星系形成和演化

在现代宇宙学的框架下对当前星系形成和演化方面的研究结果作一综合评述，内容包括知种现代化观测手段所得的结果及其物理内涵，各种模型的成功与不足之处。     

星系相互作用的一个结果：环星系形成的数值模拟

通过数值模拟相互作用星素（包括靶星系和入侵星系）的碰撞，研究环星系的形成。由于采用旋涡结构作为靶盘试验星分布函数，不同于Ｔｏｏｍｒｅ采用一系列同心圆作为试验星的分布函数，模拟得到环的结构比Ｔｏｏｍｒｅ的模拟更接近于实际环星系。     

星系形成与宇宙大尺度结构

宇宙大尺度结构的形成与星系形成密切相关,前者的研究把星系基本上视为一质点,而星系形成研究涉及到其内部结构,宇宙大尺度结构形成有两种模式,由小到大与由大到小,这两种模式被交替使用（当然不同简单的重复）很重要的一个原因是星系形成研究的推动。     

星系相互作用的一个结果:环星系形成的数值模拟

通过数值模拟相互作用星系（包括靶星系和入侵星系）的碰撞，研究环星系的形成．由于采用旋涡结构作为靶盘试验星分布函数，不同于Toomre采用一系列同心圆作为试验星的分布函数，模拟得到环的结构比Toomre的模拟更接近于实际环星系．     

椭圆星系的模拟试验

本用树形法,模拟试验由一种星系相互作用模式产生椭圆星系的可能性。     

椭圆星系的模拟试验

本文用树形法,模拟试验由一种星系相互作用模式产生椭圆星系的可能性．并用惯量张量分析方法和曲线拟合对椭球体进行结构分析,得到椭球体的半长轴相对长度．     

核盘晕星系相互作用的模拟研究

用一个星系相互作用数值模拟的综合三体模型，研究椭圆星系是否由盘星系合并而成，结果表明，主要取决于星系的盘面积和星系运行轨道面的夹角，如果夹角不等于零，盘星系的相互作用是可能形成椭圆星系的，否则还是盘星系。     

银河系卫星星系研究进展

对银河系内卫星星系进行全面的"人口普查"具有重要的意义。目前已经发现了二十几个卫星星系,其光度范围分布很广,最暗的矮星系比球状星体还暗。叙述了卫星星系的光度分布、空间分布和动力学性质。总结了观测和理论研究进展,并讨论了星流和伽玛射线在研究银河系结构和暗物质性质方面的贡献。表明了卫星星系的统计分布能用来很好地限制冷暗物质理论和星系形成的相关物理过程,同时指出当前研究的局限性和可能的发展方向。     

星系的转动与黑洞形成的判据

本文通过数值计算解出稳态星系模型，并与椭圆星系及旋涡星系核球的自转和弥散速度曲线相比较。由星系运动曲线传合出的星系半径与从光度曲线拟合出的潮汐半径不一致，后者总是前者的３倍左右，这是由于分布函数的截断方式不妥造成的。我们的结论是：该模型虽不能正确描述整个星系，但却能正确描述星系的较内部区域。我们分析了三个速度曲线资料的星系黑洞候选者：Ｍ１０４、Ｍ３１和银河系，发现它们由于有太快的自转，都不可能在达     

核盘晕星系相互作用的模拟研究

用一个星系相互作用数值模拟的综合三体模型，研究椭圆星系是否由盘星系合并而成。结果表明，主要取决于星系的盘面和星系运行轨道面的夹角。如果夹角不等于零，盘星系的相互作用是可能形成椭圆星系的，否则还是盘星系。     

On-Going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature 403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A 378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10⁸ M _⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H₂. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim 1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’ masses (HI + H₂ + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component. This revised version was published online in September 2006 with corrections to the Cover Date.     

Primordial Galaxy Formation and IGM Reionization

In this talk I will present a model for primordial galaxy formation. In particular, I will review the feedback effects that regulate the process: (i) radiative (i.e. ionizing and H₂-photodissociating photons) and (ii) stellar (i.e. SN explosions) feedback produced by massive stars. I will also address how the IGM reionization can be influenced by this population of primordial galaxies and describe a Monte Carlo method for the radiative transfer of ionizing photons through the IGM. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Epochs of Early-Type Galaxy Formation

By means of population synthesis models with variable α/Fe ratios we derive average ages, metallicities, and [α/Fe] element enhancements for a sample of 126 field and cluster early-type galaxies. We find a clear positive relation between [α/Fe] and velocity dispersion. Zero-point, slope, and scatter of this correlation are the same for cluster and field galaxies. In particular, the [α/Fe] ratios and mean ages of cluster ellipticals are positively correlated. This strongly reinforces the view that the [α/Fe] element enhancement in ellipticals is produced by star formation timescales rather than by variations of the initial mass function. These results indicate that the more massive the galaxy, the shorter is its star formation timescale, and the higher is the redshift of the bulk of star formation. This finding is not compatible with the predictions from models of hierarchical galaxy formation. The lenticular and field galaxies of the investigated sample do not follow the correlation between age and [α/Fe], but contain a non-negligible fraction of galaxies with young average ages and high [α/Fe] ratios. This revised version was published online in September 2006 with corrections to the Cover Date.     

