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.     

脉冲星的星系磁场

脉冲星的偏振信息是理解脉冲星辐射区的重要手段，利用澳大利亚的64m射电望远镜进行大量的脉冲星观测，得到了一批脉冲星的偏振轮廓和偏振参数，编辑了几乎所有发表的脉冲星轮廓资料，系统总结了脉冲星圆偏辐射的规律，为理论上解释脉冲星辐射这一重要难题提供观测依据和物理限制，利用脉冲星作为探针，研究了银河系磁场结构和模型，确定了银河系BS磁场模型，发现了银河系上下反对称的环向磁场，并首次对星系尺度的发电机类型进行判别，证认出A0型发电机运行于银河系，发现了银晕中的垂直磁场和M31及银盘中的非常延展的磁场，探测到NGC2997星系中由内到外的旋涡磁场，并提出可能有两种发电机在这个星系的不同区域运行。     

Surface color photometry of five barred spiral galaxies

Distributions of the surface brightness and the surface color of five barred spiral galaxies expressed in the form of digital maps are presented. This is the first step to determine the composition of the components of barred spiral galaxies — bar, spiral arm, inner ring and outer ring — and to obtain an accurate picture of the dynamical model of a barred spiral galaxy. We have found that (a) the bar is redder than the spiral arm and has a color similar to that of the disk and (b) the inner ring of theSB(r) type galaxy is bluer than the bar and rather resembles the spiral arm.     

Chiral Property of Spiral and Barred Spiral Galaxies in the Local Supercluster

We present an analysis of the chiral property of 667 spiral and barred spiral galaxies in the Local Supercluster (radial velocity <3 000 km s⁻¹). The arms of a galaxy (spiral or barred spiral) can be distinguished according to their orientation (leading or trailing) relative to the direction of the rotation. We use environment of each galaxy as a subsample in order to study the chiral property of galaxies. In addition, equatorial position angle distributions of leading and trailing arm galaxies are studied. We classify our database according as their morphology, diameters, radial velocities, axial ratios and magnitudes. The distribution of trailing and leading arm galaxies in the Local Supercluster is found homogeneous. A significant dominance of either trailing or leading structures is noticed within the Virgo cluster region, suggesting that the aggregation of these structures might have already started there. The rotation axes of the galaxies in the Virgo cluster is found to lie in the equatorial plane. Chirality of galaxies is found strong for the subsamples that showed a random alignment in the equatorial position angle distribution. Possible explanations of the results will be presented.     

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 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.     

3D numerical models of magnetic field evolution in galaxies interacting with the ICM

A fully three-dimensional (3D) MHD model is applied to simulate the evolution of large-scale magnetic field in galaxies interacting with the intra-cluster medium (ICM). As the model input we use a time dependent velocity field of gas clouds (HI) resulting from 3D N-body sticky-particle model of a galaxy. These clouds are affected by ram pressure due to their rapid motion through the ICM. The gas evolves in an analytically given gravitational potential which includes a dark matter halo, a disk, and a bulge component. We found that due to the interaction with the ICM the resultant magnetic field correctly reproduces the observed structures of the magnetic field forming peculiar spiral arms and magnetic features widely observed in cluster spiral galaxies. This revised version was published online in September 2006 with corrections to the Cover Date.     

The HI halo of spiral galaxies

A deep H I survey with the VLA of the spiral galaxy NGC 2403 has revealed the existence of a thick, low density layer of neutral gas surrounding the thin ‘cold’ disk. This layer has a mean rotation velocity 25–50 km s^-1 lower than that of the disk and a 10–20 km s^-1inflow towards the centre of the galaxy. In the central parts there are velocity differences from rotation of up to 150 km s^-1.Chandra observations of NGC 2403 show a diffuse, hot X-ray emitting gas component with a temperature of a few 10⁶ K. These results point at galactic fountain type of flows between disk and halo. ‘Halo’ gas with similar characteristics has also been observed in other spiral galaxies(e.g. NGC 6946, NGC 891). Such gas is probably similar to the IVCs and to some of the HVCs of the Milky Way.     

星系翘曲盘及其统计研究

河外旋涡星系外区普遍存在翘曲结构,其特征可用干翘曲参数来描述,包括翘曲角、翘曲半径、不对称度等。一些翘曲星系表已相继发表,并用于相关的统计分析。关于翘曲盘的形成已提出多种理论机制,如星系间的潮汐相互作用、星系际介质的吸积、盘与暗晕的角动量错向以及星系际磁场的作用等。     

Head-on Collision of Disk–Sphere Galaxies

Numerical simulations are performed to study the tidal effects of non-merging rapid head-on collision between a disk galaxy and a spherical galaxy. The disk consists of three components – a disk, a bulge and a halo – and the spherical galaxy is a Plummer model. The galaxies have the same dimensions with different mass ratios viz., 2, 1 and 0.5. They move in a rectilinear orbit with a relative velocity of 1000 km s⁻¹. None of the simulations leads to the merger of the galaxies by tidal capture. The results of our simulations indicate that although tidal effects are sensitive to both the mass ratio and the inclination of the disk to the orbital plane, it is the mass ratio which is more important in producing tidal damage to the less massive galaxy. The spherical galaxy undergoes considerable tidal effects if the mass of the disk is same or larger. On the other hand the collisions in which the mass of the spherical galaxy is more, result in the formation of a ring structure after the closest approach and the structure disappears by the end of the simulations.     

