Dynamics of molecular clouds on the galactic scale

We have carried out an extensive investigation into the dynamics of the molecular clouds in the disk of the Galaxy. We have used both computational methods and physical arguments to try to understand how the ensemble of molecular clouds interacts, how the clouds are affected by the gravitational field of the Galaxy and also the circumstances under which they can aggregate into giant molecular clouds (GMC's).The dynamical model is three dimensional and consists of 120,000 spherical clouds, each having a mass of 10⁴ M . It allows for the mutual gravitation between clouds, up to a cut-off distance; when two clouds collide they rebound, with a specified coefficient of restitutione. We have also developed a physically more realistic model for a cloud, supported by a magnetic field, and used it to select a suitable range of values fore. Our first paper deals with the case of an axially symmetrical galaxy. The clouds are distributed initially in a disk extending 100 pc on either side of the Galactic plane. As it evolves the system of clouds loses energy, and the disk grows thinner at a rate which depends on the value ofe. GMC's start to form once the disk is thin enough. We believe this result to be valid more generally, and that it holds also in models with spiral structure.     

Chemodynamical Modelling of Galaxy Formation and Evolution

We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 10⁹ M_⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Gravitational instability in a system of particles with mass spectrum

We investigate the conditions under which a self-gravitating system of particles of different masses may be gravitationally unstable if there is a systematic correlation between the random velocity of a particle and its mass. For an isotropic uniform medium without rotation but with mass spectrum and velocity depending on the particle mass, a situation arises where the Jeans length for such a system may be significantly smaller than for the case when some mean values are used instead of mass and velocity spectra. For a differentially rotating medium, representing a spiral galaxy, we obtain the analogue of the Toomre parameter for a heterogeneous (multi-component) system. We demonstrate that the gas system in spirals represented by an ensemble of giant molecular clouds may be considerably less stable in the case of random velocity–mass correlation than for a system with unique velocity dispersion.     

Velocities of Magneto-Elastic and Magneto-Electron Waves in Dark Molecular Clouds

We present analytic and numerical estimates for group velocity of wave motions in two models of cold interstellar medium presumably constituting the interior of cores of magnetically supported dark molecular clouds. Namely, in the model of gas-based ferrocolloidal soft matter and in the model of noncompensated electron magnetoplasma. The predictions of these models are given in juxtaposition with data on recent Zeeman measurements of the molecular linewidths detected from dark central regions of star-forming interstellar clouds.     

Formation of a Fractal Structure of Giant Molecular Clouds in the Galaxy

Observations show that molecular clouds in the interstellar medium are fractals with a dimensionality close to 2.35. A model for the formation of clouds from cloudlets ejected from stars is examined in this paper. It is shown that the motion of cloudlets in the interstellar medium is described by a model of generalized brownian motion, so that the resulting clouds should have a fractal structure. We examine the hypothesis that the fractal dimensionality of a cloud is completely determined by the way the mass of the cloudlets varies. The generalized brownian motion of an ensemble of particles is described as a random process with a time dependent parameter. The relationship among the growth of the cloudlet mass, the properties of the process by which the cloudlets move, and the fractal dimensionality of the structures resulting from this process is examined. It is shown that the fractal dimensionality of the formed clouds corresponds to the natural aggregation of mass assuming random cloudlet collisions.     

The interstellar magnetic field near the Galactic center

We review the current observational knowledge of the interstellar magnetic field within ∼150 pc ofthe Galactic center. We also discuss the various theoretical scenarios that have been put forward to explain the existing observations. Our critical overview leads to two important conclusions: (1) The interstellar magnetic field near the GC is approximately poloidal on average in the diffuse intercloud medium and approximately horizontal in dense interstellar clouds. (2) In the general intercloud medium, the field is relatively weak and probably close to equipartition with cosmic rays (B ∼ (6–20) μ G), but there exist a number of localized filaments where the field is much stronger (some filaments could possibly have B ≳ 1 mG). In dense interstellar clouds, the field is probably rather strong, with typical values ranging between a few 0.1 mG and a few mG (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

