X-ray observations. We briefly discuss these results and compare them with X-ray observations of galaxies where there is evidence of such interactions.     

Nonexistence of static conformally-flat solutions in self-creation cosmology

It is shown that the merger times of a satellite galaxy moving on the periphery of the primary galaxy obtained under the impulsive and adiabatic approximations reduce to the decay time predicted by Chandrasekhar's formula for dynamical friction if the satellite galaxy is regarded as a mass point.     

Searching for the scale of homogeneity

We introduce a statistical quantity, known as the K function, related to the integral of the two-point correlation function. It gives us straightforward information about the scale where clustering dominates and the scale at which homogeneity is reached. We evaluate the correlation dimension, D ₂, as the local slope of the log–log plot of the K function. We apply this statistic to several stochastic point fields, to three numerical simulations describing the distribution of clusters and finally to real galaxy redshift surveys. Four different galaxy catalogues have been analysed using this technique: the Center for Astrophysics I, the Perseus–Pisces redshift surveys (these two lying in our local neighbourhood), the Stromlo–APM and the 1.2-Jy IRAS redshift surveys (these two encompassing a larger volume). In all cases, this cumulant quantity shows the fingerprint of the transition to homogeneity. The reliability of the estimates is clearly demonstrated by the results from controllable point sets, such as the segment Cox processes. In the cluster distribution models, as well as in the real galaxy catalogues, we never see long plateaus when plotting D ₂ as a function of the scale, leaving no hope for unbounded fractal distributions.     

A VLA 20 and 90 centimetre radio survey of distant A-bell clusters with central cD galaxies

A radio survey, using the Very Large Array at 20 and 90 cm λ has been carried out in the direction of 46 distant Abell clusters (0.1 ≲ z ≲ 0.3) dominated by a cD galaxy (clusters classified to be Bautz-Morgan I type). A radio source coincident with the cD galaxy was detected in 16 clusters. We find that the radio luminosity function of the cD galaxies at 20cm λ, and below the luminosityP _1.4ghz ≲ 10^24.5 W Hz^-1, is similar to that of brightest ellipticals in less clustered environments. Above this luminosity, the cDs seem to have a higher probability of becoming radio sources. The effect of optical brightness on radio emission is shown to be the same for the two classes. No significantly large population of very-steep-spectrum sources with spectral index α >1.2 (flux density ∝ frequency^-α) was found to be associated with cD galaxies. A significant negative correlation is found between the radio luminosity of the cD galaxy and the cooling-time of the intra cluster medium near the galaxy. We also present evidence that the probability of radio emission from first-ranked galaxies is dependent upon their location relative to the geometrical centres of clusters and thus related to the morphological class and the evolutionary state of the clusters. We argue that both these effects are primarily caused by the dynamical evolution of these distant clusters of galaxies.     

Counting of galaxy clusters with the S-Z effect of triaxial ellipsoidal model

In the near future a large number of galaxy clusters will be observed in surveys based on the Sunyaev-Zel'dovich effect (SZE) . Both the spherical and ellipsoidal models of the dark matter of galaxy clusters are adopted to make theoretical predictions of the number of observable S-Z galaxy clusters, and to asses the effect of the cluster shape on the calculation. By means of the virial equilibrium condition the mass lower limit of the observable galaxy clusters is determined from given survey flux lower limit and the prediction of the number count of galaxy clusters is carried out by the mass function. The calculated results show that if the cosmological parameters are to be constrained by the S-Z surveys, then the uncertainty in the shape of the clusters should be taken into account.     

Mass estimation in the outer regions of galaxy clusters

We present a technique for estimating the mass in the outskirts of galaxy clusters where the usual assumption of dynamical equilibrium is not valid. The method assumes that clusters form through hierarchical clustering and requires only galaxy redshifts and positions on the sky. We apply the method to dissipationless cosmological N -body simulations where galaxies form and evolve according to semi-analytic modelling. The method recovers the actual cluster mass profile within a factor of 2 to several megaparsecs from the cluster centre. This error originates from projection effects, sparse sampling, and contamination by foreground and background galaxies. In the absence of velocity biases, this method can provide an estimate of the mass-to-light ratio on scales ∼1–10  h ⁻¹ Mpc where this quantity is still poorly known.     

