空间无碰撞激波的数值研究

无碰撞激波是空间等离子体和宇宙等离子体中的重要物理现象。文中评述了数值研究空间无碰撞激波的两种方法－粒子模拟和混合模拟，给出了准垂直和准平行无碰撞激波的数值研究结果。还指出了一些尚未解决的研究问题。     

黑洞吸积数值模拟研究的近期进展

当前,数值模拟已经成为国际上天体物理研究中最重要的研究手段之一。具体针对黑洞吸积领域,调研了数值模拟在该领域的研究情况。首先简单回顾了近20年来黑洞吸积领域数值模拟的发展过程,然后以热吸积流中的风以及超爱丁顿吸积为例,强调说明了数值模拟技术这一新的研究手段为该理论的发展起到的关键性作用。这两个方面是近年来黑洞吸积领域最重要的研究进展。随后指出了两个可望通过数值模拟技术取得重要进展的研究课题,并针对这两个问题,结合我国的实际情况,提出了如何发展黑洞吸积数值模拟的建议。     

巨分子云聚合形成N体模拟中粒子数的效应

通过N体数值模拟以不同粒子数对星系中巨分子云的形成的影响进行了研究。结果表明：在聚合形成机制下，当对不同数目的分子云取相同的平均密度时，基本分子云的有效半径与其相应质量的立方根成正比，巨分子云的碎裂率与模拟基本分子云的数目无关。     

日地空间太阳风的辐射磁流体数值模拟研究

对辐射磁流体力学(RMHD)在内日球太阳风数值模拟研究中的应用进行了简要回顾。在太阳风研究中,太阳风的起源问题是关键科学问题之一,也是近期国际重要空间计划的首要科学目标。首先简述了太阳风起源问题的现有认识,表明新生太阳风如何加速到数百千米每秒是研究太阳风起源问题的关键,它很可能是开场区日冕和太阳风加热的直接后果。简介了太阳风数值模拟中物理描述及数值实现的常用方案,表明自洽地刻画太阳风能量输运过程需要合理计入辐射效应。最后简述了多维太阳风数值模拟研究的进展,特别关注了辐射的处理,并对RMHD在内日球太阳风数值模拟研究中的应用做了展望。     

行星形成数值模拟的研究现状与未来需求Ⅱ:行星迁移、核吸积以及大气逃逸

介绍了行星形成涉及到的几个重要阶段,如行星迁移、核吸积和大气逃逸的基本物理过程和数值模拟研究现状。行星迁移会影响行星的轨道偏心率和倾角,并改变原行星盘的结构;核吸积是决定行星演化为类地行星或者类木行星的关键因素;大气逃逸对行星的气候和演化产生重要影响。这些过程均涉及到复杂的辐射磁流体动力学过程,早期的理论研究往往采用很多人为的简化假设。随着超级计算机计算能力的提高和磁流体数值模拟算法的日渐成熟,人们已经可以对这类复杂的非线性动力学问题开展直接的数值模拟研究,克服了早期理论研究中人为假设的局限。但是,目前人们开展的研究主要基于磁流体动力学数值模拟,对辐射转移如何影响磁流体动力学过程的研究还比较欠缺。强调了进行辐射磁流体动力学数值模拟的必要性和紧迫性。针对辐射磁流体数值模拟程序的开发,从辐射转移,磁场的处理,吸积盘的自引力、三维效应、非理想效应和尘埃的效应等方面提出了相应的技术需求。介绍了本研究领域内发展辐射磁流体数值模拟的策略。     

太阳耀斑研究进展和展望

简要回顾了近年来对太阳耀斑研究在某些方面所取得的进展，这些领域空间和地面观测，耀斑光谱研究，耀斑的动力学模型和MHD数值模拟等，并对耀斑研究的前景作一简短的展望。     

