The gap in the gaseous disk of the Galaxy as a manifestation of processes in the corotation region

The evolution of a system of a large number of particles (N～4×10⁴) moving in planar trajectories in the gravitational field of the Galaxy is simulated with allowance for perturbations by a spiral density wave that give rise to spiral arms (the problem of a collisional N-body system in a specified external field). The particles simulate the motion of diffuse HI clouds in the Galaxy. The spiral-wave parameters are chosen so that the corotation region lies in the vicinity of the Sun. The spiral field pumps cloud out of the corotation region, creating a trough or gap in the initially monotonic radial distribution of particles near the corotation circle. At the same time, the arms proper exhibit no conspicuous features near corotation. This trough could explain the gap in the Galactic HI distribution observed near the solar circle and can be used to locate the corotation region if combined with other data.     

The wavelet-smoothed distribution of young stellar objects in the Galactic plane

We have analyzed the distribution of young objects (open clusters, classical Cepheids, and HII regions) projected onto the plane of the Galaxy using wavelet smoothing. This smoothing technique enables investigation of the large-scale structure of the distribution of young objects. All the studied objects display a similar spiral structure, whose appearance essentially corresponds to a regular alternation of young and older stellar complexes along sections of the spiral arms. A four-arm spiral structure with its arms originating at the Galactic center is obtained if the major arms are taken to be the Carina-Sagittarius arm and an outer arm behind the Perseus arm. If all the observed arms are taken to be major arms, we obtain a 12-arm structure, in contradiction with the size of the region in which the structure is observed in the Galaxy. This discrepancy can be removed if the arms originate from a ring formed by a sufficiently long bar-like structure, rather then from the Galactic center.     

Formation of galactic shocks and the vertical structure of the gaseous disk of the galaxy in a “turbulent” interstellar medium

The effects on the formation of Galactic shocks and the vertical structure of the Galactic disk due to thermal processes in a cloudy interstellar medium as it flows through a spiral density wave in the plane of the Galactic disk are considered. The evolution of the gas is fundamentally different, depending on the thermal properties of the medium. For example, if it is compressed in the horizontal direction (parallel to the Galactic plane) by the gravitational forces of the spiral density waves responsible for the formation of spiral arms, an isothermal and adiabatic medium is swept out in the vertical direction. However, on the contrary, a medium whose volume loss function increases fairly rapidly with density and temperature is further compressed under the action of the overall gravitational field of the galaxy. This effect is referred to as “self-focusing,” and may serve as an additional mechanism to explain the recently discovered anticorrelation between the width of the atomic hydrogen layer in the Galaxy and the gas density. The difference in the vertical behavior of media with different thermal properties can be used as an indicator of the thermal properties of a particular component of the interstellar gas (atomic or molecular). Attention is drawn to the fact that Galactic shocks themselves represent a mechanism that can heat the ensemble of clouds, i.e., increase the dispersion of cloud velocities. The vertical structure of a Galactic shock front is constructed, which is in qualitative agreement with the “bow shock” inferred from radio data.     

On the spiral structure of the Milky Way Galaxy

We consider a possible scheme of the overall spiral structure of the Galaxy using data on the distribution of neutral (atomic), molecular, and ionized hydrogen. Our analysis assumes that the spiral structure is symmetric, i.e., that the spiral arms are translated into each other via a rotation about the Galactic center by 180° (a two-arm pattern) or 90° (a four-arm pattern). In the inner region, the observations are best represented with a four-arm spiral pattern, associated with all-Galaxy spiral density waves. The initial position is that of the Carina arm, reliably determined from distances to HII regions and from HI and H₂ radial velocities. This pattern continues in quadrants III and IV with weak outer HI arms; from their morphology, the Galaxy should be considered an asymmetric multi-arm spiral. The knee-like shape of the outer arms, which consist of straight segments, may indicate that these arms are transient formations that appeared due to gravitational instability in the gaseous disk. The distances between HI superclouds in the two arms that are brightest in neutral hydrogen, the Carina and Cygnus (Outer) arms, are concentrated near two values, suggesting the presence of a regular magnetic field in these arms.     

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.     