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.     

Do active galaxies have a massive halo component?

We present a dynamical model for an active galaxy. Our model is a mass model with a disk, nucleus, and halo components. Numerical calculations and theoretical evidence show, that for a fixed value of mass of the galaxy the stellar velocities in the central region decrease as the mass of halo increases. Furthermore, the motion tends to be regular while, when the halo component is absent, the majority of orbits are chaotic. The dynamical evolution of the system is also studied when mass is transported from the halo to the disk and the nucleus. Our results are compared to the recently obtained observation data for active galaxies.     

Inferring dark halo structure from observed scaling laws of late-type galaxies and LSBs

We re-examine the Fall & Efstathiou scenario for galaxy formation, including the dark halo gravitational reaction to the formation of the baryon disc, as well as continuous variations in the intrinsic halo density profile. The recently published rotation curves of low surface brightness (LSB) and dwarf galaxies together with previously known scaling relations provide sufficient information on the present-day structure of late-type disc galaxies to invert the problem. By requiring that the models reproduce all the observational restrictions we can fully constrain the initial conditions of galaxy formation, with a minimum of assumptions, in particular without the need to specify a cold dark matter (CDM) halo profile. This allows one to solve for all the initial conditions, in terms of the halo density profile, the baryon fraction and the total angular momentum. We find that a unique initial halo shape is sufficient to accurately reproduce the rotation curves of both LSB and normal late-type spiral galaxies. This unique halo profile differs substantially from that found in standard CDM models. A galactic baryon fraction of 0.065 is found. The initial value of the dimensionless angular momentum is seen to be the principal discriminator between the galaxy classes we examine. The present-day scalings between structural parameters are seen to originate in the initial conditions.     

GHASP: an Hα kinematic survey of spiral and irregular galaxies – V. Dark matter distribution in 36 nearby spiral galaxies

The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry–Perot interferometry as part of the GHASP survey. The main aim of obtaining high-resolution Hα 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The Hα velocities were combined with low resolution H  i data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere (ISO) and a Navarro–Frenk–White (NFW) profile. For the galaxies already modelled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the centre of the dark matter haloes rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and low surface brightness galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius haloes have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low-luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type.     

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.     

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．     

Mass profiles and anisotropies of early-type galaxies

We discuss the problem of using stellar kinematics of early-type galaxies to constrain the orbital anisotropies and radial mass profiles of galaxies. We demonstrate that compressing the light distribution of a galaxy along the line of sight produces approximately the same signature in the line-of-sight velocity profiles as radial anisotropy. In particular, fitting spherically symmetric dynamical models to apparently round, isotropic face-on flattened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face-on stellar disc. Such face-on stellar discs could plausibly be the cause of the radial anisotropy found in spherical models of intermediate luminosity ellipticals such as NGC 2434, 3379 and 6703.
In the light of this result, we use simple dynamical models to constrain the outer mass profiles of a sample of 18 round, early-type galaxies. The galaxies follow a Tully–Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag ( I -band) for a given mass. The most luminous galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.     

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。     

Modeling the total and polarized emission in evolving galaxies: “Spotty” magnetic structures

Future radio observations with the Square Kilometre Array (SKA) and its precursors will be sensitive to trace spiral galaxies and their magnetic field configurations up to redshift z ≈ 3. We suggest an evolutionary model for the magnetic configuration in star‐forming disk galaxies and simulate the magnetic field distribution, the total and polarized synchrotron emission, and the Faraday rotation measures for disk galaxies at z ≲ 3. Since details of dynamo action in young galaxies are quite uncertain, we model the dynamo action heuristically relying only on well‐established ideas of the form and evolution of magnetic fields produced by the mean‐field dynamo in a thin disk. We assume a small‐scale seed field which is then amplified by the small‐scale turbulent dynamo up to energy equipartition with kinetic energy of turbulence. The large‐scale galactic dynamo starts from seed fields of 100 pc and an averaged regular field strength of 0.02 μG, which then evolves to a “spotty” magnetic field configuration in about 0.8 Gyr with scales of about one kpc and an averaged regular field strength of 0.6 μG. The evolution of these magnetic spots is simulated under the influence of star formation, dynamo action, stretching by differential rotation of the disk, and turbulent diffusion. The evolution of the regular magnetic field in a disk of a spiral galaxy, as well as the expected total intensity, linear polarization and Faraday rotation are simulated in the rest frame of a galaxy at 5GHz and 150 MHz and in the rest frame of the observer at 150 MHz. We present the corresponding maps for several epochs after disk formation. Dynamo theory predicts the generation of large‐scale coherent field patterns (“modes”). The timescale of this process is comparable to that of the galaxy age. Many galaxies are expected not to host fully coherent fields at the present epoch, especially those which suffered from major mergers or interactions with other galaxies. A comparison of our predictions with existing observations of spiral galaxies is given and discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)