暗晕子结构的数值搜寻与统计性质

宇宙学数值模拟预言的暗晕及其子结构的统计性质,被广泛应用于星系形成模型、近邻宇宙学、红移巡天、宇宙大尺度结构、引力透镜和暗物质搜寻等研究领域。在精确宇宙学时代,随着天文观测能力的不断增长,人们对模型精度提出了更高的要求,因此,数值模拟及其分析工具将继续起到不可替代的巨大作用。数值模拟中物质结构的性质往往与具体的搜寻算法有关,从最初通过数值模拟发现子晕到现在的20年里,结构搜寻算法有了极大的发展,新算法不断涌现。了解不同数值搜寻算法的实现及其差异,有助于分析模拟结果的系统差和不确定性,增强研究的可重复性。通过总结现代子晕数值搜寻的基本方法,并以几种典型的搜寻算法为例,探讨了它们之间的异同,及其对相关研究的影响;回顾了子晕的基本统计性质,包括质量函数、空间分布和密度轮廓等,以及这些统计结果对搜寻算法的依赖。此外,还讨论了重子物理的影响。     

黑洞吸积盘的数值模拟

近十几年来,人们已经对吸积盘进行了深入的研究。前人很多工作都是解析工作。虽然解析工作能够简单明了地揭示盘的许多重要性质,但吸积盘的很多重要性质(例如:对流和外流)没有办法通过解析工作进行深入的研究。数值模拟是研究天体物理的重要手段。近十年来,随着计算机技术的迅速发展,数值模拟在天体物理研究中扮演了越来越重要的角色。本论文主要用数值模拟的手段研究吸积盘中一些重要的物理过程。     

基于深度学习的无碰撞引力N体数值模拟的可行性研究

提出了用深度神经网络代替快速傅里叶变换法求解无碰撞引力N体数值模拟方法PM-Tree(Partical Mesh Tree)中的势能,以提升PM-Tree方法的效率,验证深度学习方法加速无碰撞引力N体数值模拟的可行性.无碰撞引力N体数值模拟对研究星系、暗物质晕以及宇宙大尺度结构的形成和演化有重要意义.无碰撞引力N体数值...     

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。     

The connection between globular cluster systems and the host galaxies

A large number of early-type galaxies are now known to possess blue and red subpopulations of globular clusters. We have compiled a data base of 28 such galaxies exhibiting bimodal globular cluster colour distributions. After converting to a common V – I colour system, we investigate correlations between the mean colour of the blue and red subpopulations with galaxy velocity dispersion. We support previous claims that the mean colours of the blue globular clusters are unrelated to their host galaxy. They must have formed rather independently of the galaxy potential they now inhabit. The mean blue colour is similar to that for halo globular clusters in our Galaxy and M31. The red globular clusters, on the other hand, reveal a strong correlation with galaxy velocity dispersion. Furthermore, in well-studied galaxies the red subpopulation has similar, and possibly identical, colours to the galaxy halo stars. Our results indicate an intimate link between the red globular clusters and the host galaxy; they share a common formation history. A natural explanation for these trends would be the formation of the red globular clusters during galaxy collapse.     

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

Dwarf galaxies: Important clues to galaxy formation

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution. This revised version was published online in August 2006 with corrections to the Cover Date.     

An MHD gadget for cosmological simulations

Various radio observations have shown that the hot atmospheres of galaxy clusters are magnetized. However, our understanding of the origin of these magnetic fields, their implications on structure formation and their interplay with the dynamics of the cluster atmosphere, especially in the centres of galaxy clusters, is still very limited. In preparation for the upcoming new generation of radio telescopes (like Expanded Very Large Array, Low Wavelength Array, Low Frequency Array and Square Kilometer Array), a huge effort is being made to learn more about cosmological magnetic fields from the observational perspective. Here we present the implementation of magnetohydrodynamics (MHD) in the cosmological smoothed particle hydrodynamics (SPH) code gadget . We discuss the details of the implementation and various schemes to suppress numerical instabilities as well as regularization schemes, in the context of cosmological simulations. The performance of the SPH–MHD code is demonstrated in various one- and two-dimensional test problems, which we performed with a fully, three-dimensional set-up to test the code under realistic circumstances. Comparing solutions obtained using athena , we find excellent agreement with our SPH–MHD implementation. Finally, we apply our SPH–MHD implementation to galaxy cluster formation within a large, cosmological box. Performing a resolution study we demonstrate the robustness of the predicted shape of the magnetic field profiles in galaxy clusters, which is in good agreement with previous studies.     

