The magnetic-field structure in a stationary accretion disk

The magnetic-field structure in regions of stationary, planar accretion disks around active galactic nuclei where general-relativistic effects can be neglected (from 10 to 200 gravitational radii) is considered. It is assumed that the magnetic field in the outer edges of the disk, which forms in the magnetosphere of the central black hole during the creation of the relativisitic jets, corresponds to the field of a magnetic dipole perpendicular to the plane of the disk. In this case, the azimuthal field component B_φ in the disk arises due to the presence of the radial field B_ρ and the azimuthal velocity component U_φ. The value of the magnetic field at the inner radius of the disk is taken to correspond to the solution of the induction equation in a diffusion approximation. Numerical solutions of the induction equation are given for a number of cases.     

Cosmological magnetic dynamo in the early Universe

We study the behavior of large-scale magnetic fields in the early Universe influenced by an instability associated with breaking of mirror symmetry in weak interactions. It is shown that the magnetic field, whose present scale reaches about 500 m, which is negligible for galactic sizes, increases considerably if we correctly take into account the dynamics of the Universe. We conclude that this magnetic field is unlikely to provide the seed field for galactic dynamos, nearly independent of the particular instability considered.     

Formation of large-scale dust lanes in spiral arms of galaxies

We consider the interaction of interstellar dust grains with a galactic shock in the gaseous component. Typical parameters of dust grains and spiral density waves imply that the formation of large-scale dust lanes at the front of a galactic shock is possible only in models taking into account a self-focusing phenomenon. In the case of an isothermal flow of interstellar gas through a spiral arm in a model with a gaseous disk of variable thickness, dust lanes can be projected onto the region of increased gas density, although this is not associated with a galactic shock. The dust density peak derived from the classical model of a galactic shock (isothermal flow and a constant thickness of the gaseous disk) is appreciably shifted downstream of the gas flow, so that it does not outline the gas density maximum.     

Amplification of spiral density waves in gaseous and dust galactic disks as a result of interaction with the halo

The effect of the discrete structure of the halo on local oscillations of a galactic disk is analyzed. Such effects have much in common with dynamical friction. Gaseous and stellar disks are considered; in both cases, some leading spiral density waves are unstable. Bending oscillations of the disk can also be unstable when the disk interacts with the halo.     

Structure and Features of the Galactic Magnetic-Field Reversals Formation

At the present time, the induction of several microgauss in large-scale fields of the Galaxy has been repeatedly confirmed. There are numerous arguments in favor of the fact that the magnetic field exhibits so-called reversals associated with a direction change from one region to another. Such configurations are allowed in the nonlinear equations context of the dynamo theory, which describes the large-scale magnetic-field evolution. In the present study, reversals are modeled using the so-called no-z approximation based on the fact that the galactic disk is sufficiently thin. The magnetic field generation that exhibit both single and double sign changes with distance from the center is observed. From an observational point of view, one of the main methods for studying magnetic fields is to gauge the Faraday rotation measure of radio waves coming from pulsars. Its value can characterize the integral magnitude of the magnetic field, and the sign indicates its direction. A study of the pulsar distribution with large Faraday rotation measures ($$\left| {RM} \right| > 200$$ rad/m2) is presented. The results indicate that there is a region of $$4.8\;{\text{kpc}} < r < 7.3\;{\text{kpc}}$$ in the Galaxy in which the magnetic field is oriented counterclockwise. At the ring edges, the magnetic field reverses its direction. These results are in close agreement both with theoretical concepts and other studies dedicated to the structure observations of the magnetic field in the Galaxy.     

Formation of ring structures in galactic disks during close passages of galaxies

The formation of ring structures in galactic disks is investigated. It is shown that, in addition to the known mechanism of forming rings in “head-on” collisions between galaxies, ring structures can be formed during close passages of galaxies if the perturbing galaxy moves in a plane close to the equatorial plane of the perturbed disk galaxy, opposite to the direction of rotation of the disk. Numerical simulations of the formation of structures in the disk of a massive galaxy undergoing a passage with another galaxy are considered. The results of these cmputations show the formation of pronounced ring structures in the galactic disk when the initial inclination of the trajectory of the perturbing galaxy to the equatorial plane of the perturbed galaxy is no more than ~25°. However, the probability of close passages of galaxies with these parameters is small, as is the probability of head-on collisions. The characteristic time scale for the existence of pronounced rings is of order the dynamical time scale at the edge of the galaxy, 200–300 million years, close to the corresponding time for head-on collisions. The evolution of the rings has the same character in both cases: they gradually expand and move toward the periphery of the galaxy. The results of these simulations can also be applied to a close passage of one star by another star with a protoplanetary disk. According to the computation results, the characteristic time scale for the existence of pronounced rings in such a protoplanetary disk depends mainly on the size of the disk; this time scale can reach several tens of thousands of years for a disk radius of about 1000 AU. The formation of ring structures in such a disk could influence the formation and evolution of planetesimals, and possibly the character of the formation of planets and the distribution of their orbital semi-major axes.     