星系形成与演化半解析理论研究的新进展

发展了的星系形成和演化的半解析理论,可以很好地再现不同红移时宇宙恒星形成率密度（ＳＦＲ）和中性气体共动密度的最新观测结果。对该理论各个不确定性因素对结果的影响作了说尽的讨论,并指出在中等红移星系间的相互作用可能是主宰恒星形成的决定性因素,根据宇宙学模型对观测和半解析理论的影响,对宇宙学参数作出限制。     

Galaxy cluster mergers

We present the results of an Eulerian adaptive mesh refinement (AMR) hydrodynamical and N -body simulation in a Λ cold dark matter (ΛCDM) cosmology. The simulation incorporates common cooling and heating processes for a primordial gas. A specific halo finder has been designed and applied in order to extract a sample of galaxy clusters directly obtained from the simulation without considering any resimulating scheme. We have studied the evolutionary history of the cluster haloes, and classified them into three categories depending on the merger events they have undergone: major mergers, minor mergers and relaxed clusters. The main properties of each one of these classes and the differences among them are discussed. The collisions among galaxy clusters are produced naturally by the non-linear evolution in the simulated cosmological volume; no controlled collisions have been considered. We pay special attention to discuss the role of merger events as a source of feedback and reheating, and their effects on the existence of cool cores in galaxy clusters, as well as in the scaling relations.     

Galaxy density profiles and shapes – I. Simulation pipeline for lensing by realistic galaxy models

Studies of strong gravitational lensing in current and upcoming wide and deep photometric surveys, and of stellar kinematics from (integral-field) spectroscopy at increasing redshifts, promise to provide valuable constraints on galaxy density profiles and shapes. However, both methods are affected by various selection and modelling biases, which we aim to investigate in a consistent way. In this first paper in a series, we develop a flexible but efficient pipeline to simulate lensing by realistic galaxy models. These galaxy models have separate stellar and dark matter components, each with a range of density profiles and shapes representative of early-type, central galaxies without significant contributions from other nearby galaxies. We use Fourier methods to calculate the lensing properties of galaxies with arbitrary surface density distributions, and Monte Carlo methods to compute lensing statistics such as point-source lensing cross-sections. Incorporating a variety of magnification bias modes lets us examine different survey limitations in image resolution and flux. We rigorously test the numerical methods for systematic errors and sensitivity to basic assumptions. We also determine the minimum number of viewing angles that must be sampled in order to recover accurate orientation-averaged lensing quantities. We find that for a range of non-isothermal stellar and dark matter density profiles typical of elliptical galaxies, the combined density profile and corresponding lensing properties are surprisingly close to isothermal around the Einstein radius. The converse implication is that constraints from strong lensing and/or stellar kinematics, which are indeed consistent with isothermal models near the Einstein radius, cannot trivially be extrapolated to smaller and larger radii.     

Hydrodynamical Simulations of Galaxy Clusters

In my contribution I discuss the relevance that hydrodynamical simulation of clusters can play to understand the ICM physics and to calibrate mass estimates from X-ray observable quantities. Using hydrodynamical simulations, which cover quite a large dynamical range and include a fairly advanced treatment of the gas physics (cooling, star formation and SN feedback), I show that scaling relations among X-ray observable quantities can be reproduced quite well. At the sametime, these simulations fail at accounting for several observational quantities, which are related to the cooling structure of the ICM: the fraction of stars, the temperature profiles and the gas entropy in central cluster regions. This calls for the need of introducing in simulations suitable physical mechanisms which should regulate the cooling structure of the ICM.     

Star Formation Properties of the Galaxy Cluster Abell 2199

Through the morphological classifications for 290 member galaxies in the nearby galaxy Abell 2199, the star formation rates and their relations with their morphology and related physical properties are investigated in this paper. It is found that the typical star formation rate in galaxies of this galaxy cluster is strongly correlated with the H_α equivalent width, and the degree of discontinuity of the galaxy spectrum at 4000 Å is also strongly correlated with the stellar mass included in the galaxy. It is also found that star formation activities in these galaxies do not exhibit the obvious circumstance effect. This result indicates that this galaxy cluster is still situated at the stage of the violent dynamical evolution and far from the dynamical equilibrium.