类星体形成的半解析模型

以Kang等的星系形成的半解析模型为基础,研究星系和中心大质量黑洞的共同形成和演化.假设类星体的活动性由星系并合引起,通过引入黑洞吸积率,确定了红移区间0z4.5的类星体的热光度函数,并分别从爱丁顿比率、黑洞质量函数和两点相关函数3个方面对模型进行限制,模型预言的光度函数能在整个红移、整个光度范围内与观测相符合.结果显示,常数爱丁顿比率不能很好地描述黑洞吸积,爱丁顿比率需要在一定红移范围内随红移增加;星系主并合是触发类星体活动性的有效机制,而次并合只对低光度和中等光度的类星体有影响,对高光度类星体影响很小;在z=1.0处,极高光度的类星休比其他类星体具有更强的成团性.     

活动星系核中黑洞质量和吸积率分布研究

红移、中心黑洞质量和吸积率是活动星系核演化的重要参数.利用反响映射法计算了172个类星体和Seyfert星系样本的中心黑洞质量,并分析了中心黑洞的质量、红移、爱丁顿吸积率的分布,进而验证了从类星体过渡到Seyfert星系的演化.     

大质量黑洞及其寄主星系的共同演化

<正>类星体寄主星系的观测结果以及星系核球与黑洞质量间的密切相关,清晰地给出一幅星系和黑洞共同演化的图像.如何描绘出这幅图像,有大量未解决的问题,其中不乏基本问题.本文努力构建这幅美妙的图像,内容覆盖了种子黑洞的形成、大质量黑洞的自旋和负载循环的宇宙学演化、星系的恒星形成率随质量和红移的演化以及星暴和黑洞活动的相互作用.     

活动星系核黑洞核球质量及宇宙学红移关系研究

通过文献收集了90个活动星系核样本,研究黑洞质量与核球质量的关系,分析黑洞质量及核球质量比与宇宙学红移的关系,进而研究活动星系核的演化关系,结果表明:(1)在Seyfert星系中黑洞质量与核球质量无明显相关,类星体中则呈线性相关;(2)活动星系核演化序列由类星体演化到Seyfert星系。     

类星体光度和暗物质晕的统计关系

活动星系核的能量反馈是星系形成理论模型中的一个重要物理过程,与星系所处的暗物质晕质量、星系中央大质量黑洞吸积率等因素有关。当前的半解析模型预测活动星系核反馈机制主要有两种模式:射电模式和类星体模式,前者主要发生在大质量暗晕中央的大质量星系中,后者主要由较小质量星系并合导致。利用斯隆数字化巡天(SDSS)提供的目前最大的类星体光谱观测样本,结合基于SDSS构建的星系群(团)表,从统计上分析了类星体的热光度和暗晕质量的分布情况。初步分析结果显示,在大质量暗晕中,类星体的热光度和所在暗晕的质量没有相关性,类星体所在暗晕的质量分布很广,进一步证实了高光度的类星体并不存在于大质量的暗晕中。     

OJ 287的引力波辐射研究

类星体OJ 287的研究已经有100多年的历史,它呈现周期性双峰爆发现象,爆发周期为12 yr,比较好地解释观测的模型是双黑洞模型,即次黑洞绕主黑洞运动,并撞击主黑洞吸积盘从而引起爆发.此模型合理地解释了OJ 287的光变曲线并正确预言了未来爆发时间,这间接证明了广义相对论进动效应以及引力波的存在.星系中心大质量黑洞是一类重要的引力波源,而精确确定了其内部组分运动学方程的星系非常少,由于双黑洞模型提供了精确的黑洞运动轨道,故可以在这个运动轨道基础上研究其引力波辐射.在已有工作基础上,采用后牛顿近似方法首次得到了引力波辐射功率和波形随时间的演化关系,根据目前引力波探测设备IPTA(International Pulsar Timing Array)的进展情况,未来十几年内对OJ 287的引力波直接探测将成为可能.     

