Magnetized Interstellar Medium Research Articles

The role of the random magnetic fields in the ISM: HVC numerical simulations

AbstractWe know that the galactic magnetic field possesses a random component in addition to the mean uniform component, with comparable strength of the two components. This random component is considered to play important roles in the evolution of the interstellar medium (ISM). In this work we present numerical simulations associated with the interaction of the supersonic flows located at high latitude in our Galaxy (High Velocity Clouds, HVC) with the magnetized galactic ISM in order to study the effect that produces a random magnetic field in the evolution of this objects.

The magnetic structure of our Galaxy: a review of observations

AbstractThe magnetic structure in the Galactic disk, the Galactic center and the Galactic halo can be delineated more clearly than ever before. In the Galactic disk, the magnetic structure has been revealed by starlight polarization within 2 or 3 kpc of the Solar vicinity, by the distribution of the Zeeman splitting of OH masers in two or three nearby spiral arms, and by pulsar dispersion measures and rotation measures in nearly half of the disk. The polarized thermal dust emission of clouds at infrared, mm and submm wavelengths and the diffuse synchrotron emission are also related to the large-scale magnetic field in the disk. The rotation measures of extragalactic radio sources at low Galactic latitudes can be modeled by electron distributions and large-scale magnetic fields. The statistical properties of the magnetized interstellar medium at various scales have been studied using rotation measure data and polarization data. In the Galactic center, the non-thermal filaments indicate poloidal fields. There is no consensus on the field strength, maybe mG, maybe tens of μG. The polarized dust emission and much enhanced rotation measures of background radio sources are probably related to toroidal fields. In the Galactic halo, the antisymmetric RM sky reveals large-scale toroidal fields with reversed directions above and below the Galactic plane. Magnetic fields from all parts of our Galaxy are connected to form a global field structure. More observations are needed to explore the untouched regions and delineate how fields in different parts are connected.

Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2‐216

We present 1420 MHz polarization images of a -->2.5° × 2.5° region around the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian Galactic Plane Survey (CGPS). An arc of low polarized intensity (size -->0.2° × 0.7°) appears prominently in the northeast portion of the visible disk of Sh 2-216, coincident with the optically identified interaction region between the PN and the interstellar medium (ISM). The arc contains structural variations down to the ~1 -->' resolution limit in both polarized intensity and polarization angle. Several polarization-angle knots appear along the arc. By comparison of the polarization angles at the centers of the knots and the mean polarization angle outside Sh 2-216, we estimate the rotation measure (RM) through the knots to be –43 ± 10 rad m−2. Using this estimate for the RM and an estimate of the electron density in the shell of Sh 2-216, we derive a line-of-sight magnetic field in the interaction region of -->5.0 ± 2.0 -->μ G . We believe it more likely the observed magnetic field is interstellar than stellar, although we cannot completely dismiss the latter possibility. We interpret our observations via a simple model which describes the ISM magnetic field around Sh 2-216, and comment on the potential use of old PNe as probes of the magnetized ISM.

On the origin of asymmetries in bilateral supernova remnants

Aims. We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field. Methods. We perform 3D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons. Results. We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations. Conclusions. BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.

Soft gamma-ray background and light dark matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the ($S$- or $P$-wave) annihilation of light dark matter particles into ${e}^{+}{e}^{\ensuremath{-}}$. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about $4\text{ }\text{ }\mathrm{keV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

The Distribution of Pressures in a Supernova‐driven Interstellar Medium. I. Magnetized Medium

Observations have suggested substantial departures from pressure equilibrium in the interstellar medium (ISM) in the plane of the Galaxy, even on scales under 50 pc. Nevertheless, multiphase models of the ISM assume at least locally isobaric gas. The pressure then determines the density reached by gas cooling to stable thermal equilibrium. We use numerical models of the magnetized ISM to examine the consequences of supernova driving for interstellar pressures. In this paper we examine a (200 pc)3 periodic domain threaded by magnetic fields. Individual parcels of gas at different pressures reach widely varying points on the thermal equilibrium curve: no unique set of phases is found but rather a dynamically determined continuum of densities and temperatures. A substantial fraction of the gas remains entirely out of thermal equilibrium. Our results appear consistent with observations of interstellar pressures. They also suggest that the high pressures observed in molecular clouds may be due to ram pressures in addition to gravitational forces. Much of the gas in our model lies far from equipartition between thermal and magnetic pressures, with ratios ranging from 0.1 to 104 and ratios of uniform to fluctuating magnetic field of 0.5-1. Our models show broad pressure probability distribution functions with log-normal functional forms produced by both shocks and rarefaction waves rather than power-law distributions produced by isolated supernova remnants. The width of the distribution can be described quantitatively by a formula derived from the work of Padoan, Nordlund, & Jones.

