Determination of Galaxy Members in Clusters

A new empirical procedure is introduced to determine the confirmed galaxy members of a cluster. The method depends on both the projected distances of galaxies in the cluster field from the cluster centre and their radial velocities. Galaxies of the main body of the cluster are selected first, then the method works iteratively by increasing the standard values of the relative radial distance and velocity until all galaxies belong to the cluster are included. The general apparent shape of the cluster will result from the distribution of the celestial coordinates of the cluster members. This revised version was published online in July 2006 with corrections to the Cover Date.     

Capture of stars by rotating homogeneous spherical clusters

Changes of the orbit of a star passing through a homogeneous spherical cluster have been estimated. Before entering the cluster the star is supposed to move in a Keplerian parabolic orbit. When passing through the cluster the star is subject both to the attraction of the smoothed-out distribution of matter in the cluster and to dynamical friction. Due to dynamical friction the energy of the star becomes negative which leads to the elliptic-type motion of the star after leaving the cluster and to capturing the star by the cluster. The formulae for the changes of the star orbit in the cluster are given. Numerical estimation shows that open clusters transform star orbits more noticeably than do globular clusters.     

Discussions of the internal motions and mass segregation of M67 on the basis of accurate proper motions

The radius and virial mass of the old open cluster M67 are presented. The internal motion and mass segregation of the cluster are also discussed on the basis of accurate stellar proper motions obtained combining three independent proper motion catalogues of the cluster. Increases of the mean proper motion and the intrinsic dispersion of member stars with radial distance from the cluster center might suggest that the stars are escaping from the cluster. The stars in both inner and outer regions appear to be in isotropic orbits. At last, it is found that both space and velocity mass segregations exist for the old open cluster due to the dynamical evolution.     

三轴椭球模型下的SZ效应星系团计数

在不久的将来，若干基于SZ效应的巡天计划即将观测到大量的星系团．分别采用星系团暗物质的球模型和椭球模型，给出了可观测SZ星系团数目的理论预测，并讨论了形状对星系团数目不确定性的影响．在计算中，利用星系团维里平衡的条件，通过巡天观测的流量下限来确定可观测星系团质量下限，进一步由质量函数给出星系团数目预测．计算结果表明：若通过SZ巡天观测来限定宇宙学参数，形状引起的系统不确定性要予以考虑．     

疏散星团NGC 6530的光度函数和分层效应

该文利用疏散星团NGC 6530的自行观测数据和成员判别结果,给出了星团的光度函数,并详细讨论了星团的质量分层效应。分析表明,NGC 6530存在空间质量分层效应,但并不存在速度质量分层,因此星团的表观空间质量分层可能起因于星团形成时的初始条件,而不是动力学弛豫的结果。     

Discovery of high-velocity EHB stars in the globular cluster ωCentauri(NGC 5139)

We report the discovery of 45 high-velocity extreme horizontal branch(EHB) stars in the globular cluster Omega Centauri(NGC 5139).The tangential velocities of these EHB stars are determined to be in the range 93~313 km s~(-1),with an average uncertainty of ~27 km s~(-1).The central escape velocity of the cluster is determined to be in the range 60~105 km s~(-1).These EHB stars are significantly more concentrated toward the cluster core compared with other cluster members.The formation mechanisms of these EHB stars are discussed.Our conclusions can be summarized as follows:(1) A comparison of the tangential velocities of these EHB stars to the centra]escape velocity of the cluster shows that most if not all of these EHB stars are unbound to the cluster;(2) These EHB stars obtained high velocities in the central cluster region no longer than ~1 Myr ago and may be subsequently ejected from the cluster in the next ~1 Myr;(3) If the progenitors of these EHB stars were single stars,then they may have experienced a fast mass-loss process.If the progenitors were in close binaries,then they may have formed through disruptions by the intermediate-mass black hole in the cluster center.     

疏散星团的最新研究进展

疏散星团在天文学和天体物理学研究中有着多方面的重要性。近年来在观测和理论工作上都取得了显著的进展。综述了疏散星团的研究现状，对成员判别、基本参数确定、团与恒星的演化、团的结构和动力学进行了详细评述。     

Formation of massive galaxies at the centre of a cluster

The formation of massive galaxies at the centre of a cluster is discussed here. The protogalaxies move with both rotation and random velocities through the gaseous medium pervading the cluster. Each galaxy is supposed to move through a resisting medium under the general gravitational field produced by the cluster as a whole. Also, the mass of the galaxy increases by accretion all the time as it moves through the medium. Using plausible laws for density of the medium and accretion of matter and solving equation of motion, we find that the galaxy loses angular momentum in the course of the time. The loss of angular momentum drives the galaxy towards the centre of the cluster. Thus over a sufficiently long time-scale several galaxies may merge in the central region of the cluster resulting in a single massive galaxy. The process can drive rise to several massive galaxies in the central region of the cluster.     