Spatially resolved kinematics and stellar populations of brightest cluster and group galaxies

We present an examination of the kinematics and stellar populations of a sample of three brightest group galaxies (BGGs) and three brightest cluster galaxies (BCGs) in X-ray groups and clusters. We have obtained high signal-to-noise ratio Gemini/Gemini South Multi-Object Spectrograph (GMOS) long-slit spectra of these galaxies and use Lick indices to determine ages, metallicities and α-element abundance ratios out to at least their effective radii. We find that the BGGs and BCGs have very uniform masses, central ages and central metallicities. Examining the radial dependence of their stellar populations, we find no significant velocity dispersion, age, or α-enhancement gradients. However, we find a wide range of metallicity gradients, suggesting a variety of formation mechanisms. The range of metallicity gradients observed is surprising, given the homogeneous environment these galaxies probe and their uniform central stellar populations. However, our results are inconsistent with any single model of galaxy formation and emphasize the need for more theoretical understanding of both the origins of metallicity gradients and galaxy formation itself. We postulate two possible physical causes for the different formation mechanisms.     

The star formation histories of low surface brightness galaxies

We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parametrize in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high-metallicity earlier types to much younger and lower-metallicity late-type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having lower ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between an LSBG's star formation history and its K -band surface brightness, K -band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of an LSBG primarily correlates with its surface density and its metallicity correlates with both its mass and its surface density.     

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.     

The anatomy of the NGC 5044 group – II. Stellar populations and star formation histories

The distribution of galaxy properties in groups and clusters holds important information on galaxy evolution and growth of structure in the Universe. While clusters have received appreciable attention in this regard, the role of groups as fundamental to formation of the present-day galaxy population has remained relatively unaddressed. Here, we present stellar ages, metallicities and α-element abundances derived using Lick indices for 67 spectroscopically confirmed members of the NGC 5044 galaxy group with the aim of shedding light on galaxy evolution in the context of the group environment.
We find that galaxies in the NGC 5044 group show evidence for a strong relationship between stellar mass and metallicity, consistent with their counterparts in both higher and lower mass groups and clusters. Galaxies show no clear trend of age or α-element abundance with mass, but these data form a tight sequence when fitted simultaneously in age, metallicity and stellar mass. In the context of the group environment, our data support the tidal disruption of low-mass galaxies at small group-centric radii, as evident from an apparent lack of galaxies below  ∼10⁹ M_⊙  within ∼100 kpc of the brightest group galaxy. Using a joint analysis of absorption- and emission-line metallicities, we are able to show that the star-forming galaxy population in the NGC 5044 group appears to require gas removal to explain the ∼1.5 dex offset between absorption- and emission-line metallicities observed in some cases. A comparison with other stellar population properties suggests that this gas removal is dominated by galaxy interactions with the hot intragroup medium.     

Recovering physical parameters from galaxy spectra using MOPED

We derive physical parameters of galaxies from their observed spectra using MOPED, the optimized data compression algorithm of Heavens, Jimenez & Lahav. Here we concentrate on parametrizing galaxy properties, and apply the method to the NGC galaxies in Kennicutt's spectral atlas. We focus on deriving the star formation history, metallicity and dust content of galaxies. The method is very fast, taking a few seconds of CPU time to estimate ∼17 parameters, and is therefore specially suited to studying large data sets, such as the Anglo-Australian two-degree-field (2dF) galaxy survey and the Sloan Digital Sky Survey (SDSS). Without the power of MOPED, the recovery of star formation histories in these surveys would be impractical. In Kennicutt's atlas, we find that for the spheroidals a small recent burst of star formation is required to provide the best fit to the spectrum. There is clearly a need for theoretical stellar atmospheric models with spectral resolution better than 1 Å if we are to extract all the rich information that large redshift surveys contain in their galaxy spectra.     

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.