Magnetic fields in spiral galaxies

Radio polarization observations have revealed large-scale magnetic fields in spiral galaxies. The average total field strength most probably increases with the rate of star formation. The uniform field generally follows the orientation of the optical spiral arms, but is often strongest outside the arms. Long magnetic-field filaments are seen, sometimes up to a 30 kpc length. The field seems to be anchored in large gas clouds and is inflated out of the disk, e.g. by a galactic wind. The field in radio halos around galaxies is highly uniform in limited regions, resembling the structure of the solar corona. The detection of Faraday rotation in spiral galaxies excludes the existence of large amounts of antimatter. The distribution of Faraday rotation in the disks shows two different large-scale structures of the interstellar field; axisymmetric-spiral and bisymmetric-spiral, which are interpreted as two modes of the galactic dynamo driven by differential rotation     

Structure function scaling in compressible super-Alfvénic MHD turbulence

Supersonic turbulent flows of magnetized gas are believed to play an important role in the dynamics of star-forming clouds in galaxies. Understanding statistical properties of such flows is crucial for developing a theory of star formation. In this Letter we propose a unified approach for obtaining the velocity scaling in compressible and super-Alfvénic turbulence, valid for the arbitrary sonic Mach number, M(S). We demonstrate with numerical simulations that the scaling can be described with the She-Lévêque formalism, where only one parameter, interpreted as the Hausdorff dimension of the most intense dissipative structures, needs to be varied as a function of M(S). Our results thus provide a method for obtaining the velocity scaling in interstellar clouds once their Mach numbers have been inferred from observations.     

Darwin curves and galaxy arms

In the natural world, there exists one kind of structure which is beyond the scope of human laboratorial experiment. It is the structure of galaxies which is usually composed of billions of stars. Spiral galaxies are flat disk-shaped. There are two types of spiral galaxies. The spiral galaxies with some bar-shaped pattern are called barred spirals, and the ones without the pattern are called ordinary spirals. Longer-wavelength galaxy images (infrared, for example) show that ordinary spiral galaxies are basically an axi-symmetric disk that is called exponential disk. For a planar distribution of matter, Jin He defined Darwin curves in the plane as such that the ratio of the matter densities at both sides of the curve is constant along the curve. Therefore, the arms of ordinary spiral galaxies are Darwin curves. Now an important question is that: Are the arms of barred spiral galaxies the Darwin curves too? Fortunately, Jin He designed a piece of Galaxy Anatomy graphic software. With the software, not only can people simulate the stellar density distribution of barred spiral galaxies but also can draw the Darwin curves of the simulated galaxy structure. This paper shows partial evidence that the arms of galaxy NGC 3275, 4548 and 5921 follow Darwin curves.     

Long-term observations of the Seyfert galaxy NGC 1275 at ultrahigh energies by SHALON mirror Cherenkov telescopes

The Seyfert galaxy NGC 1275 is a central dominant galaxy in the Perseus cluster of galaxies. NGC 1275 is known as a powerful source of radio and X-ray radiations. In 1996, in SHALON telescope observations, a new metagalactic source of gamma radiation of ultrahigh energy > 800 GeV was detected. The position of the radiation source detected in our experiment is consistent in its coordinates with the Seyfert Galaxy NGC 1275. The results obtained at high and ultrahigh energies are necessary to understand the radiation generation processes in the entire wide energy range.     

Spiral structure and the dynamics of galaxies

A theory is proposed to explain the persistence of spiral structure in flat stellar systems. It is shown in a linear theory that spiral waves can grow spontaneously owing to star-wave interaction. The quasi-stationary spiral structure observed in actual galaxies is explained by the non-linear interaction of the growing stellar wave and the interstellar gas. It is shown that this interaction results in the time-asymptotic limit in a constant amplitude of the spiral wave. Finally we develop a quasi-linear theory of flat stellar systems that adequately can describe the properties of the stellar spiral waves on time-scales of the order of the life-time of these systems, provided the amplitude of the spiral wave does not become too large.     

Magnetised black hole as a gravitational lens

We use the Ernst–Schwarzschild solution for a black hole immersed in a uniform magnetic field to estimate corrections to the bending angle and time delay due-to presence of weak magnetic fields in galaxies and between galaxies, and also due-to influence of strong magnetic field near super-massive black holes. The magnetic field creates a kind of confinement in space, that leads to increasing of the bending angle and time delay for a ray of light propagating in the equatorial plane.     

