Spiral structure formed in a pair of interacting galaxies

The formation of spiral structure in a galaxy, as a result of the gravitational perturbation caused by a permanent companion, is studied. It is found that spiral structure appears only when a resonance exists between the rotational frequency of the stars in the galaxy and the rotational frequency of the companion galaxy. The number of spiral arms depends strongly on the particular resonance. In the case where the companion moves in an elliptic orbit, spiral arms are formed when a resonance, inside the galactic body, exists in almost all the parts of the orbit or, at least, in the largest part of it.     

Leading spiral arms,retrograde galaxy encounters and massive halos

We study, theoretically and with N-body simulations, the formation of spiral patterns in retrograde galaxy encounters. A one-armed leading spiral dominates in a disk if the tidal perturbation from the companion is large and the disk is surrounded by a massive halo. Otherwise, a trailing pattern forms. The leading arm is made up of particles in slightly elongated orbits whose turning points outline the arm. The arm rotates opposite to the disk rotation. We have found one spiral galaxy, NGC4622, with a leading arm near its nucleus. From the literature, we find that very few spirals, if any, in a sample of strongly perturbed galaxies have leading arms. A possible reason for this is that few spiral galaxies have a halo with larger mass than the disk within the visible disk.     

旋涡星系中旋臂的维持

为了避开旧物质臂理论中旋臂的缠绕困难，本文提出了旋涡星系的循环假设，并在文中提供了旋涡星系的双臂、气体层反卷、银河系中旋臂物质径向向内的速度分量和棒旋星系中棒物质沿着棒向内的流动等观测证据，进而还尝试利用此循环假设去解释旋臂物质的平自转曲线和棒旋星系的棒结构等的成因。     

早型旋涡星系M81 (NGC 3031)的形态学解构研究

利用星系解构软件GALFIT通过面亮度轮廓拟合对近邻早型旋涡星系M81 (NGC 3031)进行形态学解构,旨在探究M81星系的结构组成并对其进行形态学量化.通过6种解构模式,对M81进行了不同复杂程度的结构分解,其中最复杂的解构模式包含核球、盘、外旋臂、内旋臂、星系核5个子结构.研究结果显示, M81有一个Sérsic指数约为5.0的经典核球,其形态和光度在不同解构模式中均保持稳定; M81星系盘的Sérsic指数约为1.2,但它的形态参数和光度与是否分解内旋臂相关.不同子结构的组合对作为混合体的星系整体的形态有不可忽视的影响.星系解构的结果提供了不同解构模式适用性的建议:其中核球+盘+星系核的三成分解构适用于大样本星系的核-盘研究;而考虑旋臂的复杂解构则适合于对星系子结构的精确测量,如小样本(或个源)研究.基于Spitzer-The Infrared Array Camera (IRAC) 4.5μm的单波段图像的形态学解构研究是后续一系列研究的开始,在此基础上未来将会对M81进行多波段解构,同时研究不同子结构的光谱能量分布和星族性质,并推断M81各子结构的形成历史和演化过程.     

Maintenance of spiral arms and galactic magnetic field configurations

Of the various proposed mechanisms to maintain spiral arms in spiral galaxies, three have been supported by observations, statistics, or theories (bar, companion, extended solid-body rotation curve). It is shown here that in the presence of a central bar or oval distirtion to maintain spiral arms, the global magnetic field lines also follow the spiral shape of the arms. Excluding then barred galaxies, it is confirmed that in the presence of a companion galaxy to maintain spiral arms, the global magnetic lines in a spiral galaxy will either follow thespiral shape of the arms (when tides are larger), or else will follow thering shape of the orbit of matter crossing spiral arms (when tides are small). In the presence of an extended solid-body rotation curve to maintain spiral arms within the solid-body rotation region, the global magnetic field lines also follow the spiral shape of the arms.The results above do not favour the hypothesis that a weak intergalactic magnetic field could have been amplified enough by gravitational contraction of a protogalaxy to give rise to the observed strength of galactic magnetic fields. On the contrary, leakage of galactic magnetic fields into intergalactic/cosmological space is expected.     

