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。     

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。     

后发团盘星系自转轴方向的分布：统计样本

为研究后发团盘星系自转轴方向的分布,本文对后发团的两个星系表进行交叉证认,给出后发团中心2°.63×2°.63范围内,完整到m_p≤15.7等的138个盘星系的方位角表,其中包括94个S0星系.44个S和Irr星系。     

盘星系质量密度和扁度的分析

本文应用盘星系的倾角、半径及质量资料,求得93个盘星系的质量密度和真扁度。分析其结果得出,盘星系的质量密度从早型向晚型增大了一个量级;真扁度下降在sm,d达到极小。但去除so系及IrII之后,盘星系的这两项参数都是星系类型的单调函数。     

室女团盘星系自转向量的分布Ⅰ.室女天区310个盘星系的表

为进一步研究室女星系团盘星系自转向量的分布,本文编制了该天区有UGC发表的方位角或直径数据的310个盘星系的表(81个S0系,229个S Irr系),其中有UGC发表的方位角的星系为245个(67个S0系,178个S Irr系)。对能证认身份的所有星系的星系云,群归属均已在表中注明。本文还讨论了表列星系的一些基本统计特征,为室女团盘系自转向量分布的研究提供了迄今为止可能是最大的,3组完整的代表性样品。     

晚型旋涡星系及其盘的颜色星等关系

从Sloan数字巡天(Sloan Digital Sky Survey)第二批释放的数据(Data ReleaseTwo,简称DR2)中,选择了395个在r波段亮于15等的晚型旋涡星系作为样本,研究了这些星系及其盘的颜色星等关系.结果表明晚型旋涡星系及其盘的三个颜色g-r,r-z和g-z均与r波段的绝对星等有紧密的相关关系,越亮的星系(或盘),颜色越红,而且星系的相关性比盘的更强.     

星系振动的动力学研究进展

概括地介绍了盘星系的运动学边缘弯曲现象、盘星系中心的HI发射线空洞和棒旋星系中核球的长轴和棒的长轴不互相平行也不互相垂直等星系振动的问接观测证据;综述了星系振动动力学的理论研究情况,主要介绍了利用非静态维里方程和N体数值模拟得到的有关结果;展望了该领域今后的发展趋势,指出了建立引力自治的星系振动模型的必要性。     

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...     

盘星系的内禀扁度研究

盘星系的内禀扁度对计算星系的空间倾角非常重要．对从LEDA数据库中选取的14988个盘星系进行内禀扁度的统计分析．研究表明，盘星系的内禀扁度与星系的形态密切相关．整体上说，透镜星系和不规则星系的内禀扁度qo大于旋涡星系，早型旋涡星系的内禀扁度qo大于晚型旋涡星系，其中Scd星系的内禀扁度qo最小．利用所得的qo-T关系，还对16个已知倾角的亮星系进行了倾角计算，发现与其他方法估计的空间倾角符合得较好．     

星系盘厚度效应的研究

在三维引力Poisson方程严格解基础上，探讨了有限厚星系盘基盘的动力学性质，并进一步讨论了盘的厚度效应对银河系所需晕质量的影响。研究了扰动盘的动力学性质，通过将扰动引力势Poisson方程的严格解与林家翘、徐遐生提出的自维持密度波理论相结合，建立了三维旋涡星系有限厚盘上密度波的色散关系。在此色散关系的基础上讨论了盘的局域稳定性，研究了旋涡星系旋臂的形态、三维盘状星系密度波的群速度。研究表明厚度是星系盘研究中不容忽略的重要参量。另外在有限厚盘星系密度波色散关系的基础上还探讨了一种确定星系厚度的新方法。     

On the origin of warps and the role of the intergalactic medium

There is still no consensus as to what causes galactic discs to become warped. Successful models should account for the frequent occurrence of warps in quite isolated galaxies, their amplitude as well as the observed azimuthal and vertical distributions of the H  i layer. Intergalactic accretion flows and intergalactic magnetic fields may bend the outer parts of spiral galaxies. In this paper we consider the viability of these non-gravitational torques to take the gas off the plane. We show that magnetically generated warps are clearly flawed because they would wrap up into a spiral in less than two or three galactic rotations. The inclusion of any magnetic diffusivity to dilute the wrapping effect causes the amplitude of the warp to damp. We also consider the observational consequences of the accretion of an intergalactic plane-parallel flow at infinity. We have computed the amplitude and warp asymmetry in the accretion model, for a disc embedded in a flattened dark matter halo, including self-consistently the contribution of the modes with azimuthal wavenumbers   m = 0  and   m = 1  . Since the m = 0 component, giving a U-shaped profile, is not negligible compared to the m = 1 component, this model predicts quite asymmetric warps, maximum gas displacements on the two sides in the ratio 3 : 2 for the preferred Galactic parameters, and the presence of a fraction ∼3.5 per cent of U-shaped warps, at least. The azimuthal dependence of the moment transfer by the ram pressure would produce a strong asymmetry in the thickness of the H  i layer and asymmetric density distributions in z , in conflict with observational data for the warp in our Galaxy and in external galaxies. The amount of accretion that is required to explain the Galactic warp would give gas scaleheights in the far outer disc that are too small. We conclude that accretion of a flow with no net angular momentum cannot be the main and only cause of warps.     