Quantization effects in the plasma universe

It is suggested that a unification of the morphology of the solar system, anomalous intrinsic red shifts of quasars and galaxies, the structure of the hydrogen atom, the Einstein equations of general relativity, and Maxwell's equations can be accomplished by a basic consideration of the minimum-action states of cosmic and/or virtual vacuum field plasmas. A formalism of planetary formation theory leads naturally to a generalization which describes relativistic gravitational field theory in terms of a `pregeometry'. A virtual plasma associated with the vacuum state is postulated. It is demonstrated that the relaxed state of the virtual plasma underlies Einstein's field equation and predicts the proper form for the effective gravitational potential generated by the Schwarzschild solution of those equations. A further extension of the theory demonstrates that it also predicts the structure of the hydrogen atom described in terms of the Schrodinger equation of quantum mechanics. These concepts are applied in an attempt to explain the quantized anomalous red shifts in related galaxies as observed by H. Arp and J.H. Sulentic (1985). A possible unified field theory is suggested based on the above-mentioned concepts     

The theory of particle diffusion in a strong random magnetic field with an allowance for higher multipole moments of the distribution function

The kinetic equation including a small-scale collisional integral for the particles propagating in a strong random and regular magnetic field [29] is solved by expanding the distribution function into series in spherical harmonics of the particle momentum angles. Using methods of the quantum theory of the angular moment [41], the equations for higher multipole moments of the distribution function in the space of momentum angles are derived and solved in the stationary case for the galactic cosmic rays in interplanetary space. The observed amplitudes and phases of the diurnal variation harmonics can be explained using the results of measurements of the interplanetary magnetic field performed on board the Ulysses spacecraft [12–14] and other satellites [45, 46] with an allowance for redistribution of the interplanetary and interstellar magnetic field lines. The spatial structure of the convection and diffusion fluxes of the galactic cosmic rays is refined. Formulas taking into account a change in the Earth’s axis tilt relative to the direction toward the Sun are derived, which allow the annual changes in contributions to the diurnal variation harmonics to be determined. The equation of diffusion taking into account the 2nd harmonic is obtained, and the contribution of this effect to the relative particle density in the cosmic rays in a spherically symmetric case is analyzed.     

Ultimate energy of cosmic rays generated in supernova shells

It is shown that the ultimate energy of cosmic rays accelerated in a supernova shell due to the surfing acceleration mechanism is determined by the shell radius and the interstellar magnetic field. The ultimate energy of cosmic rays accelerated in the supernova shock does not exceed 10¹⁷ eV for typical values of the interstellar magnetic field in the vicinity of a supernova and the radii of observed supernova shells.     

Starburst galaxies as seen by gamma-ray telescopes

Starburst galaxies have a highly increased star-formation rate compared to regular galaxies and inject huge amounts of kinetic power into the interstellar medium via supersonic stellar winds, and supernova explosions. Supernova remnants, which are considered to be the main source of cosmic rays (CRs), form an additional, significant energy and pressure component and might influence the star-formation process in a major way. Observations of starburst galaxies at γ-ray energies give us the unique opportunity to study non-thermal phenomena associated with hadronic CRs and their relation to the star-formation process. In this work, recent observations of starburst galaxies with space and ground-based γ-ray telescopes are being reviewed, and the current state of theoretical work on the γ-ray emission is discussed. A special emphasis is put on the prospects of the next-generation Cherenkov Telescope Array for the study of starburst galaxies in particular and star-forming galaxies in general.     

Diffuse interstellar absorption bands

The diffuse interstellar bands (DIBs) are a large number of absorption bands that are superposed on the interstellar extinction curve and are of interstellar origin. Since the discovery of the first two DIBs in the 1920s, the exact nature of DIBs still remains unclear. This article reviews the history of the detections of DIBs in the Milky Way and external galaxies, the major observational characteristics of DIBs, the correlations or anti-correlations among DIBs or between DIBs and other interstellar features (e.g. the prominent 2175 Angstrom extinction bump and the far-ultraviolet extinction rise), and the proposed candidate carriers. Whether they are also present in circumstellar environments is also discussed. Supported by the NSF of USA (Grant No. AST-0707866), the National Natural Science Foundation of China (Grant No. 10878012), the Outstanding Overseas Young Chinese Scholarship, and the Laboratory of Particle Astrophysics, Institute of High Energy Physics, CASS     

Numerical solution of the radiation transport equation in disk geometry

We describe an efficient numerical method for solving the problem of radiation transport in a dusty medium with two dimensional (2-D) disk geometry. It is a generalization of the one-dimensional quasi-diffusion method in which the transport equation is cast in diffusion form and then solved as a boundary value problem. The method should be applicable to a variety of astronomical sources, the dynamics of which are angular-momentum dominated and hence not accurately treated by spherical geometry, e.g. protoplanetary nebulae, circumstellar disks, interstellar molecular clouds, accretion disks, and disk galaxies. The computational procedure and practical considerations for implementing the method are described in detail. To illustrate the effects of 2-D radiation transport, we compare some model results (dust temperature distributions and i.r. flux spectra) for externally heated, interstellar dust clouds with spherically symmetric and disk geometry.     