Geometry of Spiral Galaxy Arms

The previous work (He, 2001) on image analysis of over one hundred nearby galaxies indicates that for any spiral galaxy image of small inclination there is evidence of two sets of iso-ratio curves in different scales which, like curvilineal coordinate lines, expand over different galaxy components. This result sets constraint on galaxy patterns and a general equation which carries this constraint is presented in the current paper. The equation has clear geometrical meaning. Its much simplified version which applies to galaxy arms, when solved, may be a guidance to understanding galaxy patterns. This revised version was published online in July 2006 with corrections to the Cover Date.     

The gigantic interacting galaxy NGC 6872

The luminous barred galaxy NGC 6872 is one of the largest spiral galaxies known. Star formation is traced all along the arms, which extend over more than 100 kpc. The galaxy is likely to be affected by tidal perturbations from the nearby companion IC 4970 passing by on a low-inclination, prograde orbit. We have mapped the large-scale distribution and kinematics of the atomic gas in the NGC 6872/IC 4970 system and carried out N-body simulations. Our aim is to investigate the evolution of gas and stars in such close encounters, examine the influence of a dark matter halo on the length of the tidal tails, and test models of collisionally induced star formation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamical friction on a satellite of a disk galaxy: The circular orbit

In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q ₀ 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios.     

Classes of ring galaxies generated by dynamical friction

Dynamical friction is incorporated in the numerical modeling of colliding ring galaxies. We show that dynamical friction may lead to the capture of an initially unbound companion into a damped oscillatory orbit before ultimately merging with the target disk. In the late stages of its motion the companion may be virtually merged with the main galaxy while rings are still propagating in the disk. This raises the possibility that there exist (at least) two classes of ring galaxies: a class of ring galaxies with separate detached companions in which dynamical friction is too weak to have captured the collider, and a second class with no apparent companion in which the latter has either merged with the target galaxy or has disrupted. These two classes could possibly be identified with O-type and P-type ring galaxies of Few and Madore (1986).     

On spiral structure formation in a galaxy with a bar-like nucleus

A new approach to the problem of the formation of galaxy spiral structures having a rotating bar-like nucleus is offered. The process of disk formation due to matter accretion onto the disk is considered in terms of the solution of the key problem on the motion of the matter element in the equatorial plane of the galaxy in the corotation resonance region. It is shown that in the vicinity of unstable libration points high-density regions are formed, which elongate with time following the separatrix shape and forming thereby spiral arms.     

Stochastic stellar orbits in a pair of interacting galaxies

A dynamical model composed of a disk galaxy with an elliptic companion, moving in a circular orbit, is used in order to study the stellar orbits in a binary galaxy. Using the Poincare surface of section we study the evolution of the stochastic regions in the primary galaxy considering the mass of the companion or the value of the Jacobi’s integral as a parameter. Our numerical calculations suggest that the regions of stochasticity increase, as the mass of the companion or the value of the Jacobi’s integral increase. An interesting observation is that only direct orbits become stochastic.     

Distributions of Constant Directional Derivatives and a Model of Spiral Galaxy Arms

Image analysis of over one hundred nearby galaxies (He, 2003) indicates that for any spiral galaxy image of small inclination there is evidence of two orthogonalsets of iso-ratio curves in different scales which, like curvilinear coordinate lines, expand over different galaxy components. This result sets a constraint on galaxy patterns and a general model equation which carries this constraint is given in its geometric form in He (2003). In the current paper the author reformulates the geometric equation into one quasilinearsystem of 3 partial differential equations with 3 dependents (the tangent direction of one set of iso-ratio curves and the two ratio profiles along the two sets of iso-ratio curves respectively). With one profile assumed to be a nonzero constant, there are two dependents and the system of two partial differential equations is a hyperbolic one. The author uses the standard method of characteristics and the boundary condition of ideal arms which are found by setting the above constant to be zero, to solve the hyperbolic system. This provides one analytic method to model galaxy patterns. Its varying parameters will be useful for galacticarm classification and for investigation of galaxy structure.     