活动星系核与类星体中致密天体的形成与并合：伽玛爆发与引力辐射

频繁的超新星爆发可用以解释活动星系核与类星体的超高金属丰度。星团中的恒星与吸积盘相互作用导致致密天体中子星和黑洞的形成率很高,这些致密天体与红巨星很容易形成双星系统,而后形成（ＮＳ／ＢＨ,ＮＳ／ＢＨ） 双星由于引力辐射最终导致并合而形成伽玛射线爆发。我们估计了伽玛射线爆的爆发频率,发现与观测相符。     

高红移大质量星系的形成与演化

星系形成及演化一直是天体物理研究中最重要的领域之一.近10 yr来星系形成及演化的研究取得了突破性进展,主要包括:(1)近邻星系在颜色-星等图上呈现双峰分布,早型星系普遍颜色较红,而晚型星系颜色偏蓝;并且近邻宇宙红星系的总质量相对于红移z=1时至少增长了1倍,意味着存在蓝星系到红星系的转化过程.这一转化过程是如何发生的成为现代星系形成演化领域最重要的问题之一;(2)星系中心超大质量黑洞质量跟核球质量有很强的相关,意味着黑洞演化跟星系演化有着紧密的关系.黑洞的活动(活动星系核)如何影响星系演化也成为了亟待解决的问题之一.然而近邻宇宙中的大质量星系基本都已停止了剧烈的恒星形成活动和黑洞活动,因此,要回答这些问题,我们需要仔细研究宇宙早期红移在z≈2的星系性质.在这一红移处,星系中的恒星形成和黑洞增长均处于高峰期.     

AGN吸积盘辐射区半径的多方法研究

通过收集数据,用标准α盘模型法、短时标光变法、连续波混响滞后法、微引力透镜法分析了活动星系核吸积盘辐射区半径,并对4种方法进行分析讨论。结果表明:(1)将耀变体和类星体参数比较,发现两个子类的中心黑洞质量、光度和吸积盘辐射区半径都没有明显差别。选择高光度源进行研究,出现了两个子类以上参数无明显区别的情况;(2)短时标光变法得到的吸积盘辐射区半径比和黑洞质量没有明显的相关性;(3)连续波混响滞后法得到的吸积盘辐射区半径比对黑洞质量的依赖性高于标准α盘模型法,对于无法确定中心黑洞质量的源,可以利用连续波混响滞后法得到吸积盘辐射区半径;(4)微引力透镜法得到的吸积盘辐射区半径比和黑洞质量没有明显的相关性,且该方法只适用于存在微引力透镜效应的类星体;(5)这些方法从观测上证明了吸积盘辐射区半径与黑洞质量存在相关性,验证了标准模型成立。为进一步在观测上寻找这4种方法所需要的源提供了理论指导,对活动星系核的吸积盘辐射区半径研究有重要意义。     

第一次称量类星体中的黑洞

天文学家第一次确定了位于可观测宇宙边缘附近的一个类星体的大黑洞的质量。估计这个黑洞和类星体是大爆炸后10亿年里形成的,由它们可以洞悉早期宇宙的情况以及黑洞和星系曾有过什么变化。天文学家用莫纳克亚山上的英国红外望远镜观测了类星体J1148+5251。装在望远镜上的成像光谱仪测量了来自这一天体的红外光谱。在离子MgⅡ,即失去一个电子的镁原子的发射线上有一个特殊加强。     

A unified model for the evolution of galaxies and quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few per cent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 10⁷ yr. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z ∼2 to z =0 is a result of the combination of three effects: (i) a decrease in the merging rate; (ii) a decrease in the amount of cold gas available to fuel black holes, and (iii) an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Ly α systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starburst galaxies is inextricably linked to the hierarchical build-up of galaxies.     

Formation of Starburst Galaxies in Merging Clusters

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process in cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations that show the dynamical evolution of a gas-rich, late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that the time-dependent tidal gravitational field during merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region and finally triggers a secondary starburst. This result provides a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma Cluster as one observational example indicating the global tidal effects of group–cluster merging. Our numerical results further suggest a causal link between the observed excess of blue galaxies in distant clusters and the cluster virialization process through hierarchical merging of subclusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Semi-analytical Model of Quasar Formation

On the basis of Kang et al.’s semi-analytical model of galaxy formation and evolution, the joint formation and evolution of galaxies and their central massive black holes are studied. It is assumed that the activity of quasars is caused by merging of galaxies. Via the introduction of the mass accretion rate of black holes, the bolometric luminosity function of quasars with the redshifts in the region of 0 < z < 4.5 is ascertained. With the respective limitations of the three factors, i.e., the Eddington ratio, black-hole mass function and two-point correlation function, the luminosity function predicted by the model may coincide with observations in the entire range of luminosity. This result reveals that the constant Eddington ratio cannot well describe the accretion of black holes, so the Eddington ratio has to be increased with the redshift in a certain range of redshift. The major merging of galaxies is the effective mechanism of triggering the quasar activity, while the minor merging can merely affect the quasars with low and intermediate luminosities. Its effect on the high-luminosity quasars is very small. At the place of z=1, the quasars with extremely high luminosities possess more intense properties of clustering than other quasars.     

