高红移星系特征与星系环境

本文在星系有偏袒形成的冷暗物质模型框架下,假定星系都在红移～5时形成,研究了星系群和星系团的形成对星系演化的影响。由示冷暗物质模型中各种不同尺度扰动的幅度相近,星系群和星系团的维里化与星系的维里化在时间上比较接近,在星系群或星系团的维里化过程中获得巨大维里速度的星系仍然富有气体,因此,当它们因为频繁的互相碰撞,或者因为进入星系团中心区的稠密星系际介质受到巨大冲压,或者是被附近射电星系的喷流所产生的高压茧状体所包容,都可能激发活动性,造成恒星的爆发性形成.高红移星系团中观测到的B-O效应和高红移射电星系的光学像与射电喷流同轴向现象都可以据此得到解释.     

星系团Abell 2199的恒星形成性质

通过对近邻星系团Abell 2199中290颗成员星系进行形态分类,研究这些星系的恒星形成率及其与形态和相关物理特性之间的关系.该星系团中星系的特征恒星形成率与Ha等值宽度、星系光谱在4000A处的跃变程度以及星系所包含的恒星质量之间有较强的相关性.这些星系的恒星形成活动没有表现出明显的环境效应,表明该星系团仍处在剧烈的动力学演化阶段,远没有达到动力学平衡.     

恒星形成星系主序关系的研究进展

恒星形成星系的恒星形成率与其恒星质量的相关关系被称为恒星形成星系主序关系,是描述星系演化的基本关系之一。准确测定不同红移处主序关系的斜率、弥散和零点能够对理解星系恒星形成活动的演化及其物理过程提供关键的观测限制。已有的研究揭示,恒星形成星系的整体恒星形成率从z■2到z■0下降为原值的1/30,气体消耗时标却由5亿年增至15亿年;主序关系的斜率在大质量和小质量星系段有变化,反映出决定恒星形成活动的物理过程有系统差别;而星系可能经历多个阵发性的恒星形成爆发活动,有助于更好地解释主序关系的弥散。随着观测能力的提升,对高红移宇宙(z■1~3)的恒星形成星系的研究也更加深入。恒星形成星系主序关系的特征随红移的变化规律为理解星系演化提供关键的观测约束。     

宇宙画廊

长蛇星系团 图片来自NASAAPOD,http：／／apod．nasa．gov／apod／mage／1205／20120330AC0106042L9-81au960：jpg 恒星组成的系统叫作星系,星系也会组成更大的系统,比如位于长蛇座天区的附近,就有一个著名的星系团——长蛇星系团。它就好像一个西瓜,星系是其中的西瓜籽,稀薄的星系际物质和星际气体好比西瓜瓤。不过,星系团并不是静止的,星系之间虽然因为引力作用聚集在一起,但仍然保持快速运行,它们的周围还聚集着众多的暗物质。     

不同层次天体的质光比研究进展

利用天体的质光比和测光信息可以得到不同层次天体的质量信息,利用恒星的质光关系可以计算单星的质量。星团和星系的表面亮度分布反映其质量分布,因此可以通过研究质光比来了解星团和星系的动力学演化。由于星系中存在大量暗物质,因此,星系的质光比普遍比星团的大。星系团的质光比普遍比星系的更大,说明星系团中暗物质的比例更高。介绍了恒星、星团、星系、星系群和星系团等不同层次天体的质光比测量方法及其研究进展,并简单阐述这一领域未来的研究方向。     

星系的恒星速度弥散度的归算方法

星系的恒星视向速度分布是星系动力学模型的重要观测约束,其特征参数包括速度弥散度、分布轮廓以及红移。这些参数对研究星系的动力学、结构和演化以及中央黑洞的质量等都具有重要的价值。该文全面总结了从星系光谱归算星系的恒星视向速度分布及其弥散度的各种方法,以及对观测和处理的一些要求。这些方法都假设星系谱线可看作是模板星光谱经多普勒位移并加宽后的线性叠加。提取尽可能多的星系内部恒星运动信息、减少模板星失配的影响、简化误差分析,是这些方法追求的目标。     

星系际物质

星系际物质许梅概说星系际物质（ＩｎｔｅｒｇａｌａｃｔｉｃＷｅｔｔｅｒ，简称ＩＧＭ），顾名思义，是存在于星系之间的物质。它们有的聚集于相邻的星系之间，构成星系间的物质桥；有的位于星系团内，组成星系团的隐匿物质；有的位于星系团之间，形成星系团际物质。有的...     

