Lost and found dark matter in elliptical galaxies

There is strong evidence that the mass of the Universe is dominated by dark matter, which exerts gravitational attraction but whose exact nature is unknown. In particular, all galaxies are believed to be embedded in massive haloes of dark matter. This view has recently been challenged by the observation of surprisingly low random stellar velocities in the outskirts of ordinary elliptical galaxies, which has been interpreted as indicating a lack of dark matter. Here we show that the low velocities are in fact compatible with galaxy formation in dark-matter haloes. Using numerical simulations of disk-galaxy mergers, we find that the stellar orbits in the outer regions of the resulting ellipticals are very elongated. These stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories. The elongated orbits, combined with the steeply falling density profile of the observed tracers, explain the observed low velocities even in the presence of large amounts of dark matter. Projection effects when viewing a triaxial elliptical can lead to even lower observed velocities along certain lines of sight.     

Supermassive black holes do not correlate with dark matter haloes of galaxies

Supermassive black holes have been detected in all galaxies that contain bulge components when the galaxies observed were close enough that the searches were feasible. Together with the observation that bigger black holes live in bigger bulges, this has led to the belief that black-hole growth and bulge formation regulate each other. That is, black holes and bulges coevolve. Therefore, reports of a similar correlation between black holes and the dark matter haloes in which visible galaxies are embedded have profound implications. Dark matter is likely to be non-baryonic, so these reports suggest that unknown, exotic physics controls black-hole growth. Here we show, in part on the basis of recent measurements of bulgeless galaxies, that there is almost no correlation between dark matter and parameters that measure black holes unless the galaxy also contains a bulge. We conclude that black holes do not correlate directly with dark matter. They do not correlate with galaxy disks, either. Therefore, black holes coevolve only with bulges. This simplifies the puzzle of their coevolution by focusing attention on purely baryonic processes in the galaxy mergers that make bulges.     

A geometric measure of dark energy with pairs of galaxies

Observations indicate that the expansion of the Universe is accelerating, which is attributed to a ‘dark energy’ component that opposes gravity. There is a purely geometric test of the expansion of the Universe (the Alcock–Paczynski test), which would provide an independent way of investigating the abundance (Ω(X)) and equation of state (W(X)) of dark energy. It is based on an analysis of the geometrical distortions expected from comparing the real-space and redshift-space shape of distant cosmic structures, but it has proved difficult to implement. Here we report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows us to determine that the Universe is flat. By alternately fixing its spatial geometry at Ω(k)≡0 and the dark energy equation-of-state parameter at W(X)≡-1, and using the results of baryon acoustic oscillations, we can establish at the 68.3% confidence level that and -0.85>W(X)>-1.12 and 0.60<Ω(X)<0.80.     

Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales

Most of the matter in the Universe is not luminous, and can be observed only through its gravitational influence on the appearance of luminous matter. Weak gravitational lensing is a technique that uses the distortions of the images of distant galaxies as a tracer of dark matter: such distortions are induced as the light passes through large-scale distributions of dark matter in the foreground. The patterns of the induced distortions reflect the density of mass along the line of sight and its distribution, and the resulting 'cosmic shear' can be used to distinguish between alternative cosmologies. But previous attempts to measure this effect have been inconclusive. Here we report the detection of cosmic shear on angular scales of up to half a degree using 145,000 galaxies and along three separate lines of sight. We find that the dark matter is distributed in a manner consistent with either an open universe, or a flat universe that is dominated by a cosmological constant. Our results are inconsistent with the standard cold-dark-matter model.     