星系磁场和星系激波演化的一种模式

本文求解了二维不定常磁流体力学方程组,从而讨论星系磁场和星系激波的起源和演化过程。初始均匀的磁场将缠卷而在星系盘中形成螺旋形的结构,其中磁场和星际气体是冻结在一起的。分析了星际磁场对形成星系激波的影响。由于磁场不是很强,它对星系激波的影响不是非常强。     

Magnetic behavior of Ba3Cu3Sc4O12

The chain-like system Ba(3)Cu(3)Sc(4)O(12) has potentially interesting magnetic properties due to the presence of Cu(2+) and a structure-suggested low dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C(p), (45)Sc nuclear magnetic resonance (NMR), muon spin rotation (μSR), neutron diffraction measurements and electronic structure calculations for Ba(3)Cu(3)Sc(4)O(12). The onset of magnetic long-range antiferromagnetic (AF) order at T(N) ～ 16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T(N) to the applied magnetic field H (T(N) ～ 0 K for H = 70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature (θ(CW) ～ 65 K) in the susceptibility (from a 100 to 300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations, where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out the one-dimensional magnetic behavior suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.     

Spiral magneto-electron waves in interstellar gas dynamics

We discuss possible observational consequences resulting from the propagation of transverse magneto-electron waves in the interstellar medium. We briefly describe a magnetohydrodynamic model for the cyclotron waves with emphasis on their analogy with hydrodynamic inertial waves. It is shown that the cyclotron waves are heavily damped in the interstellar medium and, therefore, cannot affect the gas dynamics of star-forming molecular clouds. We developed an analytical model of the helicoidal magneto-electron waves based on the electromagnetic induction equation for the magnetic flux density driven by the Hall and Ohmic components of the electric field generated by flows of thermal electrons. It is established that the helicons can propagate in the interstellar medium without any noticeable attenuation. The presented numerical estimates for the group velocity of the intercloud helicons suggest that spiral circularly polarized magneto-electron waves of this type can be responsible for the broadening of molecular lines detected from dark interstellar clouds.     

Radio emission from interstellar neutral hydrogen

The emission line for neutral hydrogen at 1420 Mc/s is the only line so far discovered in radioastronomy. Since its mechanism of origin is completely understood, observations of this line provide direct information about conditions in interstellar space such as temperatures, densities and velocities. Extensive investigations of our own Milky Way system have shown clearly that it is a spiral galaxy similar to, but rather smaller than, the great spiral nebula in Andromeda. Our knowledge of the spiral structure of galaxies is far from complete; hydrogen-line measures of high-speed expansions in the centre of the Milky Way system may provide a clue to the understanding of this problem. In addition, determinations of the hydrogen content of different types of galaxy reinforce current theories which suggest that irregular galaxies evolve through the spiral form to elliptical galaxies.     

Magnetic and Magneto-Optical Properties in Paramagnetic NdF3 Under High Magnetic Field

In this paper, we first theoretically report the magnetic and magneto-optical properties in paramagnetic media under high external magnetic field. Considering the action of the external magnetic field H_e and indirect exchange interaction H_v, the characteristic of the magnetic saturation and the property of the Faraday rotation to be nonlinear with external magnetic field are presented in paramagnetic NdF₃. In terms of our theory, the indirect exchange interaction plays an important role in the magnetization M and the Faraday rotation θ in NdF₃ under high external magnetic field. The theory is in good agreement with experimental results. On the other hand, a reasonable explanation for the temperature dependence of the ratio of the Verdet constant to the magnetic susceptibility V/χ is obtained.     

Peculiar ferrimagnetism associated with charge order in layered perovskite GdBaMn2O5.0

The magnetic properties of GdBaMn2O5.0, which exhibits charge ordering, are studied from 2 to 400 K using single crystals. In a small magnetic field applied along the easy axis, the magnetization M shows a temperature-induced reversal which is sometimes found in ferrimagnets. In a large magnetic field, on the other hand, a sharp change in the slope of M(T) coming from an unusual turnabout of the magnetization of the Mn sublattices is observed. Those observations are essentially explained by a molecular field theory which highlights the role of delicate magnetic interactions between Gd3+ ions and the antiferromagnetically coupled Mn2+/Mn3+ sublattices.     

RX J0720.4-3125 as a possible example of magnetic field decay in neutron stars

We consider the possible evolution of the rotation period and magnetic field of the X-ray source RX J0720.4-3125, assuming that this source is an isolated neutron star accreting from the interstellar medium. The magnetic field of the source is estimated to be 10⁶–10⁹ G (the most probable value is about 2·10⁸ G), and it is difficult to explain the observed rotational period 8.38 s without invoking the hypothesis of magnetic field decay. For calculations we used the model of ohmic dissipation of the field in the core of the neutron star. Estimates for the accretion rate (10^?14–10^?16 M_⊙/yr), velocity of the source through the interstellar medium (10–50 km/s), and neutron star age (2·10⁹–10¹⁰ yrs) are obtained.