The formation of a disk galaxy within a slowly growing dark halo

The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment of the disk ISM with metal-rich gas from the bulge. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation and evolution of disk galaxies

I review several of the current issues in the theory of disk galaxy formation. There is still much to be done, observationally and theoretically, before we can expect to approach an understanding of disk galaxies that is reliable enough to make robust predictions about the high redshift universe. This revised version was published online in August 2006 with corrections to the Cover Date.     

The general equilibria of a spinning satellite in a circular orbit

Let a rigid satellite move in a circular orbit about a spherically symmetric central body, taking into account only the main term of the gravitational torque. We shall investigate and find all solutions of the following problem: Let the satellite be permitted to spin about an axis that is fixed in the orbit frame; the satellite need not be symmetric, the spin not uniform, and the spin axis not a principal axis of inertia. The complete discussion of this type of spin reveals that the cases found by Lagrange and by Pringle - and the well-known spin about a principal axis of inertia orthogonal to the orbit plane — are essentially the only ones possible; the only further (degenerate) case is uniform spin of a two-dimensional, not necessarily symmetric satellite about certain axes that are orthogonal to the plane containing the body and to the orbit of the satellite around the central body.     

On the true circular orbit of a satellite

The osculating orbit of a planetary satellite moving in the equatorial plane of the central body under the influence of a rotational symmetric perturbation force is elliptical in first order approximation even if the true orbit is always circular. The satellite motion is influenced by a resonance effect due to this perturbing force. An inclined true satellite orbit cannot be circular.     

Regular precessions of a gyrostatic satellite in a circular orbit

In the present paper, the regular precessions about the mass center are studied, for a Volterra-type gyrostatic satellite in a circular orbit under a Newtonian force field. All the possible regular precessions are determined.     

The evolution of disk galaxies in clusters

We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10 cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present here the first results of our study, and discuss the implications of our data in the context of current ideas and models of galaxy formation and evolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

The kinematic properties of stars in an oscillating Kuzmin disk

We study the kinematic properties of stars under the combined potential of a Kuzmin disk with a simple radial oscillation and a logarithmic halo. The results are: 1) There exist stable, ordered and near-circular orbits. 2) The effect of the oscillating disk is greater on orbits with smaller angular momenta and on that departly greatly from the near-circular orbits. 3) Most of the motion in the disk is ordered motion. 4) Orbits that depart greatly from the near-circular orbits generally have chaotic motion and may eventually escape. But the actual fraction escaped in one Hubble time is small. 5) Disk oscillation may be one of the mechanisms for the formation and long-term maintenance of some star clusters; the larger the amplitude, the greater may be the number of clusters; for a given disk galaxy, there may be more clusters with small than with large angular momenta.     

The chemo-dynamical evolution of a disk galaxy

I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z= 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. Correspondingly, the inner halo is the oldest component, followed by the outer halo, the bar/bulge, the thick and the thin disk. The bulge in the model consists of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar. This revised version was published online in August 2006 with corrections to the Cover Date.     

The irregular galaxy Cassiopeia 1 – a new member of the Local group

A photometry in the V, R, I (Cousins) system has been performed for stars in the galaxy Cassiopeia 1. The resulting colour-agnitude (CM) diagram of Cas 1 reveals the existence of blue stars and the absence of red ones. From an analysis of the CM diagram of the galaxy it is concluded that A_v = 4.0 mag, and the distance modulus (m – M)_o = 24.5 mag, corresponding to a distance of 790 kpc. The visual magnitude of the galaxy is V = 14^m.62 (M_v = −13^m.8) and the colour index (V – R) = 0.89 mag. From the distance determination the galaxy Cassiopeia 1 is a member of the Local group.     

Photometric observations of galaxy clusters from a new X-ray survey

We present the first results of our photometric observations of galaxy clusters from a new ROSAT X-ray survey of clusters. The observations were begun in the fall of 2000 with the 1.5-m telescope (AZT-22) that recently became operational at TUBITAK National Observatory (Turkey). The first series of observations was carried out in September–November 2000. Magnitudes of the brightest galaxies were measured for seven clusters, and photometric estimates of their redshifts were obtained. These first observations show that the necessary photometry for most galaxy clusters from the X-ray survey can be performed even during this year.     

The influence of mergings on galaxy evolution

The evolution of the galaxy mass function MF (and luminosity function LF) depending on merging of galaxies is discussed. The richness and masses of most massive (and most luminous) galaxies of a cluster are chosen as a characteristic of the bright edge of LF. Mergers are simulated by a Monte-Carlo method. The probability of merging depends on the masses of galaxies. The ratio of the current number of galaxies to the initial one plays the role of the time scale. Transformation to real time and redshift is realized with help of the Smoluchowsky kinetic equation (SE) solution, describing the merging process and making possible to find the dependence of the galaxy number on time. The dependencies of richness, masses and magnitude differences of most massive and brightest galaxies of a cluster on redshift have been obtained. Creation of cD-galaxy has been retraced on small redshifts. The masses of the second and less massive cluster galaxies grow, but after the creation of the cD-galaxy they begin to decrease. Comparison of obtained results with data of the catalogues is provided. Correspondence in mentioned dependencies is observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the periodic solutions of a rigid dumbbell satellite in a circular orbit

The aim of the present paper is to provide sufficient conditions for the existence of periodic solutions of the perturbed attitude dynamics of a rigid dumbbell satellite in a circular orbit.     