A dynamical condition for a relativistic galaxy cluster model

In an attempt to give a coherent interpretation of the secondary maximum in the density distribution of clusters of galaxies we use an approximate metric tensor proposed by other authors, with the purpose of building a relativistic generalization of the isothermal models of galaxy clusters.Although such a generalization gives rise to oscillations in the density distribution, the quantitative agreement with the observational data is unsatisfactory.The analysis of the metric tensor we have used brings us to point out that (i) the approximation on which the metric is based is not suitable for describing an actual galaxy cluster and (ii) the dynamical conditions of clusters require inclusion of a cosmological expansion, and of anisotropic distribution function in the phase-space.     

The baryon mass distribution for nearby galaxy clusters

Based on ROSAT X-ray data, we constructed the baryon mass function for a statistically complete sample of galaxy clusters at redshifts of 0.01–0.1. Since the derived function is similar to the total mass function for clusters, it can be used to determine cosmological parameters.     

星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。     

Role of Gas Dynamical Friction in the Evolution of Embedded Stellar Clusters

Two puzzles associated with open clusters have attracted a lot of attention – their formation, with densities and velocity dispersions that are not too different from those of the star forming regions in the galaxy, given that the observed Star Formation Efficiencies (SFE) are low and, the mass segregation observed/inferred in some of them, at ages significantly less than the dynamical relaxation times in them. Gas dynamical friction has been considered before as a mechanism for contracting embedded stellar clusters, by dissipating their energy. This would locally raise the SFE which might then allow bound clusters to form. Noticing that dynamical friction is inherently capable of producing mass segregation, since here, the dissipation rate is proportional to the mass of the body experiencing the force, we explore further, some of the details and implications of such a scenario, vis-à-vis observations. Making analytical approximations, we obtain a boundary value for the density of a star forming clump of a given mass, such that, stellar clusters born in clumps which have densities higher than this, could emerge bound after gas loss. For a clump of given mass and density, we find a critical mass such that, sub-condensations with larger masses than this could suffer significant segregation within the clump.     

Correlation between baryon mass and intergalactic gas temperature in nearby galaxy clusters

We measured the correlation between baryon mass and intracluster gas temperature in nearby galaxy clusters selected from the ROSAT All-Sky Survey. The mass of the intracluster gas was determined directly from an analysis of X-ray images. A correlation was found between the gas mass and the mass of the cluster stellar matter, which was used to determine the total baryon mass (i.e., gas + stars). The mass was measured within the radii corresponding to overdensities of 324 and 500 relative to the mean baryon density inferred from the theory of primordial nucleosynthesis. The measured correlation between baryon mass and temperature is close to that predicted by a self-similar theory of cluster formation: M ∝ T ^3/2.     

Stellar Relics from the Early Galaxy

We reviewed the recent progress in the field of stellar/galactic archeology, which is a study of the relics from the early galaxy. The oldest and most pristine objects that can be observed in the galaxy are the low mass metal poor stars of the Milky Way. They were formed during the early phases, when the ISM might have been polluted only by the Pop-III supernovae. With the recent large spectroscopic surveys (e.g. HK survey by Beers and collaborators, the Hamburg-ESO survey by Christlieb and collaborators and Sloan Digital Sky Survey) it has been possible to get clues on the nature of the first stars that has contributed to the heavy elements. Most of these metal-poor low mass stars also retain their signature of the early dynamical evolution of the galaxy, which can be studied through their orbits around the galaxy and spatial distribution. Here, we discuss the connection between the chemical and the kinematical properties of metal-poor stars in order to probe the early galaxy formation. We also discuss about the globular clusters, the satellite galaxies around the Milky Way and its possible contribution to the formation of the galaxy halo.     

'Galaxy associations' as possible common features of galaxy clusters

The results are here presented of an analysis of the subclustering in a sample of galaxy clusters from the European Southern Observatory (ESO) Nearby Abell Cluster Survey (ENACS), along with complementary data from other studies. The analysis is performed by using the S-tree method, enabling us to study the hierarchical properties of clusters by the detection of the main physical cluster and of its subgroups. The results indicate (a) systematically lower genuine cluster velocity dispersions than were known from previous studies, and (b) the existence of 2–3 subgroups in each cluster. As a result of certain properties of a new class of dynamical entities, we denote these subgroups as galaxy associations ; these may become an essential challenge for the formation mechanisms of galaxy clusters.     

