Magnetic Field in Selected Directions of the Galaxy, Direction of the Spiral Arm of Sagittarius

Faraday rotation data on 40 pulsars are used in a detailed study of the magnetic field and its fluctuations in the direction of the spiral arm of Sagittarius. These results mostly agree with standard models for the galactic magnetic field. A magnetic field on the order of 3.2 G is directed from galactic longitude l ₀=55° (toward the sun). However, an asymmetry has been found in the degrees of rotation relative to a plane lying in the southern hemisphere parallel to the galactic plane and at a distance of 50-60 pc from it. All the pulsars with measures of dispersion greater than 30 pc·cm^-3 and lying to the north of this plane have positive measures of rotation which increase linearly with distance, while the pulsars lying to the south of this plane have unusually absolutely low negative measures of rotation. We propose that the spiral arm of Sagittarius lies entirely to the north of this plane, while the negative measures of rotation of the pulsars below this plane are caused by the magnetic field of the halo of the southern hemisphere of the galaxy. The magnetic field in the arm of Sagittarius is regular to a great extent and its fluctuating component is roughly half the regular component.     

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。     

Magnetic fields and the dynamics of spiral galaxies

We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H  i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength.
Furthermore, we find that the introduction of a cold H  i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.     

Magnetic and optical spiral arms in the galaxy NGC 6946

The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at λλ 3.5 and 6.2 cm, in broadband red light, in the λ 21.1 cm H  i line and in the optical Hα line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5–12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3–0.7 at r ≃5 kpc, decreases by a factor of two at r =5–6 kpc and remains low at 0.2–0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3–0.6 in a wide radial range r =1.5–12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields.     

Outer pseudoring in the galaxy

The kinematics of the Sagittarius (R = 5.7 kpc),Carina (R = 6.5 kpc), Cygnus (R = 6.8 kpc), and Perseus (R = 8.2 kpc) arms suggests the existence of two spiral patterns in the Galaxy that rotate with different speeds. The inner spiral pattern that is represented by the Sagittarius arm rotates with the speed of the bar, Ω_b = 60 ± 5 km s⁻¹ kpc⁻¹, while the outer spiral pattern that includes the Carina, Cygnus, and Perseus arms rotates with a lower speed, Ω_s = 12–22 km s⁻¹ kpc⁻¹.The existence of an outer slow tightly wound spiral pattern and an inner fast spiral pattern can be explained by numerically simulating the dynamics of outer pseudorings. The outer Lindblad resonance of the bar must be located between the Sagittarius and Carina arms. The Cygnus arm appears as a connecting link between the fast and slow spiral patterns.     

Mid-infrared polarization studies of SgrA: a three-dimensional study of the central parsec

Thermal emission from magnetically aligned dust grains produces the observed mid-infrared polarization in the northern arm and east–west bar of SgrA West; recent arcsecond-resolution imaging polarimetry at 12.5 μm of these ionized filaments is presented, which confirms and extends previous studies. A lower limit ∼2 mG is found for the magnetic field in the northern arm and the IRS16 complex appears to be displaced from the northern arm by ∼ 0.15 pc along the line of sight. It is shown that the physical conditions in the ionized filaments of the central parsec lead to a very uniform grain alignment that is directed along the local magnetic field. The position angle of polarized emission will then be at right angles to the projection of the field direction on the plane of the sky and its amplitude a measure of the component of field along the line of sight; this makes possible a partial reconstruction of the field in three dimensions. We present the first application of the use of polarimetry in this way. This partial reconstruction is compared with the H92α observations of Roberts et al. and the implications are that the northern arm and east–west bar do not define either an orbital path or a spiral arm but rather represent a tidally stretched structure in free fall about SgrA^★ with significant deviations from a single plane, and most likely represent the inner ionized rim of a more extended neutral cloud.     

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.     

Simulation of the apparent anomalies of the galactic structure

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.     

Magnetic fields and radio polarization of barred galaxies

In order to simulate evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo we apply a three-dimensional (3D) MHD numerical model. We solve a induction equation using a time-dependent velocity field of molecular gas resulting from self-consistent 3D N-body simulations of a galactic disk. The gaseous halo rotates differentially co-rotating with the disk. In our model we introduce the dynamo process causing the amplification of the magnetic field as well as the formation of field structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed structures of polarization B-vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms, is demonstrated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sweeping-out of dust from spiral galaxy as a result of “magnetic sling” effect

A new mechanism of sweeping out of dust grains beyond galactic disks both in the radial direction along the galactic plane and in the vertical, cross-disk direction is proposed. The mechanism is driven by the interaction of dust grains with the bisymmetric nonstationary magnetic field of the galaxy, whose lines are curved and corotate with the stellar spiral density wave responsible for the arms. We attribute the radial transfer of interstellar dust grains in the plane of galactic disks to the fact that charged dust grains are “glued” to magnetic field lines and are therefore pushed outward because of the rotation of magnetic field lines and their tilt with respect to the radial direction parallel to the disk plane. In addition, dust is swept out vertically in the cross-disk direction because of the drift motion in crossed magnetic and gravitational fields (both are parallel to the galactic plane). Numerical computations of the motion of dust grains in real magneto-gravitational fields with the allowance for the drag force from interstellar gas show that the time scale of dust grain transport beyond galactic disks is on the order of 1 Gyr or shorter.     