Interaction of the dark-matter cusp with the baryonic component in disk galaxies

The influence of the formation and evolution of a (disk) galaxy on the matter distribution in the dark-matter halo is considered. Calculations of the evolution of an isolated dark-matter halo were carried out with and without including a baryonic component. N-body simulations (for the dark-matter halo) and gas-dynamical numerical simulations (for the baryonic gas) were used for this analysis. Star formation, feedback, and heating and cooling of the interstellar medium were taken into account in the gas-dynamical calculations. The results of these numerical simulations with high spatial resolution indicate that 1) including the star formation resolves the so-called cusp problem (according to CDMcosmological models, the density distribution in the central regions of the dark-matter halo should have a distinct peak (cusp), which is not shown by observations); 2) the interaction of the dark matter with dynamical substructures of the stellar-gas galactic disk (spiralwaves, a bar) affects the shape of the dark-matter halo. In particular, the calculated dark-matter distribution in the plane of the disk is more symmetric when the baryonic component is taken into account.     

The Impact of Nucleosynthesis Models on Models of the Chemical Evolution of Disk Galaxies

The impact of uncertainties in the relative efficiency of nucleosynthesis of various elements in stars on models of the chemical evolution of disk galaxies is studied using a single-zone model for the galactic evolution. The dependences of the abundances of ¹²C, ¹⁴N, ¹⁶O, and ⁵⁶Fe on nucleosynthesis models are compared. The influence of the uncertainty in iron production by Type Ia supernovae on its abundance in a galaxy is also considered. It is concluded that differences in nucleosynthesis models can appreciably affect the results of modeling the early stages of galactic evolution, but this influence becomes insignificant at ages t> 10⁹ yr. Uncertainties in the amount of iron ejected by Type Ia supernovae do not significantly influence the total galactic abundance of iron.     

Polarimetric differences between Schwarzschild and Kerr black holes in active galactic nuclei

If the linear polarization of the optical emission of active galactic nuclei (AGNs) arises in magnetized accretion disk (the Milne problem), the degree of polarization should depend strongly on the spin of the central black hole. For the same black hole luminosities and masses, the polarization is substantially higher for rotating Kerr than for non-rotating Schwarzschild black holes. Statistically, this means that the majority of AGNs displaying appreciable linear polarization should have Kerr black holes. The spin dependence of the polarization is due to the fact that the radius of the innermost stable circular orbit r _isco depends on the spin—this radius is three gravitational radii for a Schwarzschild black hole, and a factor of six smaller for a rapidly rotating black hole. This means that the magnetic field in the region of emergence of the optical emission, which decreases with distance from r _isco , is higher for a non-rotating than for a rapidly rotating black hole. This higher magnetic field gives rise to strong Faraday depolarization, explaining the effect considered here.     

Magnetic-field structure in the accretion disks of semi-detached binary systems

The results of three-dimensional MHD numerical simulations are used to investigate the characteristic properties of the magnetic-field structures in the accretion disks of semi-detached binary systems. It is assumed that the intrinsic magnetic field of the accretor star is dipolar. Turbulent diffusion of the magnetic field in the disk is taken into account. The SS Cyg system is considered as an example. The results of the numerical simulations show the intense generation of a predominantly toroidal magnetic field in the accretion disk. Magnetic zones with well defined structures for the toroidal magnetic field form in the disk, which are separated by current sheets in which there ismagnetic reconnection and current dissipation. Possible observational manifestations of such structures are discussed. It is shown that the interaction of a spiral precessional wave with the accretor’s magnetosphere could lead to quasi-periodic oscillations of the accretion rate.     

Dynamics of gaseous disks in a non-axisymmetric dark halo

The dynamics of a galactic disk in a non-axisymmetric (triaxial) dark halo is studied in detail using high-resolution, numerical, hydrodynamical models. A long-lived, two-armed spiral pattern is generated for a wide range of parameters. The spiral structure is global, and the number of turns can be two or three, depending on the model parameters. The morphology and kinematics of the spiral pattern are studied as functions of the halo and disk parameters. The spiral structure rotates slowly, and its angular velocity varies quasi-periodically. Models with differing relative halo masses, halo semi-axis ratios, distributions of matter in the disk, Mach numbers in the gaseous component, and angular rotational velocities of their halos are considered.     