Recent investigations on disk galaxies in massive halos

Numerical experiments undertaken to investigate the longevity and behavior of dark-lane elliptical galaxies are described. This is dynamically the same problem as a disk galaxy in a massive halo. Spiral galaxies are disks from a dynamical point of view. A disk of particles embedded in a self-consistent galaxy provides the basic model used for the experiments. This model is applicable to ordinary disk galaxies if the disk is interpreted as the visible galaxy and the galaxy is interpreted as the massive halo thought to be present around disk galaxies. Fully three-dimensional fully self-consistentn-body computer programs that can handle 100,000 particles are used for the experiments. The background galaxy is oblate, and the disk is inclined to the axis of the oblate galaxy, so the disk precesses differentially to produce a warp. A surprising result is that the galaxy center shifted, leaving the disk center orbiting around the galaxy center. This produces interesting phenomena reminiscent of observations in the region of the Galactic center.     

Dynamical evolution of galactic disks driven by interaction with a satellite

Dynamical evolution of galactic disks driven by interaction with satellite galaxies, particularly the problem of the disk warping and thickening is studied numerically. One of the main purpose of the study is to resolve the long standing problem of the origin of the disk warping. A possible cause of the warp is interaction with a satellite galaxy. In the case of the Milky Way, the LMC has been considered as the candidate. Some linear analysis have already given a positive result, but one had to wait for a fully self-consistent simulation as a proof. I have accomplished the numerical simulations with a million particles, by introducing a hybrid algorithm, SCF-TREE. Those simulations give us quantitative estimates for the Milky Way system. We have found an example in which large warp amplitudes are developed. We also found that the warp amplitudes depend on the halo distribution. Among our three models, the most massive and spherical halo is preferable for the observable warp excitation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Contribution of the Large Magellanic Cloud to the Galactic warp

Multi-scale interaction between the LMC, the Galactic halo, and the disk is examined with N-body simulations, and precise amplitudes of the Galactic warp excitation are obtained. The Galactic models are constructed most realistically to satisfy available observational constraints on the local circular velocity, the mass, surface density and thickness of the disk, the mass and size of the bulge, the local density of the halo matter at the solar radius, and the mass and orbit of the LMC. The mass of the halo within R=50 kpc is set to about 5×10¹¹ M_⊙. Since the observational estimate of the mass distributed in outer region has large ambiguity, two extreme cases are examined; M(<170 kpc)=2.1 and 0.9×10¹² M_⊙. LMC is orbiting in a ellipse with apocentric radii of 100 kpc, thus the main difference between our two models is the mass density in the satellite orbiting region, so that our study can clarify the role of the halo on excitation of the warp.By using hybrid algorithm (SCF–TREE) I have succeeded to follow the evolution with millions of particles. The orbiting satellite excites density enhancement as a wake, and the wake exerts a tidal force on the disk. Because of the additional torque from the wakes in the halo, the amplitudes of the induced warps are much larger than the classical estimate by Hunter and Toomre [ApJ 155 (1969) 747], who considered only the direct torque from the LMC. The obtained amplitudes of m=0, 1, 2 warps in the larger halo model show very good agreement with the observed amplitude in the Milky Way. This result revives the LMC as a possible candidate of the origin of the Galactic warp. Our smaller halo model, however, yield only weak warps in all the harmonic modes. Therefore, the halo still has significant influence on excitation of warp even in the interaction scenario for excitation of warps.     