Disk Thicknesses and Some Parameters of 108 Non-Edge-On Spiral Galaxies

We present disk thicknesses, some other parameters and their statistics of 108 non-edge-on spiral galaxies. The method for determining the disk thickness is based on solving Poisson's equation for a disturbance of matter density in three-dimensional spiral galaxies. From the spiral arms found we could obtain the pitch angles, the inclination of the galactic disk, and the position of the innermost point (the forbidden region with radius r 0 to the galactic center) of the spiral arm, and finally the thickness.     

The distribution of the galactic diffuse radio background

The distribution of the galactic radio emission is analyzed and a three component model (Halo, Base Disk and Spiral Arms) of the observed radiation examined.To reach agreement between temperature measured at low and high galactic latitudes we need: (a) a low emissivity spherical halo, (b) the Sun inside the local spiral arm somewhere between the arm axis and the arm outer edge, and (c) free circulation of the radiating electrons between the three emitting regions.     

Ultra-high energy cosmic rays in the galactic corona

On the basis of recent new information on regular and chaotic magnetic fields in coronae of spiral galaxies, we discuss propagation of ultra-high energy cosmic rays of energies exceeding 10¹⁷ eV in the galactic corona. It is shown that the expected regular magnetic field is able to confine to the corona protons of energies up to 3×10¹⁹ eV. Chaotic magnetic fields of the corona play an important role in dynamics of cosmic-ray protons of energy up to 7×10¹⁸ eV.     

Massive star formation in the southern Milky Way

During the past decade we have compiled a large molecular line data base of massive star forming regions in the southern Milky Way. These regions are confined into giant molecular clouds that trace the galactic spiral arms. Their radial distribution has a pronounced peak midway between the Sun and the galactic center, which in the IV quadrant corresponds to the location of the Norma Spiral arm. We study in some detail one of the foremost regions of massive star formation in the Norma arm, using millimeter continuum and line emission maps obtained with the SEST, APEX, and ASTE telescopes. It is a multiple system evolving along a complete GMC core, candidate for future ALMA observations.     

The radio luminosity of pulsars and the distribution of interstellar electron density

The radio luminosities of pulsars are given as functions of their period and the time variation of the period. The parameters of that dependence are calculated and independent distances are determined for pulsars. The average electron densities toward the pulsars are determined from the known dispersion measures. The results obtained are used to study the large-scale electron density distribution in the Galaxy. The distribution maximum lies in the vicinity of the Sagittarius spiral arm. The electron density falls off exponentially in the regions between spiral arms. This revised version was published online in July 2006 with corrections to the Cover Date.     

The radio luminosity of pulsars and the distribution of electron density in the galaxy

Radio luminosities of pulsars depend on their periods and derivatives of periods. The parameters of these dependences and the independent distances for 288 pulsars are determined. Known dispersion measures are used for determination of the mean electron densities in the direction of pulsars. The results obtained are used for investigation of the large-scale distribution of electron concentration in the galaxy. The maximum value of that distribution is found at a distance of 9 kpc from the galactic center in the Sagittarius arm. In the interarm regions electron density decreases roughly exponentially.Published in Astrofizika, Vol. 38, No. 4, pp. 587–592, October–December, 1995.     

The origin of our galactic magnetic field

A serious difficulty with the standard alpha‐omega theory of the origin of galactic magnetic fields involves the question of flux expulsion. This is intimately related to flux freezing. The alpha‐omega theory is shown in the context of the giant superbubble explosions that have a large impact on the physics of the interstellar medium. It is shown that superbubbles alone can duplicate the processes of the alpha‐omega dynamo and produce exponential growth of the galactic magnetic field. The possibility of the blow‐out of pieces of the magnetic field is discussed and it is shown that they have the potential to solve the flux‐expulsion problem. However, such an explanation must lead to apparent ‘gaps’ in the field in the galactic disc. These gaps are probably unavoidable in any dynamo theory and should have important observable consequences, one of which is an explanation for the escape of cosmic rays from the disc (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cosmic Ray Nonlinear Processes in Space Plasma: Applications to the Dynamic Heliosphere

Influence of cosmic ray pressure and kinetic stream instability on space plasma dynamics and magnetic structure are considered. It is shown that in the outer Heliosphere are important dynamics effects of galactic cosmic ray pressure on solar wind and interplanetary shock wave propagation as well as on the formation of terminal shock wave of the Heliosphere and subsonic region between Heliosphere and interstellar medium. Kinetic stream instability effects are important on distances more than 40–60 AU from the Sun: formation of great anisotropy of galactic cosmic rays in about spiral interplanetary magnetic field leads to the Alfven turbulence generation by non isotropic cosmic ray fluxes. Generated Alfven turbulence influences on cosmic ray propagation, increases the cosmic ray modulation, decreases the cosmic ray anisotropy and increases the cosmic ray pressure gradient in the outer Heliosphere (the later is also important for terminal shock wave formation). This revised version was published online in July 2006 with corrections to the Cover Date.     

3D global simulations of galactic magnetic fields and gas flows

The evolution of three-dimensional (3D), dynamo excited galactic magnetic fields under the influence of a time-dependent gas flow in spiral arms is already well investigated. Our principal goal is to check how the dynamo-driven turbulent magnetic fields affect the gas flows. Numerical solutions of the full set of 3D MHD equations for dynamos in spiral galaxies are presented. Further we try to investigate the nonlinear evolution of magnetic instabilities in a global galactic model. The model includes differential rotation, eddy diffusivity and tensorial alpha-effect. In a first step the flow is driven by a prescribed gravitational potential. The vertical density stratification and the radial-azimutal spiral pattern are taken closely to observational data. We use a modified variant of the highly parallelized time-stepping ZeusMP code for the simulations of global galactic magnetic fields and gas flows. This revised version was published online in August 2006 with corrections to the Cover Date.