A Model for the Cartwheel Ring Galaxy

A new model of the Cartwheel galaxy is presented, based on the nearly central inclined collision of a rigid companion galaxy with a self-gravitating disc containing both stars and gas. We assume that the intruder is the most distant companion (G3 in J. Higdon's notation). Two rings and a transient network of gaseous spokes develop after the passage of the companion. The expansion velocity of the outer ring is in agreement with the one observed for the atomic gas. At the end of the simulation, the position and velocity of the companion agree with those of G3. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of leading spiral arms in retrograde galaxy encounters

We study, theoretically and withN-body simulations, the formation of spiral structures in retrograde galaxy encounters. A one-armed leading spiral dominates in a disc if (i) the tidal perturbation from the retrograde companion is large enough, and (ii) the disc is surrounded by a massive halo. From the literature we find that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with a larger mass than the disc mass.     

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．     

The possible origin of the spiral-arm instability

Physical arguments suggest the spiral arms may be manifestations of the galaxy not being in dynamical equilibrium — in the sense that the kinetic energy of tis stars and gas is less relative to its binding energy than that dictated by the virial theorem. Without constant cooling of the galactic disk (i.e., a progressive increase in the binding energy of the galaxy) such a departure from dynamical equilibrium would be corrected and the spiral arms destroyed in about 10⁹ yr due to an increase in the velocity dispersion of the stars in the disk resulting from their interacting with the spiral arms. The rate of cooling required to maintain the spiral arms, about 6×10⁴ L , may be provided by mass loss from stars in the disk population. The cooling arises from the average scale-heights and velocities of these stars being larger than that of the gas in the disk, so that there is a net loss of kinetic energy and an increase in the binding energy of the galaxy due to the ejected gas settling down to a lower terminal velocity and scale-height in the galactic disk.     

NGC 7468: A galaxy with an inner polar disk

We present our spectroscopic observations of the galaxy NGC 7468 performed at the 6-m Special Astrophysical Observatory telescope using the UAGS long-slit spectrograph, the MPFS multi-pupil fiber spectrograph, and the IFP scanning Fabry-Perot interferometer. We found no significant deviations from the circular rotation of the galactic disk in the velocity field in the regions of brightness excess along the major axis of the galaxy (the putative polar ring). Thus, these features are either tidal structures or weakly developed spiral arms. However, we detected a gaseous disk at the center of the galaxy whose rotation plane is almost perpendicular to the plane of the galactic disk. The central collision of NGC 7468 with a gas-rich dwarf galaxy and their subsequent merging seem to be responsible for the formation of this disk.     

盘状星系形成中的角动量问题

目前的星系形成理论认为，盘状星系由暗晕中的重子物质冷却并坍缩而成。在这一演化图像中，重子物质的角动量及其分布起着关键作用，它直接决定了盘状星系的结构。然而，在盘状星系形成和演化的各种解析、半解析模型及数值模拟中，出现了一系列与角动量有关的问题。其中包括角动量灾变(angular momentum catastrophe)及角动量分布不匹配(mismatch of angulamomentum profile)等。近年来，为解决此类问题，人们进行了大量的研究，引入并建立了各种机制和模型。详细表述了星系形成与演化中两类主要的角动量问题，并系统地综述了目前针对此类问题所提出的各种可能的解决方法和途径。     

星系形态分类的研究进展

传统的哈勃星系形态分类法可以很好地对近邻的亮星系进行分类,但对低面亮度星系、矮椭球星系、矮旋涡星系以及高红移星系等都已无能为力。德沃古勒分类系统、叶凯士分类系统和范登伯分类系统是在哈勃分类法的基础上进行了发展和细化,利用光的中心聚集度或光度级等作为星系形态分类的参数。模型化分类系统试图定量地测量星系形态参数,但需要假定星系面亮度分布满足一定的形式,如r~(1/4)律、指数律等。最近几年,又有一些学者提出了非模型化分类系统,给出了若干个可以直接测量星系形态的结构参数,如:聚集度指数C、非对称指数A、簇聚指数S、基尼系数G及矩指数M_(20)。这些参数可以反映星系的形成历史、恒星形成、与其他星系的相互作用、已经发生或正在进行的并合活动等。它们不仅可以有效地给出近邻星系的分类特性,还能用于测量高红移星系的形态。该文介绍了不同的星系形态分类方法,比较了各类方法的优点和不足。在此基础上,最后介绍了基于非模型化分类系统的星系形态分类的研究进展。