The onset of warps in Spitzer observations of edge-on spiral galaxies

We analyse warps in the nearby edge-on spiral galaxies observed in the Spitzer /Infrared Array Camera (IRAC) 4.5-μm band. In our sample of 24 galaxies, we find evidence of warp in 14 galaxies. We estimate the observed onset radii for the warps in a subsample of 10 galaxies. The dark matter distribution in each of these galaxies are calculated using the mass distribution derived from the observed light distribution and the observed rotation curves. The theoretical predictions of the onset radii for the warps are then derived by applying a self-consistent linear response theory to the obtained mass models for six galaxies with rotation curves in the literature. By comparing the observed onset radii to the theoretical ones, we find that discs with constant thickness can not explain the observations; moderately flaring discs are needed. The required flaring is consistent with the observations. Our analysis shows that the onset of warp is not symmetric in our sample of galaxies. We define a new quantity called the onset-asymmetry index and study its dependence on galaxy properties. The onset asymmetries in warps tend to be larger in galaxies with smaller disc scalelengths. We also define and quantify the global asymmetry in the stellar light distribution, that we call the edge-on asymmetry in edge-on galaxies. It is shown that in most cases the onset asymmetry in warp is actually anticorrelated with the measured edge-on asymmetry in our sample of edge-on galaxies and this could plausibly indicate that the surrounding dark matter distribution is asymmetric.     

Galactic dynamos and galactic winds

Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 μG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 μG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The halo scale height and the electron lifetime allow to estimate the wind speed. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. There is no observation yet of a halo with a large-scale coherent dynamo pattern. A global wind outflow may prevent the operation of a dynamo in the halo. Halo regions with high degrees of radio polarization at very large distances from the disk are excellent tracers of interaction between galaxies or ram pressure of the intergalactic medium. The observed extent of radio halos is limited by energy losses of the cosmic-ray electrons. Future low-frequency radio telescopes like LOFAR and the SKA will allow to trace halo outflows and their interaction with the intergalactic medium to much larger distances.     

The local environment of H ii galaxies

We have carried out an investigation of the environments of low redshift H  ii galaxies by cross-correlating their positions on the sky with those of faint field galaxies in the Automatic Plate Measuring Machine (APM) catalogues. We address the question of whether violent star formation in H  ii galaxies is induced by low-mass companions by statistically estimating the mean space density of galaxies around them. We argue that even if low-mass companions were mainly intergalactic H  i clouds, their optical counterparts should be detectable at faint limits of the APM scans.
A significantly positive signal is detected for the H  ii galaxy–APM galaxy angular cross-correlation function, but the amplitude is poorly determined. The projected cross-correlation function has a higher signal-to-noise ratio, and suggests that the amplitude is slightly lower than for normal field galaxies. This implies that these bursting dwarf galaxies inhabit slightly lower density environments than those of normal field galaxies, consistent with other studies of emission-line galaxies. This suggests that in these dwarf starburst galaxies, star formation is not always triggered by tidal interactions, and a significant fraction must have a different origin.     

Intergalactic Pressure and the Maximal Extent of Gaseous Haloes of Field Galaxies

The problem of influence of the ambiental intergalactic pressure on extended gas associated with normal field galaxies is briefly discussed. An empirically established characteristic radius of absorption can not be explained in the context of simple two-phase halo model as a consequence of pressure equilibrium of the hot halo with the smooth intergalactic medium. This conclusion is based upon the stringent constraints on the temperature and density of the intergalactic plasma obtained through the CMBR measurements. On the other hand, ram-pressure stripping caused by peculiar motions of galaxies does present a viable alternative for the hot halo truncation. It is shown that for a particular set of chosen parameters, a simple model is capable of producing the absolute upper limit on the extent of gas associated with the galaxy. The values obtained are compatible with the results of the recent QSO absorption-line studies, and are significantly higher than the radius at which the cooling timescale of the halo gas equates the dynamical timescale.     

Tidal torques and galactic warps

%We study the evolution of galactic disks subject to tidal torques motivated by cosmological N-body simulations using analytic and numerical techniques. We find that self-gravitating disks subject to these torques resemble observed warped galaxies. The warps develop at a local surface density of 70 M _⊙ pc^-2 and move out through the disk at a rate that depends on the surface density of the disk. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of projection on the observed gas velocity fields in barred galaxies

The problem of determining the pattern of gas motions in the central regions of disk spiral galaxies is considered. Two fundamentally different cases—noncircular motions in the triaxial bar potential and motions in circular orbits but with orientation parameters different from those of the main disk—are shown to have similar observational manifestations in the line-of-sight velocity field of the gas. A reliable criterion is needed for the observational data to be properly interpreted. To find such a criterion, we analyze two-dimensional nonlinear hydrodynamic models of gas motions in barred disk galaxies. The gas line-of-sight velocity and surface brightness distributions in the plane of the sky are constructed for various inclinations of the galactic plane to the line of sight and bar orientation angles. We show that using models of circular motions for inclinations i>60° to analyze the velocity field can lead to the erroneous conclusions of a “tilted (polar) disk” at the galaxy center. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photometric and dynamical axes. As an example, we consider the velocity field of the ionized gas in the galaxy NGC 972.