宇宙常数和星系团

在Ｘ射引线观测提高了星系团质量和半径测量精度的新形势下，本讨论了宇宙常数不等于零的平坦宇宙冷暗物质结构形成模型，利用球对称扰动区在宇宙常数不为零时的动力学方程的解，估计了星系团形成红移与宇宙常数是否为零的关系，计算了星系团质量函数随红移的演化。     

宇宙常数与宇宙年龄问题

本用球对称扰动模型导出了星系暗晕的平均密度与形成时间的关系，并由此估算银河系的形成时间ｔｖ，我们把球状星团的年龄作为银河系年龄ｔＧ的代表。则ｔＧ＋ｔＶ是宇宙年龄，对Ωλ＝０，０．０７和０．８的平坦宇宙模型，本计算并讨论了能与它相洽的哈毂常数的范围，结果表明，若哈勃常数大到８０ｋｍ·ｓ＾－１·Ｍｐｃ＾－１左右，引入宇宙常数并不一定能解决宇宙年龄的矛盾。     

宇宙常数与宇宙年龄问题

本文用球对称扰动模型导出了星系暗晕的平均密度与形成时间的关系 ,并由此估算银河系的形成时间tV.我们把球状星团的年龄作为银河系年龄tG 的代表 ,则tG tV 是宇宙年龄 .对Ωλ=0 ,0 .7和 0 .8的平坦宇宙模型 ,本文计算并讨论了能与它相洽的哈勃常数的范围 .结果表明 ,若哈勃常数大到 80km·s- 1 Mpc- 1 左右 ,引入宇宙常数并不一定能解决宇宙年龄的矛盾     

超星系团的宇宙乐章

天文学家正在努力了解宇宙的大尺度结构——它们是如何形成以及是如何影响宇宙膨胀的？ 在最大的尺度上,我们的宇宙看上去就犹如午后无风的湖面,平静、波澜不惊。但是在数亿光年或者更小的尺度上,宇宙则呈现出了物质和巨洞之间的随机、混杂分布。这是一个物质聚集成团的宇宙。     

星系团的引力透镜效应及宇宙微波背景辐射的各向异性

本研究了Ｐｏｉｓｓｏｎ随机分布的星系团作为引力透镜天体,其横向本动速度引起的运动引力透镜效应对宇宙背景辐射（ＣＢＲ）温度涨落各向异性的影响。对星系团的密度结构及其引力透镜效应,我们采用Ｈｅｒｎｑｕｉｓｔ模型,并在此基础上,利用星系团的统计理论,在冷蝉 物质及等曲率重子扰动模型框架下,计算了运动引力透镜效应对ＣＢＲ温度涨落各向异性的贡献。     

宇宙画廊 测绘星系团质量分布

我们银河系的直径大约是1O～12万光年,对我们来说,这样的尺度已经大得很难想象了。与银河系比起来,星系团的大小简直就是超乎想象。星系团通常包含成百上千个星系,它们在引力的作用下聚集在一起,其尺寸足有几千万光年。这些星系团是正在碰撞的星系和暗物质之间复杂的相互作用形成的,暗物质不参与电磁相互作用,既不发光也不反射光,所以我们无法直接探测到它们的存在。探测极其遥远星系团的质量分布就更是难上加难了。     

非零宇宙常数Global Monopole的引力场

本文讨论了膨涨宇宙处于宇宙常数不为零时ＧｌｏｂａｌＭｏｎｏｐｏｌｅ存在的可能性、稳定性及求其引力场的解。最后分析了ＧｌｏｂａｌＭｏｎｏｐｏｌｅ的宇宙演化。     

非零宇宙常数Global Monopole的引力场

本文讨论了膨胀宇宙处于宇宙常数不为零时Ｇｌｏｂａｌ Ｍｏｎｏｐｏｌｅ存在的可能性，稳定性及求其引力场的解，最后分析了Ｇｌｏｂａｌ Ｍｏｎｏｐｏｌｅ的宇宙演化。     

倾斜冷暗物质谱与宇宙中可观测结构

为了缓解标准冷暗物质模型在解释一些观测现象时所遇到的困难，本考察了倾斜冷暗物质谱模型。在ＢＢＫＳ原初扰动场统计理论的基础上，本根据天体质量函数的计算公式和背景－天体相互作用的估计方法，计算了星系群和星系团的质量函数，并与最新观测质量函数作了对比，又分析和计算了在各种空洞尺度上天体密度的变化。计算结果表明，倾斜冷暗物质谱模型在与ＣＯＢＥ卫星观测结果相容的前提下，能在上述两个方面缓解标准冷暗物质模     

Counterrotating Galaxies and Memory of Cosmological Initial Conditions

As it is known, a good number of galaxies are observed to have counterrotating cores. A popular scenario to explain the formation of such galaxies is based on a secondary process of merging of galaxies with their satellites, or gas infall, or merger events between galaxies. An alternative mechanism, proposed by Voglis et al., 1991, and by Harsoula and Voglis 1998, could also be responsible for the formation of these galaxies directly from cosmological initial conditions (direct scenario). The novel mechanism was demonstrated by using quiet cosmological initial conditions in N-body simulations. In the present paper we extend our N-body simulations using clumpy initial conditions and show that this mechanism still works to create counterrotating galaxies. Counterrotation is a result of the considerable amount of memory of initial conditions surviving for times comparable to the Hubble time, despite the large degree of instability of individual orbits and the dramatic redistribution and mixing of the particles in phase space. We show, for example, that the particles remember, in a statistical sense, not only their distance from the center of mass (memory of energy), but also the initial orientation of their position relative to the direction of an external tidal field, which determines the sign and the amount of angular momentum that is transferred to the particles of the system.     