The mass content in the region of the cluster NGC 1750

Kinematical information has been used to solve the Jeans’ equation to provide a mass estimate for the open cluster NGC 1750. Using published data we have also applied the Michie–King model to estimate the mass in the cluster. The comparison of these masses gives the best estimation of mass in the region of this cluster as 1.01×10⁴ M_⊙, which indicates that the cluster may be a massive cluster. Checks of the density distribution and isotropy of the stellar velocity distribution are given.     

Velocity dispersion estimates of APM galaxy clusters

We present 83 new galaxy radial velocities in the field of 18 APM clusters with redshifts between 0.06 and 0.13. The clusters have Abell identifications and the galaxies were selected within 0.75  h ⁻¹ Mpc in projection from their centres. We derive new cluster velocity dispersions for 13 clusters using our data and published radial velocities.
We analyse correlations between cluster velocity dispersions and cluster richness counts as defined in Abell and APM catalogues. The correlations show a statistically significant trend although with a large scatter, suggesting that richness is a poor estimator of cluster mass irrespectively of cluster selection criteria and richness definition. We find systematically lower velocity dispersions in the sample of Abell clusters that do not fulfil APM cluster selection criteria, suggesting artificially higher Abell richness counts owing to contamination by projection effects in this subsample.     

Revisiting the open cluster Lo 807

We have conducted an astrometric study of the open cluster Lo 807 using proper motion data from UCAC2 catalog. We applied the maximum likelihood method to determine cluster membership by using the proper motion data. The Padova isochrones lines with solar metallicity were fitted upon the Color Magnitude Diagram in near-infrared to derive physical parameters of the cluster. We do not detect significant variable reddening within the cluster. The present result is compared with our previous study.     

Influence of cluster environments on X-ray cluster morphology

We investigated the influence of environment on cluster morphology by examining the alignment of the cluster position angle with respect to the nearest neighbour cluster. The cluster position angle and ellipticity were measured using high spatial resolution X-ray data taken from the Chandra archive, while the nearest neighbour sample was extracted from the Abell cluster catalogue. We found high confidence for an alignment when neighbour distance (Dn) was less than  70  h ⁻¹  Mpc, while we found no significant variations of the confidence level at various values of Dn less than  70  h ⁻¹  Mpc.     

Age determination of the open cluster NGC 2287 (M41)

We have estimated the age of the open cluster NGC 2287 (M41) by taking into account the rotation of the member stars.TheUBV photoelectric measurement values were taken from Eggen (1974). Besides the Main-Sequence stars this cluster has seven red giants, all of them above the turn-off point.Having obtained the theoretical Hertzsprung-Russell diagram from observed colour-magnitude diagram, we have discussed two methods, given in the literature, to estimate the age of the cluster under consideration. The first method we have followed is to determine the turn-off point on the cluster Main-Sequence. The other is fitting the position of the red giants with the evolutionary tracks of rotating stars with different masses on the Hertzsprung-Russell diagram.We have estimated the cluster age based on the turn-off point as 3.10×10⁷ yr. From the fitting of the evolutionary tracks of rotating stars to the cluster Hertzsprung-Russell diagram, we infer that the age of the cluster is about 8.00×10⁷ yr.     

A2111: a z=0.23 Butcher–Oemler cluster with a non-isothermal atmosphere and normal metallicity