暗能量与暗物质相互耦合的观测限制

暗能量是一种引起宇宙加速彭胀的排斥力,探索暗能量的性质是目前天文学和理论物理领域中最突出的问题之一。提供选取暗能量状态参数为常量和变量(随红移变化)的2种模型,并考虑在模型中加入一种被广泛采用的暗能量耦合形式Q=3αHρm进行数值模拟讨论。应用最新的观测数据对耦合模型的相关参数进行限制,得到了暗因子耦合强度的上限。对常量和变量耦合模型,在1σ,2σ和3σ置信度内,代表耦合强度的参数分别为α≤0.0176(0.0225)(0.0273)和α≤0.0206(0.0278)(0.0345)。研究表明,这2种模型可给出与目前的观测结果相一致的参数范围。     

A class of compact dwarf galaxies from disruptive processes in galaxy clusters

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.     

暗物质模型简介

本文简单介绍了一些关于暗物质的模型研究,首先概述了暗物质的发现和大致的研究框架,并介绍了一些自然的暗物质候选者,如轴子和超对称模型中的Neutralino等,随后我们讨论一些具体模型和由实验诱导的一些模型.     

暗物质间接探测现状

本文简介了暗物质间接探测的最新进展.主要介绍近年来PAMELA（Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics）卫星所探测到的宇宙线正电子超出的问题,及相关的理论解释.本文还介绍了PAMELA对于宇宙线反质子的测量结果,ATIC和Fermi卫星对于正负电子总能谱的测量结果等.另一个重要的结果来自于Fermi和地面实验对于伽马射线的测量结果,这些结果对暗物质性质给出较强的限制.     

The moment of truth for WIMP dark matter

We know that dark matter constitutes 85 per cent of all the matter in the Universe, but we do not know of what it is made. Amongst the many dark matter candidates proposed, WIMPs (weakly interacting massive particles) occupy a special place, because they arise naturally from new theories that seek to extend the standard model of particle physics. With the advent of the Large Hadron Collider at CERN, and a new generation of astroparticle experiments, the moment of truth has come for WIMPs: either we will discover them in the next five to ten years, or we will witness their inevitable decline.     

X-ray clusters of galaxies as tracers of structure in the Universe

Clusters of galaxies are visible tracers of the network of matter in the Universe, marking the high-density regions where filaments of dark matter join together. When observed at X-ray wavelengths these clusters shine like cosmic lighthouses, as a consequence of the hot gas trapped within their gravitational potential wells. The X-ray emission is linked directly to the total mass of a cluster, and so can be used to investigate the mass distribution for a sizeable fraction of the Universe. The picture that has emerged from recent studies is remarkably consistent with the predictions for a low-density Universe dominated by cold dark matter.     

暗物质存在性的数学预测

分别运用变形Poincar 群法、Boltzmann方程分析与中微子味残存概率法阐述了暗物质的存在性,并介绍了暗物质模型.     

Gravitational redshift of galaxies in clusters as predicted by general relativity

The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the Lambda Cold Dark Matter (ΛCDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web. General relativity has not hitherto been tested on cosmological scales independently of the assumptions of the ΛCDM model. Here we report an observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based on archival data. Our measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy (the f(R) theory), but is inconsistent with alternative models designed to avoid the presence of dark matter.     

浅论宇宙中的暗物质

本文以兹威基对暗物质的首次发现为出发点 .回顾了物理学工作者研究暗物质所走过的坎坷之路 .以天文观测的事实和粒子物理学研究的新成果为根据 ,推测了暗物质存在的空间区域及形态 .     

从万有引力出发对黑洞、暗物质和暗能量的分析与理解

万有引力具有两个基本特性：普适性和纯粹吸引作用,重力系统的能量则必须是正定的．从万有引力出发,在牛顿力学的基础上进行简单推导得到了一些关系式,对黑洞、暗物质和暗能量等一些物理课题进行一定程度的分析和理解．     

从万有引力出发对黑洞、暗物质和暗能量的分析与理解

万有引力具有两个基本特性:普适性和纯粹吸引作用,重力系统的能量则必须是正定的.从万有引力出发,在牛顿力学的基础上进行简单推导得到了一些关系式,对黑洞、暗物质和暗能量等一些物理课题进行一定程度的分析和理解.