Distribution of the relative plasma turbulence level in the galaxy

The statistical dependence of τ/(DM)² (the ratio of the broadening of a pulsar pulse due to scattering in the interstellar medium to the square of the pulsar’s dispersion measure) on the pulsar’s dispersionmeasure, Galactic coordinates, age, and the angular distance to the nearest supernova remnant are studied. This parameter describes the relative level of electron density fluctuations in the turbulent interstellar plasma. It is shown that the interstellar plasma turbulence level is three orders of magnitude higher in the spiral arms of the Galaxy than outside the arms. The plasma turbulence level is approximately an order of magnitude higher in the Galactic arms, in regions within ?0.3° of supernova remnants, than outside these regions. We conclude that the source of energy for the turbulence in the Galactic arms is supernova explosions in the denser medium there.     

Velocity field in the spiral-wave pattern observed in rotating shallow-water experiments

The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered.     

Global repeating events in the history of the Earth and the motion of the Sun in the Galaxy

Chronological analyses of correlations between certain global repeating events (mass extinctions of marine organisms, meteorite impacts, and flashes in the frequency of geomagnetic reversals) during the Phanerozoic Eon and the motion of the solar system in the Galaxy are presented for five rotationally symmetrical models for the regular Galactic gravitational field. Thirteen of sixteen mass-extinction events can be described by a repetition interval of 183±3 million years. This is in agreement with the anomalistic period (interval between two subsequent passages of the Sun through the apocenter of its Galactic orbit) in the model of Allen and Martos. The positions of the minima and maxima in Gaussian functions approximating the frequency distribution for geomagnetic reversals also agree with the times of passage of the Sun through the apocenter and pericenter, respectively, of its Galactic orbit in this model. The maximum in the distribution of the deviations of the dates of mass extinctions from the nearest dates of impacts of large, crater-forming bodies is close to zero, providing evidence that many such events are correlated. As a rule, extinctions follow impact events. The impacts of large bodies have occurred most often when the solar system passes through the Galactic plane, while mass extinctions occur more often at some distance from the Galactic plane (about 40 pc). As a rule, intervals of increases in the frequency of geomagnetic reversals coincide with dates of impacts of large bodies. At the same time, these intervals do not show a clear correlation with the dates of mass extinctions. The intensity of mass extinctions, like the energy released by impacts, is consistently higher in periods when the Sun is moving from the apocenter toward the pericenter of its orbit, than when it is moving from the pericenter toward the apocenter. Thus, there is evidence for a variety of relationships between repeating global events in the Phanerozoic and the motion of the Sun in the Galaxy. Long-period variations in the frequency of geomagnetic reversals are correlated with the orbital motion of the Sun, and increases in the frequency of geomagnetic reversals are correlated with impacts. Mass extinctions are correlated with the impacts of large bodies, whose motions may have been perturbed by clouds of interstellar material concentrated toward the Galactic plane and by the shock front associated with the Perseus spiral arm, through which the solar system passes. The velocity of the Sun relative to the spiral pattern is estimated.     

The phenomenon of “self-focusing” in the gaseous disk of the Galaxy

We consider perturbations in interstellar gas excited by the gravitational field of the spiral-density wave that is responsible for the Galactic arms, taking into account thermal effects. Under the conditions of fairly efficient cooling, the reaction of the gas to the perturbing field is non-trivial: the thickness of the gaseous layer is reduced in the region of the Galactic disk where the density of the gas is enhanced. We call this effect “self-focusing,” and explain it using observational results for the Galactic radio emission in the 21 cm line. Under our assumptions, we find the control parameter (δ) governing the relationship between perturbations of the thickness of the gaseous disk and the gas density in the vicinity of the Galactic equator, i.e., this parameter shows whether the correlation between these quantities is positive or negative, and provides important additional information on the thermal properties of the medium. It can be used as a diagnostic in joint studies of Galactic structure and large-scale features of the interstellar gas. Estimates for the typical Galactic parameters show that the effect of self-focusing should be clearly manifest in the Galaxy.     