Collisionless Slowing Down of Nova and Supernova Shells in Magnetized Interstellar Medium

The collisionless interaction of an expanding plasma cloud with a magnetized background plasma is examined in the framework of a 3D kinetic-hydrodynamic model. The slowing down of a hydrogen cloud is studied for high Alfven-Mach numbers and magneto-laminar interaction parameters. A particle-in-cell method is used to study the dynamics of the magnetic field, plasma cloud, background plasma, and collisionless shock wave generated by the intense particle flux. A numerical simulation is consistent with the nonstationary interactions between the plasma shells formed during nova and supernova explosions and the interstellar plasma medium.

Magnetohydrodynamic density waves in a composite disc system of interstellar medium and cosmic ray gas

Multiwavelength observations from radio to soft X-ray bands of large-scale galactic spiral structures offer synthesized and comprehensive views of nearby disc galaxies. In the presence of a massive dark matter halo, the density-wave dynamics on galactic scales involves the stellar disc, the gas disc of the interstellar medium (ISM), the magnetic field and the cosmic ray gas (CRG). In this paper, we explore the dynamic and electromagnetic interplay between the magnetized ISM disc and the CRG disc so that structural and diagnostic features of optical, infrared and synchrotron radio-continuum emissions from a spiral galaxy can be understood physically. On time-scales of galactic density waves, cosmic rays collectively may be treated as a relativistically hot tenuous gas fluid that is tied to the large-scale mean magnetic field in transverse bulk motions but moves otherwise differently along the magnetic field relative to the ISM. For both fast and slow magnetohydrodynamic (MHD) density waves in a composite disc system of magnetized ISM and CRG, the minute CRG mass-density enhancement is phase shifted relative to the enhancement of the parallel magnetic field. Owing to the extremely small number of cosmic rays, the large-scale magnetic field enhancement dominates in synchrotron radio-continuum emissions (as if the CRG is almost unperturbed) for spiral structural manifestations. In addition to the fast and slow MHD density waves, there also exists a suprathermal MHD wave mode by which CRG adjusts itself with an effective suprathermal speed of sound close to the speed of light c.

Multiwavelength Structural Manifestations of Fast Magnetohydrodynamic Density Waves in Spiral Galaxies

In a spiral galaxy, disks of stars and of magnetized interstellar medium (ISM) are dynamically coupled by gravity. The theory of magnetohydrodynamic (MHD) density waves in such a composite rotating disk system provides a basic framework for studying large-scale dynamics and multiwavelength diagnostics of galactic structures. As swirllike, trailing spiral structures with broad, fuzzy arms (regular or irregular) manifest in neutral hydrogen H I disks that are usually larger than optical spiral patterns in disk galaxies, spiral MHD density waves in H I gas disks should persist within and beyond optical patterns. Sporadic diffusions of relativistic cosmic-ray electrons beyond optical patterns may cause radio continuum arms to occasionally extend across optical spiral patterns and correlate with H I spiral arms at larger radii. We here report the first H I observations of the southern spiral galaxy NGC 2997 to support the fast MHD density wave (FMDW) scenario and to confirm the prior prediction that the isolated, polarized radio continuum arm discovered in the southeast quadrant is indeed associated with a broad segment of the H I arm. For multiwavelength observations of spiral galaxies, we emphasize the important perspective that large-scale magnetic fields of MHD density waves play the key role in organizing correlated spiral structures of the various underlying ISM components. Meanwhile, we briefly discuss the nature of circumnuclear structures of NGC 2997 in reference to those of NGC 1097.

Three‐dimensional Magnetohydrodynamic Numerical Simulations of Cloud‐Wind Interactions

We present results from three-dimensional numerical simulations investigating the magnetohydrodynamics of cloud-wind interactions. The initial cloud is spherical, while the magnetic field is uniform and transverse to the cloud motion. A simplified analytical model that describes the magnetic energy evolution in front of the cloud is developed and compared with simulation results. In addition, it is found that the interaction of the cloud with a magnetized interstellar medium results in the formation of a highly structured magnetotail. The magnetic flux in the wake of the cloud organizes into flux ropes, and a reconnection current sheet is developed as field lines of opposite polarity are brought close together near the symmetry axis. At the same time magnetic pressure is strongly enhanced at the leading edge of the cloud from the stretching of the field lines that occurs there. This has an important dynamical effect on the subsequent evolution of the cloud, since some unstable modes tend to be strongly enhanced.