Polygonal structure of spiral galaxies

We have carried out numerical simulations of hydrodynamical processes occurring in the disks of spiral galaxies. The initial state of the disk is an equilibrium stellar-gaseous configuration. The spherical component is described by a standard analytical model for the gravitational potential. The behavior of the modeled disk in the presence of an external perturbation is analyzed. The results of numerical simulations of stellar-gaseous galactic disks aimed at studying the formation of polygonal structures in spiral galaxies are presented. The possible influence of spur-like formations on the appearance of polygonal structure is studied.     

Response of First-Order Sequence to the Galactic Year Cycle in the Phanerozoic:An Important Progress and Topic

The sequence which corresponds to the first-order sea-level cycle is called the first-order sequence, and the first-order sequence may have an internal relationship with the galactic year cycle, in other words, the galactic year cycle may control the first-order sequence macroscopically. The superevents in the Phanerozoic, such as the concentration change of atmospheric gas carbon dioxide, the alternation of "calcite sea" and "aragonite sea" in the hydrosphere, and the reversal of magnetic poles, are cyclical. They have obvious correlation in genesis, and they are likely to be controlled by the complicated genetic relationship between galactic annual cycles and various superevents. Over the years, researches have shown that when the solar system moves around the galactic center, the acceleration of gravity (value of G) changes accordingly, and the solar system expands (or shrinks). The Earth, a member of the solar system, forms a change in potential energy and absorbs (or releases) heat. At the same time, the gravitational differentiation changes as a result of the value of G change, which leads to the expansion of the mantle and the contraction of the core; when the energy accumulates to a certain degree, the superplume will be activated, which will affect the plate pattern on the earth. In recent years, the periodic changes of these superevents in Phanerozoic have attracted extensive attention and discussion. Paying attention to these wise understandings and existing problems will provide important thinking process and research clues for in-depth research.     

Fine vertical structure of the galactic gaseous disk

The three-dimensional evolution of an ensemble of N particles (N = 8 × 10⁵) in the external gravitational field of a galaxy perturbed by a spiral density wave is considered. The particles simulate clouds of interstellar gas, and inelastic two-body collisions between them are taken into account. The three-dimensional structure of the gaseous galactic layer and the vertical profile of the spiral arms are computed. It is shown that: (1) the local thickness of the gaseous galactic disk has a minimum where the volume gas density has a maximum (the maximum density of the interstellar medium is shifted downstream relative to the galactic shock front), (2) the configuration of the vertical profile of the spiral arms changes radically when the corotation region is crossed. Our first result explains the negative correlation between the thickness of the gas layer and the density derived from neutral-hydrogen observations. The second result can be used in the next generation of neutral-hydrogen observations to localize the corotation radius in the Galaxy.     

Disk accretion onto a magnetized star

An exact solution is found for the interaction of a rotating magnetic field that is frozen into a star with a thin, highly conducting accretion disk. The disk pushes the magnetic-field lines towards the star, compressing the stellar dipole magnetic field. At the corotation radius, where the Keplerian and stellar rotational frequencies are equal, a current loop appears. Electric currents flow in the magnetosphere only along two particular magnetic surfaces, which connect the corotation region and the inner edge of the disk with the stellar surface. It is shown that a closed current surface encloses the magnetosphere. The disk rotation is stopped at some distance from the stellar surface, equal to 0.55 of the corotation radius. The accretion from the disk spins up the stellar rotation. The angular momentum transferred to the star is determined.     