Starburst Galaxies in Clusters

The nature of the starburst phenomenon in galaxies is investigated using a narrow-band colour system designed to study colour evolution in distant clusters. Work on zero redshift, luminous far-IR galaxies, calibrated by starburst models, demonstrates the usefulness of this colour system in isolating starburst from normal star-forming colours, and also demonstrates a strong correlation with far-IR colours despite reddening effects. The same colour system applied to distant clusters finds that a majority of the faint blue cluster population are starburst dwarf galaxies, probably the progenitors of the current population of dwarf ellipticals in nearby clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmic Rays and Clusters of Galaxies

The presence of a significant population of relic relativistic electrons - created at an early epoch of the Universe - has been invoked to explain the diffuse EUV emission excess observed in a number of galaxy clusters. While the postulated inverse Compton scattering of the 3° K background radiation by cosmic ray electrons might indeed be utilized as an important diagnostic tool for the physical nature of the intracluster cosmic rays, it is shown here that continuous generation plus reacceleration would be necessary if the conditions on the observed energy spectral distribution and energy supply rate are to be met in the case of clusters with large radio halos. This revised version was published online in July 2006 with corrections to the Cover Date.     

Varying Fine-Structure Constant and the Cosmological Constant Problem

We start with a brief account of the latest analysis of the Oklo phenomenon providing the still most stringent constraint on time variability of the fine-structure constant α. Comparing this with the recent result from the measurement of distant QSO's appears to indicate a non-uniform time-dependence, which we argue to be related to another recent finding of the accelerating universe. This view is implemented in terms of the scalar-tensor theory, applied specifically to the small but nonzero cosmological constant. Our detailed calculation shows that these two phenomena can be understood in terms of a common origin, a particular behavior of the scalar field, dilaton. We also sketch how this theoretical approach makes it appropriate to revisit non-Newtonian gravity featuring small violation of Weak Equivalence Principle at medium distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Star Formation History of Dwarf Galaxies over Cosmological Timescales

Dwarf galaxies may play a key role in the formation and evolution of bigger systems. This makes it a topic of major interest to know how they form and evolve and, in particular, how their star formation histories (SFHs) have proceeded since their birth. For nearby galaxies, thecolour–magnitude diagram (CMD) contains stars formed over their full lifetime. It is hence a fossil record of their SFHs. Analysis via synthetic CMDs provides a powerful tool to retrieve them. In this paper, I briefly present the critical issues related to synthetic CMD analysis and make a summary of the currently available results for the SFH extending over the full lifetimes of galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Orientation of Galaxies in Clusters

A detailed analysis of the geometry of galaxies in clusters has been undertaken in both two and three dimensional space. The procedure was applied to the three Abell clusters A1644, A548 E and A548 W. No significant alignment trend of galaxies in clusters confirmed in three dimensional space was found. This result is consistent with the mixed dark matter model MDM of galaxy formation. The result is also consistent with Peebles suggestion that protogalaxies acquired angular momentum from tidal torques exerted by their neighbours in the early universe. The amount of angular momentum predicted by this mechanism could be described by a single dimensionless papameter λ≈ 0.8. N-body experiments have shown that λ has approximately normal distribution with mean value about 0.07 and with a standard deviation of about 0.03. From the analyic fit to flat rotation curves it is found that tidal torque can provide the observed angular momentum if the mean collapse factor is about 20 and the mean halo-to-disk mass ratio is of order 10. The MDM model does not predict any systematic relation between the tidal torques among the halos of galaxies and the large scale structures such as the clusters, superclusters of galaxies and giant voids in between. Further work in this area is still required. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gas in Groups and Clusters of Galaxies

Groups and clusters contain a large fraction of hot gas which emitsX-ray radiation. This gas yields information on the dynamical stateand on the total mass of these systems. X-ray spectra show that heavyelements are present in the gas. As these metals must have beenproduced in the cluster/group galaxies and later transported into thegas, the metallicity is a good tracer for the transport processes. Severalpossible processes, that transport gas from the small potential wellsof the galaxies into the clusters and groups, are discussed.     

Photometric Properties of Clusters of Galaxies

We present CCD photometry of 16 Abell clusters and one cluster candidate found in POSS-II field 861. The images were taken at the 0.9 m Telescope at Cerro Tololo, in the g, r and i filters of the Gunn–Thuan system. We tested the idea proposed by Garilli et al. (1996) that there is a population of unusually red galaxies which could be associated with either the field or clusters. Garilli et al. (1996) suggest that these galaxies have anomalously red colours, but we find that these objects are all near the limiting magnitude of the images (20^m<r<22^m) and have colours that are consistent with those expected for stars or field galaxies at z∼0.7. This revised version was published online in July 2006 with corrections to the Cover Date.