A unified theory for the formation of galactic structures

A new theory for the formation of the main structures of galaxies is proposed: these structures are viewed as low-frequency normal modes in disks consisting of precessing stellar orbits. Mathematically, the theory is based on an integral equation in the form of a classical eigenvalue problem, with the eigenvalues being equal to the angular velocities Ω_p of the modes. Analysis of the general properties of the master integral equation (without finding concrete solutions) shows that it admits two types of solutions: barlike and spiral. The numerical algorithms are discussed and particular solutions of the integral equation are presented. If resonance interaction can be neglected, the bar mode represents a neutral perturbation of the disk. This mode can be amplified by the effect of the long-range gravitational field of the mode on stars located in the vicinity of the corotation and outer-Lindblad resonances. Spiral perturbations are waves with zero total angular momentum, and spiral modes are excited at the inner-Lindblad resonance. The approach proposed is compared to currently accepted mechanisms for the formation of galactic structures. In particular, Toomre's application of the swing amplification mechanism to explain the formation of global modes is critically discussed. In addition, we show that it is not correct to simulate the real stellar velocity dispersion in a galaxy using softened gravity.     

On the Formation of Spiral Arms in Dwarf Galaxies

Astronomy Reports - Spiral structure (both flocculent and Grand Design types) is very rarely observed in dwarf galaxies because the formation of spiral arms requires special conditions. In this...     

Peculiarities of α-element abundances in Galactic open clusters

A catalog compiling the parameters of 346 open clusters, including their metallicities, positions, ages, and velocities has been composed. The elements of the Galactic orbits for 272 of the clusters have been calculated. Spectroscopic determinations of the relative abundances, [el/Fe], for 14 elements synthesized in various nuclear processes averaged over data from 109 publications are presented for 90 clusters. The compiled data indicate that the relative abundances of primary α elements (oxygen and magnesium) exhibit different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits in clusters with high, elongated orbits satisfying the criterion (Z_max² + 4e²)^1/2 > 0.40 and in field stars of the Galactic thin disk (Z_max is the maximum distance of the orbit from the Galactic plane in kiloparsec and e is the eccentricity of the Galactic orbit). Since no systematic effects distorting the relative abundances of the studied elements in these clusters have been found, these difference suggest real differences between clusters with high, elongated orbits and field stars. In particular, this supports the earlier conclusion, based on an analysis of the elements of the Galactic orbits, that some clusters formed as a result of interactions between high-velocity,metal-poor clouds and the interstellar mediumof theGalactic thin disk. On average, clusters with high, elongated orbits and metallicities [Fe/H] < -0.1 display lower relative abundances of the primary a elements than do field stars. The low [O, Mg/Fe] ratios of these clusters can be understood if the high-velocity clouds that gave rise to them were formed of interstellar material from regions where the star-formation rate and/or the masses of Type II supernovae were lower than near the Galactic plane. It is also shown that, on average, the relative abundances of the primary a elements are higher in relatively metal-rich clusters with high, elongated orbits than in field stars. This can be understood if clusters with [Fe/H] > -0.1 formed as a result of interactions between metal-rich clouds with intermediate velocities and the interstellar medium of the Galactic disk; such clouds could form from returning gas in a so-called “Galactic fountain.”     

Structure of the accretion disk in the cataclysmic variable UX UMa in the transition to its quiescent state

Our analysis of BV RI light curves for the cataclysmic variable UX UMa obtained in intermediate activity states, in the transition between the active and quiescent states of the system on March 12, 1997 and May 3, 2000, shows that the shapes of these light curves cannot be adequately described using the standard hot-spot model. A model with a “hot line” near the edge of the disk and a two-armed spiral structure on the disk surface reproduces much better out-of-eclipse variations in the light curves. The presence of an extended hot line can explain the anomalous shape of the I light curve on March 12, 1997. The decrease in the observed luminosity of the system between March 12, 1997 and May 3, 2000 could be due to a decrease in the disk luminosity by a factor of 2–2.5; the higher disk luminosity on the earlier date is associated with appreciable deviations of the radial temperature distribution of the disk material from that for the standard model. The phases and depths of dips in the out-of-eclipse sections of the UX UMa light curves are due primarily to the parameters of the complex shape of the accretion disk, which has a spiral structure located mainly near its outer edge. The contribution of the spiral arms in the V filter reaches 20–50% of the total disk radiation. The crest of the first spiral wave in our model maintains its approximate position in azimuth; this structure could represent a bulge in a halo at the outer edge of the disk near orbital phases φ ～ 0.7, in the direction of the continuation of the extended shock in the disk itself. The position of the crest of the second spiral arm changes with time. This structure may represent a one-armed spiral wave near the apastron of the weakly elliptical disk. Finally, the observations testify to the presence of another spiral arm that is les clearly manifest in terms of both its luminosity and its height above the unperturbed disk surface. Thus, in an intermediate activity state of UX UMa, the surface of the accretion disk is distorted by the action of a two-armed spiral structure in the outer regions of the disk, which is asymmetric in both its luminosity and dimensions, and a bulge at the disk edge in the region of its interaction with the inflow to the disk.     