Large-scale magnetohydrodynamic density-wave structures in the Andromeda nebula

The Andromeda galaxy (also referred to as M31 or NGC 224) shows approximately axisymmetric large-scale structures which are identified here with fast magnetohydrodynamic (MHD) density waves in a simple theoretical model characterized by a rotating composite system of a massive spherical halo (dark matter included), a stellar disc and a magnetized gas disc. For fast MHD density waves inside the Lindblad resonance, enhancements of stellar and gas surface mass densities as well as the azimuthal magnetic field are all roughly in phase. Based on this analysis and considerations of radiation processes in a magnetized interstellar medium, the observed overall correlation of annulus structures of M31 at many electromagnetic wavelengths can be naturally explained. In particular, the enhanced magnetic field (inferred from the synchrotron radio continuum) tends to align roughly along the magnetic torus of M31 as revealed by radio polarization studies. From observational estimates of physical parameters, the composite system of M31 is found to be quite stable – a fact that may account for the observed low global star formation rate in M31.

Interaction of Planetary Nebulae with a Magnetized ISM

We study the interaction of planetary nebulae with a magnetized interstellar medium (ISM) and show that the ISM magnetic field cannot be ignored in analysis of the interaction of planetary nebulae with (PNs) with the ISM. We show that stretching and bending of magnetic field lines can increase the effective pressure of the ISM. In particular, near the stagnation point of the flow of the ISM around the nebula, the ISM magnetic pressure can overcome the thermal pressure, leading to strong MHD instabilities even in isothermal flows. At early stages, when the planetary nebula outer regions are still neutral, the ISM magnetic field can penetrate the nebula via ambipolar diffusion. As a result, when pulled downstream by the flow, the magnetic field will strip the outer layers of the nebula. The Rayleigh-Taylor and Kelvin-Helmholtz instabilities with wavenumbers along the magnetic field lines are suppressed. Therefore, the ISM magnetic field can break the cylindrical symmetrical nature of the interaction.

ON THE DYNAMICAL EVOLUTION OF THE FRACTAL STRUCTURE OF INTERSTELLAR MEDIUM

Two-dimensional compressible turbulence in a self-gravitating, magnetic interstellar medium is calculated as an initial value problem. It is shown that even if the initial density distribution is homogeneous and the initial velocity distribution contains only a few Fourier components, the nonlinear interaction among the Fourier components will generate more and more Fourier components and lead to a turbulent and fractal structure in the interstellar medium. The calculations are carried out for three different initial states. In order to see the time evolution, detailed density distributions and fractal dimensions of the density contours are calculated at three moments of time for each of the initial states. The results show that the fractal dimension remains almost the same (~1.4–1.5), although the detailed density distribution has changed considerably. The insensibility of the fractal dimension of density contours to both the initial conditions and the evolution time is in good agreement with observations of molecular clouds in the interstellar medium.

Hot bubbles in a magnetic interstellar medium - Another look at the soft X-ray background

An attempt is made to understand the origin and properties of an isolated local bubble of hot gas using models of explosive events in a magnetic interstellar medium which mimic the local bubble in terms of the C band X-ray surface brightness and radius. The residual bubble of hot gas reaches a maximum size with an internal pressure below ambient, and then shrinks to smaller volume and a pressure nearly equal to ambient. The X-ray brightness reaches its minimum at the time of maximum radius and rises thereafter. It is found that the bubble growth must have been confined by a probably unacceptably large external pressure in order for the hot gas in the maximally extended cavity to radiate at a rate like that observed in the soft X-ray background.

Physical reasons and consequences of a three-dimensionally structured heliosphere

In this article we have discussed reasons both of solar and of interstellar origin giving rise to a pronounced three-dimensional structure of the expanding solar wind and thus of the global configuration of the heliosphere. Our present observational knowledge on these structurings is reviewed, and all attempts to theoretically model these solar wind structures are critically analysed with respect to their virtues and flaws. It is especially studied here by what mechanisms interstellar imprints on the actual type of solar wind expansion can be envisaged. With concern to this aspect it hereby appears to be of eminent importance that the solar system maintains a relative motion with a submagnetosonic velocity of about 23km/sec with respect to the ambient magnetized interstellar medium corresponding to a magnetosonic Mach number of about 0.5.

The formation of giant cloud complexes by the Parker-Jeans instability

A previously derived dispersion relation is used to evaluate dominant modes in the Parker-Jeans instability, with a view to its proposition as the mechanism of giant cloud complex formation near the OB associations. Results show that clouds as massive as about one million solar masses may form as quickly as 12 million years in the approximately 5/cu cm, high-density environments associated with spiral-density wave shocks. It is also determined that the masses of the clouds that form when density exceeds about 3/cu cm are insensitive to magnetic field strength, cosmic ray pressure, and ambient density, and that these masses are essentially the Jeans mass in a magnetic interstellar medium. The similarity of the local OB associations may be explained by the occurrence of a characteristic mass.