We report results from an X-ray study of the Abell 2111 galaxy cluster using the Advanced Satellite for Astrophysics and Cosmology ( ASCA ) and the ROSAT Position Sensitive Proportional Counter (PSPC). By correcting for the energy-dependent point-spread function of the ASCA instruments, we have examined the temperature structure of the cluster. The core of the cluster within 3 arcmin is found to have a temperature of 6.46±0.87 keV, significantly higher than the value of 3.10±1.19 keV in the surrounding region of r =3–6 arcmin. This radially decreasing temperature structure can be parametrized by a polytropic index of γ ≃ 1.45. The X-ray morphology of the cluster appears elongated and clumpy on scales ≤1 arcmin. These results, together with earlier ROSAT and optical studies which revealed that the X-ray centroid and ellipticity of A2111 shift with spatial scale, are consistent with the hypothesis that the cluster is a dynamically young system. Most likely, the cluster has recently undergone a merger, which may also be responsible for the high fraction of blue galaxies observed in the cluster. Alternatively, the temperature structure may also be caused by the gravitational potential of the cluster. We have also measured the emission-weighted abundance of the X-ray-emitting intracluster medium as 0.25±0.14 solar. This value is similar to those of nearby clusters that do not show a large blue galaxy fraction, indicating that star formation in disc galaxies and subsequent loss to the medium do not drastically alter the average abundance of a cluster. This is consistent with recent results, which indicate that cluster abundances have remained constant since at least z ∼0.3.     

The effect of the dynamical state of clusters on gas expulsion and infant mortality

The star-formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (>50 Myr) time. There is an often quoted critical SFE of ～30% for a cluster to survive gas expulsion. I reiterate that the SFE is not the critical factor; rather, it is the dynamical state of the stars (as measured by their virial ratio) immediately before gas expulsion. If the stars in a star cluster are born in an (even slightly) cold dynamical state, then the survivability of a cluster can be greatly increased.     

Constraining the Mass Distribution of Cluster Galaxies by Weak Lensing

Analysing the weak lensing distortions of the images of faint background galaxies provides a means to constrain the average mass distribution of cluster galaxies and potentially to test the extent of their dark matter haloes as a function of the density of their environment. The observable image distortions are a consequence of the interplay between the effects of a global cluster mass distribution and the perturbations resulting from individual cluster galaxies. Starting from a reconstruction of the cluster mass distribution with conventional techniques, we apply a maximum likelihood method to infer the average properties of an ensemble of cluster galaxies. From simulations this approach is found to be reliable as long as the galaxies including their dark matter haloes only contribute a small fraction to the total mass of the system. If their haloes are extended, the galaxies contain a substantial mass fraction. In this case our method is still applicable in the outer regions of clusters, where the surface mass density is low, but yields biased estimates of the parameters describing the mass profiles of the cluster galaxies in the central part of the cluster. In that case it will be necessary to resort to more sophisticated strategies by modelling cluster galaxies and an underlying global mass distribution simultaneously. We conclude that galaxy–galaxy lensing in clusters provides a unique means to probe the presence and extent of dark haloes of cluster galaxies.     

Keck imaging of the globular cluster systems in the early-type galaxies NGC 1052 and 7332

The presence of two globular cluster subpopulations in early-type galaxies is now the norm rather than the exception. Here we present two more examples for which the host galaxy appears to have undergone a recent merger. Using multi-colour Keck imaging of NGC 1052 and 7332 we find evidence for a bimodal globular cluster colour distribution in both galaxies, with roughly equal numbers of blue and red globular clusters. The blue ones have similar colours to those in the Milky Way halo and are thus probably very old and metal-poor. If the red globular cluster subpopulations are at least of solar metallicity, then stellar population models indicate young ages. We discuss the origin of globular clusters within the framework of formation models. We conclude that recent merger events in these two galaxies have had little effect on their overall globular cluster systems. We also derive globular cluster density profiles, global specific frequencies and, in the case of NGC 1052, radial colour gradients and azimuthal distribution. In general these globular cluster properties are normal for early-type galaxies.     

Magnetic fields in the 3C 129 cluster

We present multifrequency VLA observations of the two radio galaxies 3C 129 and 3C 129.1 embedded in a luminous X-ray cluster. These radio observations reveal a substantial difference in the Faraday Rotation Measures (RMs) toward 3C 129.1 at the cluster centre and 3C 129 at the cluster periphery. After deriving the density profile from available X-ray data, we find that the RM structure of both radio galaxies can be fitted by a tangled cluster magnetic field with strength 6 μG extending at least 3 core radii (450 kpc) from the cluster centre. The magnetic field makes up a small contribution to the total pressure (5 per cent) in the central regions of the cluster. The radio morphology of 3C 129.1 appears disturbed on the southern side, perhaps by the higher pressure environment. In contrast with earlier claims for the presence of a moderately strong cooling flow in the 3C 129 cluster, our analysis of the X-ray data places a limit on the mass deposition rate from any such flow of < 1.2 M_⊙ yr⁻¹.     

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.