A stochastic model for correlations between central black hole masses and galactic bulge velocity dispersions

We consider a cosmological model in which part of the Universe, Ω_h～10^?5, is in the form of primordial black holes with masses of ～ 10⁵ M _⊙. These primordial black holes were the centers for growing protogalaxies, which experienced multiple mergers with ordinary galaxies and with each other. The galaxy formation is accompanied by the merging and growth of central black holes in the galactic nuclei. We show that the recently discovered correlations between central black hole masses and galactic bulge parameters naturally arise in this scenario.     

On the physical connections between galaxies of different types

Galaxies can be classified in two broad sequences which are likely to reflect their formation mechanism. The 'main sequence', consisting of spirals, irregulars and all dwarf galaxies, is probably produced by gas settling within dark matter haloes. We show that the sizes and surface densities along this sequence are primarily determined by the distributions of the angular momentum and formation time of dark haloes. They are well reproduced by current cosmogonies provided that galaxies form late, at z  ≲ 2. In this scenario, dwarf ellipticals were small 'discs' at z  ∼ 1 and become 'ellipticals' after they fall into cluster environments. The strong clustering of dwarf ellipticals is then a natural by-product of the merging and transformation process. The number of dwarf galaxies predicted in a cluster such as Virgo is in good agreement with the observed number. On the other hand, the 'giant branch', consisting of giant ellipticals and bulges, is probably produced by the merging of disc galaxies. Based on the observed phase-space densities of galaxies, we show that the main bodies of all giant ellipticals can be produced by dissipationless mergers of high-redshift discs. However, high-redshift discs, although denser than present-day ones, are still not compact enough to produce the high central phase-space density of some low-luminosity ellipticals. Dissipation must have occurred in the central parts of these galaxies during the merger which formed them.     

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.     

星系的IRAS颜色

本文以369个在四个波段上都被探测到了的IRAS亮星系为依据,讨论了星系的IRAS颜色。根据IRAS颜色对它们做了分类,用简单的双黑体模型,可以很好拟合绝大多数IRAS亮星系的辐射。     

Winds and infalling gas in Lyman break galaxies

A model for gas outflows is proposed which simultaneously explains the correlations between the (i) equivalent widths of low-ionization and Lyα lines, (ii) outflow velocity, and (iii) star formation rate observed in Lyman break galaxies (LBGs). Our interpretation implies that LBGs host short-lived (30 ± 5 Myr) starburst episodes observed at different evolutionary phases. Initially, the starburst powers a hot wind bound by a denser cold shell, which after ≈5 Myr becomes dynamically unstable and fragments; afterwards the fragment evolution is approximately ballistic while the hot bubble continues to expand. As the fragments are gravitationally decelerated, their screening ability of the starlight decreases as the ultraviolet (UV) starburst luminosity progressively dims. LBG observations sample all these evolutionary phases. Finally, the fragments fall back on to the galaxy after ≈60 Myr. This phase cannot be easily probed as it occurs when the starburst UV luminosity has already largely faded; however, galaxies dimmer in the UV than LBGs should show infalling gas.     

Continuum Evn and Merlin Observations of Ultra Luminous Infrared Galaxies

Radio imaging of ULIR galaxies is ideal to explore the connection between the starburst and the AGN phenomenon since it is unaffected by dust obscuration, and provides the required high angular resolution to distinguish between an AGN and starburst emission. We have made combined 18 cm radio continuum, EVN and MERLIN observations of 13 ULIRGs that have the parsec and deci-parsec scale resolution necessary to distinguish between an AGN and supernovae remnants at the centres of these galaxies, and assess the contribution of each to the total energy distribution. Images of three galaxies are presented here.     

Bursts of star formation caused by disk-halo mass exchange

It is well known that galaxies accumulating large quantities of gas undergo violent bursts of star formation. This is believed to be due to tidal interactions of galaxies leading to the infall of gas into their central regions. Bursts of star formation in this scenario are transitory phenomena and can be induced only by external sources.However, in some cases there is no direct evidence of tidal interactions in starburst galaxies.We discuss another possibility of bursting phenomena in galaxies connected with nonlinear feedback processes in mass-exchange between components of star-forming region. We consider a three-component model including cold clouds, warm gas and massive stars and take into account the delay processes in the transformation of hot gas ejected by massive stars and evaporated from cold phase, into the warm phase. Self-regulating mechanism of phase transition of small clouds into warm gas due to heating radiation of massive stars is also taken into account.The analysis of stability of the system shows that it could be unstable even in case of a small efficiency in the birth of massive stars. The evolution of unstable nonlinear perturbations leads to the development of self-sustained nonlinear oscillations of star formation.