星系中心发现暗物质

观测星系时仅能看到其总质量的一部分,剩下的是由神秘的暗物质构成的,这些暗物质通常作为一个巨大的晕存在于星系周围,延伸的范围比可见恒星要远得多。然而,英国科学家发现了暗物质聚集在中心的古老的化石星系。这些化石星系是整个星系团坍缩     

X射线冷核星系团研究进展

冷核是指在一部分X射线星系团中出现的低温高亮的核状结构。冷核星系团在X射线表面亮度轮廓、温度轮廓、中心冷却时间、质量沉积率、熵轮廓等多个方面具有与非冷核星系团截然不同的观测特征。星系团冷核的发现距今已有40年,人们对冷核的认识经历了由冷流模型到稳定冷核模型的转变。但是直到今天,冷核的产生机制和维持条件仍未完全弄清。研究冷核对于理解活动星系核产能机制、星系际元素增丰过程、星系团的形成与演化、大尺度结构形成等众多领域具有非常重要的意义。     

极贫金属晕星中Eu元素的起源

快中子过程(r-过程)发生在富中子的环境中.但是它所在的具体天体物理环境并不是很清楚.随着观测的进展,近年发现了很多的极贫金属的晕星.它们有两个特征:一是快中子元素超丰,并且相对丰度与太阳上的一致.另一个特征是当金属丰度一样时,星体中快中子元素的含量有大的弥散性.这为研究r-过程起源提供了一种独特的途径.模拟计算的方法,用于研究星系演化的过程,以及相伴发生的恒星的快中子核素含量的弥散性,可用来了解快中子元素的起源这样得出的星系演化模型,不仅包含了气体区域自发的恒星形成,而且考虑了超新星爆发激发的恒星形成.结果显示,低质量端的超新星应是快中子核素的产生地.同时,超新星引起的星系演化的不均匀性不足以解释观测到的晕星快中子元素含量的弥散,因此这个问题还有待于更进一步研究.     

How the extinction of extragalactic background light affects surface photometry of galaxies, groups and clusters

The faint regions of galaxies, groups and clusters hold important clues about how these objects formed, and surface photometry at optical and near-infrared wavelengths represents a powerful tool for studying such structures. Here, we identify a hitherto unrecognized problem with this technique, related to how the night sky flux is typically measured and subtracted from astronomical images. While most of the sky flux comes from regions between the observer and the target object, a small fraction – the extragalactic background light (EBL) – comes from behind. We argue that since this part of the sky flux can be subjected to extinction by dust present in the galaxy/group/cluster studied, standard reduction procedures may lead to a systematic oversubtraction of the EBL. Even very small amounts of extinction can lead to spurious features in radial surface brightness profiles and colour maps of extended objects. We assess the likely impact of this effect on a number of topics in extragalactic astronomy where very deep surface photometry is currently attempted, including studies of stellar haloes, starburst host galaxies, disc truncations and diffuse intragroup/intracluster light. We argue that EBL extinction may provide at least a partial explanation for the anomalously red colours reported for the haloes of disc galaxies and for the hosts of local starburst galaxies. EBL extinction effects also mimic truncations in discs with unusually high dust opacities, but are unlikely to be the cause of such features in general. Failure to account for EBL extinction can also give rise to a non-negligible underestimate of intragroup and intracluster light at the faintest surface brightness levels currently probed. Finally, we discuss how EBL extinction effects may be exploited to provide an independent constraint on the surface brightness of the EBL, using a combination of surface photometry and direct star counts.     

Galaxy evolution in clusters

The galaxy populations in present-day clusters are distinctly different from those of the field, indicating that environment plays a strong role in galaxy evolution. This review discusses some of the recent observations of moderate to high redshift clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies which formed early in the cluster history, as well as field galaxies which have had their star formation truncated upon falling into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate in different clusters, perhaps because of the recent merging history of the clusters. We also present a recent analysis of population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of dwarf galaxy disruption in semi-analytic models