Integral field spectroscopy with GEMINI: Extragalactic star cluster in NGC1275

Studies of globular cluster systems play a critical role in our understanding of galaxy formation. Imaging with the Hubble Space Telescope has revealed that young star clusters are formed copiously in galaxy mergers, strengthening theories in which giant elliptical galaxies are formed by the merger of spirals [e.g. Whitmore, B.C., Schweizer, F., Leitherer, C., Borne, K., Robert, C., 1993. Astronomical Journal. 106, 1354; Miller, B.W., Whitmore, B.C., Schweizer, F., Fall, S.M., 1997. Astronomical Journal. 114, 2381; Zepf, S.E., Ashman, K.M., English, J., Freeman, K.C., Sharples, R.M., 1999. Astronomical Journal. 118, 752; Ashman, K.M., Zepf, S.E., 1992. Astrophysical Journal. 384, 50]. However, the formation and evolution of globular cluster systems is still not well understood. Ages and metallicities of the clusters are uncertain either because of degeneracy in the broad-band colors or due to variable reddening. Also, the luminosity function of the young clusters, which depends critically on the metallicities and ages of the clusters, appears to be single power-laws while the luminosity function of old clusters has a well-defined break. Either there is significant dynamical evolution of the cluster systems or metallicity affects the mass function of forming clusters. Spectroscopy of these clusters are needed to improve the metallicity and age measurements and to study the kinematics of young cluster systems. Therefore, we have obtained GMOS IFU data of 4 clusters in NGC1275. We will present preliminary results like metallicities, ages, and velocities of the star clusters from IFU spectroscopy.     

The clustering of colour-selected galaxies

We present measurements of the angular correlation function of galaxies selected from a B _J ∼23.5 multicolour survey of two 5°×5° fields located at high galactic latitudes. The galaxy catalogue of ∼4×10⁵ galaxies is comparable in size to catalogues used to determine the galaxy correlation function at low redshift. Measurements of the z ∼0.4 correlation function at large angular scales show no evidence for a break from a power law, although our results are not inconsistent with a break at ≳15 h⁻¹ Mpc. Despite the large fields-of-view, there are large discrepancies between the measurements of the correlation function in each field, possibly caused by dwarf galaxies within z ∼0.11 clusters near the South Galactic Pole.
Colour selection is used to study the clustering of galaxies from z ∼0 to z ∼0.4. The galaxy correlation function is found to depend strongly on colour, with red galaxies more strongly clustered than blue galaxies by a factor of ≳5 at small scales. The slope of the correlation function is also found to vary with colour, with γ∼1.8 for red galaxies and γ∼1.5 for blue galaxies. The clustering of red galaxies is consistently strong over the entire magnitude range studied, although there are large variations between the two fields. The clustering of blue galaxies is extremely weak over the observed magnitude range, with clustering consistent with r ₀∼2 h⁻¹ Mpc. This is weaker than the clustering of late-type galaxies in the local Universe, and suggests that galaxy clustering is more strongly correlated with colour than morphology. This may also be the first detection of a substantial low-redshift galaxy population with clustering properties similar to faint blue galaxies.     

基于Shapelets基函数的引力透镜质量重构方法

引力透镜效应是探测星系团物质分布的有效方法之一.目前,利用引力透镜数据重构星系团质量分布的主流方法可以分为两大类,即参数法和非参数法.在实际研究工作中,受限于质量模型假设和计算分辨率等方面的影响,现有的重构算法仍有诸多亟需解决的问题.基于Shapelets基函数的引力透镜质量重构方法通过基函数来实现引力透镜质量重构,使用Shapelets基函数分解引力透镜势,以引力透镜中多重像的位置和背景星系椭率畸变为限制条件来迭代求解基函数系数从而得到透镜体的质量分布.通过拟合一个模拟的NFW (Navarro,Frenk and White)透镜系统测试了新方法的可行性,结果表明新方法可以在整体上重构出透镜体的质量分布,并拟合出接近真实的源位置,能够为星系团质量测量提供一套灵活且高效的重构算法.