Seyfert nuclei as sources of ultrahigh-energy cosmic rays

Particles can be accelerated to ultrahigh energies E≈10²¹ eV in moderate Seyfert nuclei. This acceleration occurs in shock fronts in relativistic jets. The maximum energy and chemical composition of the accelerated particles depend on the magnetic field in the jet, which is not well known; fields in the range ～5–1000 G are considered in the model. The highest energies of E≈10²¹ eV are acquired by Fe nuclei when the field in the jet is B≈16 G. When B～(5–40) G, nuclei with Z<10 are accelerated to E≤10²⁰ eV, while nuclei with Z≥10 acquire energies E≥2×10²⁰ eV. Only particles with Z≥23 acquire energies E≤10²⁰ eV when B～1000 G. Protons are accelerated to E<4×10¹⁹ eV, and do not fall into the range of energies of interest for any magnetic field B. The particles lose a negligible amount of their energy in interactions with infrared photons in the accretion disk; losses in the thick gas-dust torus are also negligible if the luminosity of the galaxy is L≤10⁴⁶ erg/s and the angle between the normal to the galactic plane and the line of sight is sufficiently small, i.e., if the axial ratio of the galactic disk is comparatively high. The particles do not lose energy to curvature radiation if their deviations from the jet axis do not exceed 0.03–0.04 pc at distances from the center of R≈40–50 pc. Synchrotron losses are small, since the magnetic field frozen in the galactic wind at R≤40–50 pc is directed (as in the jet) primarily in the direction of motion. If the model considered is valid, the detected cosmic-ray protons could be either fragments of Seyfert nuclei or be accelerated in other sources. The jet magnetic fields can be estimated both from direct astronomical observations and from the energy spectrum and chemical composition of cosmic rays.     

Nonresonance spiral responses in disk galaxies

The behavior of the gravitational potential outside the region where the main spiral arms of galaxies are located is investigated. The characteristic features of this behavior include nearly circular extensions of the main arms, which typically have an angular extent of 90°. It is natural to interpret these quarter-turn spirals as the response of the galactic disk to the gravitational potential of the main spiral arms. The theoretical models are supported by observational data for the brightness distributions in both normal (NGC 3631) and barred (NGC 1365) galaxies.     

The nature of active radio galaxies in the cluster A569

We investigate the nature of bright radio sources with known radio spectra in the direction of the nearby cluster of galaxies A569 (z=0.0193). The optical identifications of the sources show that 45% of these radio sources are associated with compact galaxies. A substantial fraction of these galaxies have active nuclei, with the radio emission concentrated toward the galactic center. Some of the cluster galaxies have radio halos, with appreciably weaker radio powers and spectral indices α=0.95±0.2. We compute the magnetic fields in the nuclei and halos of the galaxies for the adopted distance to A569. As expected, the magnetic fields in the galactic halos make a smooth transition to the intergalactic field, while the magnetic fields in the central regions of the galaxies rise sharply toward the nucleus.     

Stellar structure of irregular galaxies: Edge-on galaxies

Stellar photometry obtained using the Hubble Space Telescope is used to study the distributions of the number densities of stars of various ages in 12 irregular and dwarf spiral galaxies viewed edge-on. Two subsystems can be distinguished in all the galaxies: a thin disk comprised of young stars and a thick disk containing a large fraction of old stars (primarily red giants) in the system. Variations of the stellar number density in the thin and thick disks in the Z direction perpendicular to the plane of the galaxy follow an exponential law. The size of the thin disk corresponds to the visible size of the galaxy at the μ = 25 mag/arcsec² isophote, while the thick disk is a factor of two to three larger. In addition to a thick disk, the massive irregular galaxy M82 also has a more extended stellar halo that is flattened at the galactic poles. The results of our previous study of 12 face-on galaxies are used together with the new results presented here to construct an empirical model for the stellar structure of irregular galaxies. Original Russian Text ? N.A. Tikhonov, 2006, published in Astronomicheskiĭ Zhurnal, 2006, Vol. 83, No. 7, pp. 579–588.     

A possible mechanism for the formation of tilted disks in intermediate polars

Using 3D gas dynamics, we numerically simulate accretion-disk formation in typical cataclysmic variable intermediate polars with dipolar magnetic fields (B _a = 10⁵?5 × 10⁵ G) and misaligned white-dwarf magnetic and rotation axes. Our simulations confirm that a significant misalignment of the axes results in a significant misalignment of the disk to the orbital plane. However, over time, this disk tilt disappears: early in the simulation, the initial particle positions in the rarefied tilted disk are governed solely by the magnetic field of the white dwarf. Due to the increasing disk mass and hence increasing disk gas pressure, the tilted disk eventually becomes decoupled from the magnetic field. The tidal action of the donor leads to a retrograde (i.e., nodal) precession of the tilted disk’s streamlines, and the disk becomes twisted. When the disk tilt is greater than 4°, the incoming gas stream no longer strikes the disk rim (i.e., bright shocked region). Matter is now transported over and under the disk rim to the inner regions of the disk. Over time, the increased mass of inner parts of the disk due to the action of the colinear gas stream returns the inner-disk regions to a colinear configuration. Meanwhile, the outer regions of the tilted, twisted disk become warped. Our simulations suggest that the lifetime of an intermediate polar’s tilted disk could be several tens to thousands of orbital periods.