We present results for a galaxy formation model that includes a simple treatment for the disruption of dwarf galaxies by gravitational forces and galaxy encounters within galaxy clusters. This is implemented a posteriori in a semi-analytic model by considering the stability of cluster dark matter subhaloes at   z = 0  . We assume that a galaxy whose dark matter substructure has been disrupted will itself disperse, while its stars become part of the population of intracluster stars responsible for the observed intracluster light. Despite the simplicity of this assumption, our results show a substantial improvement over previous models and indicate that the inclusion of galaxy disruption is indeed a necessary ingredient of galaxy formation models. We find that galaxy disruption suppresses the number density of dwarf galaxies by about a factor of 2. This makes the slope of the faint end of the galaxy luminosity function shallower, in agreement with observations. In particular, the abundance of faint, red galaxies is strongly suppressed. As a result, the luminosity function of red galaxies and the distinction between the red and the blue galaxy populations in colour–magnitude relationships are correctly predicted. Finally, we estimate a fraction of intracluster light comparable to that found in clusters of galaxies.     

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。     

The growth and assembly of a massive galaxy at z∼ 2

We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.     

Evolutionary synthesis models for galaxy transformation in clusters

The galaxy population in rich local galaxy clusters shows a ratio of one quarter elliptical galaxies, two quarters S0 galaxies, and one quarter spiral galaxies. Observations of clusters at redshift 0.5 show a perspicuously different ratio, the dominant galaxy type are spiral galaxies with a fraction of two quarters while the number of S0 galaxies decreases to a fraction of one quarter (Dressler et al. 1997). This shows an evolution of the galaxy population in clusters since redshift 0.5 and it has been suspected that galaxy transformation processes during the infall into a cluster are responsible for this change. These could be merging, starburst or ram-pressure stripping. We use our evolutionary synthesis models to describe various possible effects of those interactions on the star formation of spiral galaxies infalling into clusters. We study the effects of starbursts of various strengths as well as of the truncation of star formation at various epochs on the color and luminosity evolution of model galaxies of various spectral types. As a first application we present the comparison of our models with observed properties of the local S0 galaxy population to constrain possible S0 formation mechanisms in clusters. Application to other types of galaxies is planned for the future. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polarization of resonance X-ray lines from clusters of galaxies

It is known that resonant scattering can distort the surface brightness profiles of clusters of galaxies in X-ray lines. We demonstrate that the scattered line emission should be polarized and possibly detectable with future X-ray polarimeters. Spectrally resolved mapping of a galaxy cluster in polarized X-rays could provide valuable independent information on the physical conditions, in particular element abundances and the characteristic velocity of small-scale turbulent motions, in the intracluster gas. The expected degree of polarization is of the order of 10 per cent for the richest regular clusters (e.g. Coma) and clusters whose X-ray emission is dominated by a central cooling flow (such as Perseus and M87/Virgo).     

Searching for the brightest stars in galaxies outside the Local Group

This paper introduces a technique for searching for bright massive stars in galaxies beyond the Local Group.To search for massive stars,we processed the results of stellar photometry from the Hubble Space Telescope(HST) images using the DAOPHOT and DOLPHOT packages.The results of such searches are demonstrated with examples of the galaxies DDO 68,M94 and NGC 1672.In the galaxy DDO 68,the LBV star changes its brightness,and massive stars in M94 can be identified by excess in the Ha band.For the galaxy NGC 1672,we measure the distance for the first time by the TRGB method,which enabled determining the luminosities of the brightest stars,likely hypergiants,in the young star formation region.So far,we have performed stellar photometry on HST images of 320 northern sky galaxies located at a distance less than 12 Mpc.This allowed us to identify 53 galaxies with probable hypergiants.Further photometric and spectral observations of these galaxies are planned to search for massive stars.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

星系形成的半解析模型研究进展

基于目前流行的冷暗物质等级成团理论的星系形成的半解析模型取得了很大进展．在引入一系列合理的关于重子物质物理过程的假设，如气体冷却、恒星形成率、超新星能量反馈以后，结合N体数值模拟，星系形成的半解析模型一方面能较成功地预言许多与观测符合的近邻星系的性质，如光度函数、星系的两点相关函数、Thlly-Fisher关系、星系团中椭圆星系的颜色一星等关系等；另一方面还能较成功地预言宇宙的恒星形成历史，以及一部分高红移星系的性质，如Lyman-Break星系的数目随红移的分布．但是最近一些观测表明，半解析模型在高红移预言的大质量星系较少，且红星系数目也较观测少．重点讨论半解析模型成功与不足之处，并指出解决这些问题可能的途径。