Flattened Jaffe models for galaxies

This paper introduces a class of galactic models which extend Jaffe's spherical models to axisymmetric systems, and then studies the properties of their densities and two-integral even distribution functions. The models have finite total mass and finite densities which, at large distances, decay radially like r ⁻⁴ except on the major axis, and like r ⁻³ on the major axis. The more flattened the galaxy, the stronger is the dependence of the even distribution functions on the angular momenta of its stars. Their distribution functions can be obtained by using the maximum entropy principle or assuming the anisotropy of the models. In particular, some formulae analogous to those of Hunter & Qian are obtained to calculate two-integral odd distribution functions, and they can be applied to obtain the distribution functions under the assumption of anisotropy for the oblate models.     

Oscillations In Galaxies

Oscillations in galaxies have been investigated by numerical simulations. The various models used have density distributions corresponding to that of polytrope of index n in the range 0 ≤ n ≤ 4 and their evolution has been followed for more than 70 crossing times. The kinetic energy shows regular and smooth oscillations for models with n = 0, 1 and 2 whereas in other models it shows noisy oscillation. The oscillation in kinetic energy is observed to have a period of 3 crossing time irrespective of the density and size of the galaxy. The amplitude of oscillation is seen to decrease as the central density of the galaxy increases. This revised version was published online in July 2006 with corrections to the Cover Date.     

人为动力泥沙灾害类型及其特征初步研究

人为动力泥沙灾害是人类活动无意地导致地表物质侵蚀、输移、沉积过程中的渐近或超临界现象 ,它具有独特的社会学及灾害学属性。按人为驱动力形式、灾害所在地貌部位、灾害的表现形式及具体灾害现象 ,人为动力泥沙灾害系列可划分为 :流域水系、沟道坡面、河道及平原河口海岸四个泥沙灾害类 ;各灾害类包括侵蚀型、搬运型、堆积型、复杂型及关联型五种泥沙灾害型 ;共 54个灾害种。这是一种比较综合、系统的人为动力泥沙灾害类型划分。从流水地貌、泥沙运动及灾害学相结合出发 ,人为动力泥沙灾害具有以下特征 :1地理地带基础上的加剧性特征 ;2人文环境影响下的渐变性与急变性特征 ;3盲目性驱动下的无序性特征 ;4复杂性和非线性特征等。人文环境的变化是人为动力泥沙灾害产生的主要原因。只有加强管理力度 ,提高人民的环境保护意识 ,才能控制人为泥沙灾害的发生。     

成都市可持续发展模拟与调控

利用系统动力学模型,以成都市经济、人口、资源、环境四个子系统之间的相互制约关系为基础,对整个系统进行分析和模拟,结果表明符合实例情况和发展趋势,因此该系统具有相当的可信度。通过各种参数的调整和输入,研究了整个系统未来的可持续发展概况及调控对策。该种模拟和预测方法,对于区域可持续发展的定量研究提供一定的途径。     

The mass distribution in the innermost regions of spiral galaxies

We use high-spatial resolution (100 pc) rotation curves of 83 spiral galaxies to investigate the mass distribution of their innermost kpc. We show that, in this region, the luminous matter completely accounts for the gravitational potential and no dark component is required. The derived I-band disk mass-to-light ratios agree well with those obtained from population synthesis models and correlate with color in a similar way. We find strict upper limits of 10⁷ M for the masses of compact bodies at the center of spirals, ruling out that these systems host the remnants of the quasar activity.     

The extended rotation curve and the dark matter halo of M33

We present the 21-cm rotation curve of the nearby galaxy M33 out to a galactocentric distance of 16 kpc (13 disc scalelengths). The rotation curve keeps rising out to the last measured point and implies a dark halo mass ≳5×10¹⁰ M_⊙. The stellar and gaseous discs provide virtually equal contributions to the galaxy gravitational potential at large galactocentric radii, but no obvious correlation is found between the radial distribution of dark matter and the distribution of stars or gas.
Results of the best fit to the mass distribution in M33 picture a dark halo which controls the gravitational potential from 3 kpc outward, with a matter density which decreases radially as R ^−1.3. The density profile is consistent with the theoretical predictions for structure formation in hierarchical clustering cold dark matter (CDM) models, and favours lower mass concentrations than those expected in the standard cosmogony.     

Time evolution of galactic warps in prolate haloes

A recent observation with the Hipparcos satellite and some numerical simulations imply that the interaction between an oblate halo and a disc is inappropriate for the persistence of galactic warps. Following on from this , we have compared the time evolution of galactic warps in a prolate halo with that in an oblate halo. The haloes were approximated as fixed potentials, while the discs were represented by N -body particles. We have found that the warping in the oblate halo continues to wind up, and finally disappears. On the other hand, for the prolate halo model, the precession rate of the outer disc increases when the precession of the outer disc recedes from that of the inner disc, and vice versa. Consequently, the warping in the prolate halo persisted to the end of the simulation by retaining the alignment of the line of nodes of the warped disc. Therefore, our results suggest that prolate haloes could sustain galactic warps. The physical mechanism of the persistence of warp is discussed on the basis of the torque between a halo and a disc and that between the inner and outer regions of the disc.     

Approximate third integrals for axisymmetric potentials using local Stäckel fits

We use a set of Stäckel potentials to obtain a local approximation for an effective third integral in axisymmetric systems. We present a study on the feasibility and effectiveness of this approach. We apply it to three trial potentials of various flattenings, corresponding to nearly ellipsoidal, discy and boxy density isophotes. In all three cases, a good fit to the potential requires only a small set of Stäckel potentials, and the associated Stäckel third integral provides a very satisfactory, yet analytically simple, approximation to the trial potentials effective third integral.     

The local density of matter mapped by Hipparcos

We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102±0.010 pc⁻³, which may be compared with an estimate of 0.095 M_⊙ pc⁻³ in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.