The structure of matter flows in semi-detached binaries after the termination of mass transfer

We present the results of three-dimensional gas-dynamical simulations of matter flows in semi-detached binaries after termination of the mass transfer between the components of the system. The structure of the residual accretion disk is studied. When the mass transfer has ended, the quasi-elliptical disk becomes circular and its structure changes: tidal interactions result in the formation of a second arm in the spiral shock wave. In addition, a condensation (blob) moving through the disk with variable velocity is formed. The blob is maintained by interactions with the arms of the spiral shock and exists essentially over the entire lifetime of the disk. We also show that, for a viscosity corresponding to α～0.01 (typical for observed accretion disks), the lifetime of the residual disk is about 50 orbital periods.     

The galactic constants and rotation curve from molecular-gas observations

We obtained the photometric distances and radial velocities for the molecular gas for 270 star-forming regions and estimated the distance to the Galactic center from ten tangent points to be R₀ = 8.01 ± 0.44 kpc. Estimates of R₀ derived over the last decade are summarized and discussed; the average value is R₀ = 7.80 ± 0.33 kpc. We analyze deviations from axial symmetry of the gas motion around the Galactic center in the solar neighborhood. Assuming a flat rotation curve, we obtain Θ₀ ～ 200 km/s for the circular velocity of the Sun from regions beyond the Perseus arm. We used these Galactic constants to construct the Galactic rotation curve. This rotation curve is flat along virtually its total extent from the central bar to the periphery. The velocity jump in the corotation region of the central bar in the first quadrant is 20 km/s. We present analytical formulas for the rotation curves of the Northern and Southern hemispheres of the Galaxy for R₀ = 8.0 kpc and Θ₀ = 200 km/s.     

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.     

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.     

Star formation and the kinematics of gas in the disk of NGC 628

The radial profile of the star-formation rate (SFR) in the galaxy NGC 628 is shown to be modulated by a spiral-density wave. The radial profile of the velocity of gas inflow into the spiral arm is similar to the radial distribution of the surface density of the SFR. The position of the corotation resonance is determined along with other parameters of the spiral-density wave via a Fourier analysis of the azimuthal distribution of the observed radial velocities in annular zones of the disk of NGC 628. The radial profile of the surface density of the SFR is determined using the empirical SFR—linear size relation for star-formation complexes (giant HII regions) and measurements of the coordinates, Hα fluxes, and the sizes of HII regions in NGC 628.     

Reconciling the Earth's stratigraphic record with the structure of our galaxy

The passage of our Solar System through the spiral arms has been implicated as a contributor to global environmental perturbations.The suggestion of a consistent structure within the arms,informed by density wave theory,raises the possibility of repeating patterns of events at each arm crossing.Here we test the hypothesis that the structure of the arms of our galaxy influences the stratigraphic record on Earth.We construct independent structural and temporal models and colbine these to compare the timings of arm tracers,materials from the earliest Solar System and events on Earth,including the largest extinctions.We find that a recurring sequence of events across the four arms emerges with an average arm-passing time of 188 million years.We suggest that the multiple temporal overlaps of events across arms,and their alignment with arm tracers and the earliest Solar System,presents an opportunity for a greater understanding of both Earth-based phenomena and galactic structure.     

Numerical simulations of a multi-arm gaseous configuration in a global two-arm spiral gravitational field

The results of numerical simulations of supersonic, gas-dynamical processes occurring in the disks of spiral galaxies are presented. Qualitative and quantitative pictures of morphological features in the structure formed by a two-armed gravitational potential are given. A gas-dynamical interpretation of a quasi-stationary configuration containing two shock fronts separated by a contact discontinuity is discussed. This type of supersonic configuration with three jumps can form and produce a spiral pattern with six arms displaying global two-fold symmetry. An economical method has been used to calculate the complex, non-stationary, supersonic gas flows using a fully conservative difference scheme to solve the appreciably divergent gas-dynamical equations in the Euler variables in an axially symmetric approximation.