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。     

The evolution of groups of very light stars in the galaxy

The evolution of the velocity distribution function of a group of zero mass bodies in the gravitational field of a stellar cluster or a galactic disk is considered. We use the model of a purely discontinuous random process.     

GEO卫星定轨可视化软件设计

介绍了基于Windows系统开发的GEO卫星定轨可视化软件,该软件是采用Microsoft Visual Studio 2005软件平台,利用Visual Basic.NET编程技术开发设计的,具有预处理观测数据资料、解算GEO卫星精密轨道、分析和图形化轨道解算结果等功能。该软件界面友好、可操作性强、方便省时,有效地提高了GEO卫星定轨工作效率。     

Gravitational lensing by stars in a galaxy halo: Theory of combined weak and strong scattering

The theory of gravitational lensing of background quasars by stars in the halo of a galaxy is considered. In the limiting case of small ‘optical depth’, only one star is close enough to the beam to cause strong scattering, and the effect of all the other stars is treated as a perturbation with both systematic and random components. The perturbation coming from weak scattering can increase the number of images and the amplification in those cases where the amplification is already high; such events are preferentially selected in flux limited observations. The theory is applicable to the apparent association of background quasars with foreground galaxies. A comparison with earlier work on the same problem is given. The relevance of these results to gravitational lensing by galaxies as perturbed by random inhomogeneities surrounding the ray path is also briefly discussed.     

Dynamical friction and the escape of a compact supermassive object shot from the center of a galaxy

A numerical study has been made of the motion of a compact object consisting of a supermassive black hole with a dense cluster of stars around through a galaxy which has recoiled from the center of the latter as a result of anisotropic emission of gravitational radiation or asymmetrical plasma emission. We find that the effect of dynamical friction on its motion through the galaxy (mass10¹¹ M ) estimated using the impulsive approximation technique, is minimal for an object mass 10⁹ M and for recoil taking place at a velocity larger than that of escape. A velocity 1.1 times the escape velocity is needed for the object to escape from the galaxy, whereas for velocities of recoil less than this critical velocity, damped oscillatory motion ensures. The energy exchange of the object with the galaxy is not large enough to cause appreciable change in the internal energy of the latter.     

Study of the galaxy DDO 68: New evidence for its youth

DDO 68 (UGC 5340) is the second most metal-poor star-forming galaxy (12 + log(O/H) = 7.14). Its peculiar optical morphology and its HI distribution and kinematics are indicative of a merger origin. We use the u, g, r, and i photometry based on the SDSS images of DDO 68 to estimate its stellar population ages. The Hα images of DDO 68 were used to select several representative regions without nebular emission. The derived colors were analyzed by comparison with the PEGASE2 evolutionary tracks for various star formation (SF) scenarios, including the two extreme cases: (i) an instantaneous starburst and (ii) continuous SF with a constant rate. The (u − g) and (g − r) colors for all of the selected regions are consistent with the scenario of several “instantaneous” SF episodes with ages between ∼0.05 and ∼1 Gyr. The total mass of the visible stars in DDO 68 was estimated by comparing the colors and fluxes of the observed stellar subsystems with PEGASE2 models to be ∼2.4 × 10⁷ M _⊙. This accounts for ∼6% of the total baryonic mass of the galaxy. All of the available data are consistent with the fact that DDO 68 is a very rare candidate for young galaxies. The bulk of its stars were formed during the recent (with the first encounter ∼1 Gyr ago) merger of two very gas-rich disks. DDO 68 is closest in its properties to cosmologically young low-mass galaxies. This article was submitted by the authors in English.     

The formation of a ring galaxy during a head-on collision between a disk and a spherical galaxy

The tidal force effects of a spherical galaxy passing head-on through a disk galaxy have been studied at various regions of the disk galaxy and for various orientations of the disk galaxy with respect to the direction of relative motion of the two galaxies. The density distribution of the disk galaxy is taken to be, (r)=_ce^–4r/R , where _c is the central density andR is the radius of the disk. The density distribution of the spherical galaxy is taken to be that of a oolytrope of indexn=4. It is found that as a result of the collision, through the central parts and the outer parts of the disk galaxy remain intact, the region in between these two regions disrupts. Thus a ring galaxy with a nucleus embedded in the ring-i.e., a ring galaxy of the RN-type, is formed.     

Tests and constraints on theories of galaxy formation and evolution

Tests and constraints on current theories of galaxy formation and evolution are presented. They are derived from observations of the Northern Hubble Deep Field with WFPC2 and NICMOS. Photometric redshifts, extinctions and spectral energy distributions are calculated for all objects in the field. The tests and constraints are derived from the output of this analysis. The tests of the predictions from hierarchical and pure luminosity evolution galaxy evolution calculations favor the hierarchical model. Constraints are provided by the current luminosity function and its evolution to a redshift of 6. This revised version was published online in August 2006 with corrections to the Cover Date.