星系中原子和分子气体的半解析模型

星际气体是星系中重子物质的重要组成部分,其中的分子气体（主要是分子氢H₂）以及原子气体（主要是中性氢HI）对于星系中发生的各个物理过程至关重要。本文在前人的星系形成和演化的半解析模型基础上,加入了描述星系盘中分子气体和原子气体成分的物理模型,来研究分子气体和原子气体对于星系形成和演化所起的作用。我们主要使用了马普天体物理所Munich Group的L-Galaxies半解析星系形成模型,并借鉴了星系化学演化模型的方法,把半解析模型中的每一个星系盘分成了多个同心圆圈,然后在每个圈中分别追踪气体下落、分子气体和原子气体转化、恒星形成、金属增丰、超新星爆发加热冷气体等发生在星系盘上的物理过程,并且每个同心圈都是独立演化的。在我们的模型中,一个基本假设是每个时间步内气体都是以指数形式下落到星系盘上,并且直接叠加在已有的气体径向面密度轮廓之上,其中指数盘的标长r_d正比于星系所在暗物质晕的维里半径r_vir与旋转参量λ的乘积。我们的模型使用了两种描述分子气体形成的模型:一种是基于Krumholz等人解析模型的结果,其中分子气体的比例与局域气体面密度以及局域气体金属丰度相关;另一种是分子气体比例与星际压强相关的模型,根据Obreschkow等人的近似,分子气体的比例与气体面密度以及恒星质量面密度相关。由于恒星形成过程发生在星际巨分子云之中,并且根据Leroy等人的观测结果,恒星形成率面密度近似正比于分子气体的面密度,因此我们在模型中使用了与分子气体面密度相关的恒星形成规律。     

Molecular cloud abundances and anomalous diffusion

The chemistry of molecular clouds has been studied for decades, with an increasingly general and sophisticated treatment of the reactions involved. Yet the treatment of turbulent diffusion has remained extremely sketchy, assuming simple Fickian diffusion with a scalar diffusivity D. However, turbulent flows similar to those in the interstellar medium are known to give rise to anomalous diffusion phenomena, more specifically superdiffusion (increase of the diffusivity with the spatial scales involved). This paper considers to what extent and in what sense superdiffusion modifies molecular abundances in interstellar clouds. For this first exploration of the subject we employ a very rough treatment of the chemistry and the effect of non‐uniform cloud density on the diffusion equation is also treated in a simplified way. The results nevertheless clearly demonstrate that the effect of superdiffusion is quite significant, abundance values at a given radius being modified by order of unity factors. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetic Monte Carlo method for simulating astrochemical kinetics: Hydrogen chemistry in diffuse clouds

We perform numerical simulations of the molecular hydrogen production on the surface of interstellar dust grains and its dissociation by the ultraviolet background in conditions typical for the interstellar medium. The kinetic version of the Monte Carlo method is used for the modeling of the catalytic chemical reactions on the surface of the dust fraction and in the surrounding medium. Our simulations show the importance of the interstellar dust particles for hydrogen chemistry in diffuse molecular clouds.     

Dust metamorphosis in the galaxy

Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations.     

Influence of gamma-ray emission on the isotopic composition of clouds in the interstellar medium

We investigate one mechanism of the change in the isotopic composition of cosmologically distant clouds of interstellar gas whose matter was subjected only slightly to star formation processes. According to the standard cosmological model, the isotopic composition of the gas in such clouds was formed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in the Universe. The dispersion in the available cloud composition observations exceeds the errors of individual measurements. This may indicate that there are mechanisms of the change in the composition of matter in the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction and production rates of light isotopes (D, ³He, ⁴He) under the influence of photonuclear reactions triggered by the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction and production of light elements depending on the spectral characteristics of the gamma-ray emission. We show that in comparison with previous works, taking into account the influence of spectral hardness on the photonuclear reaction rates can increase the characteristic radii of influence of the gamma-ray emission from AGNs by a factor of 2?C8. The high gamma-ray luminosities of AGNs observed in recent years increase the previous estimates of the characteristic radii by two orders of magnitude. This may suggest that the influence of the emission from AGNs on the change in the composition of the medium in the immediate neighborhood (the parent galaxy) has been underestimated.     

Turbulent molecular clouds

Stars form within molecular clouds but our understanding of this fundamental process remains hampered by the complexity of the physics that drives their evolution. We review our observational and theoretical knowledge of molecular clouds trying to confront the two approaches wherever possible. After a broad presentation of the cold interstellar medium and molecular clouds, we emphasize the dynamical processes with special focus to turbulence and its impact on cloud evolution. We then review our knowledge of the velocity, density and magnetic fields. We end by openings towards new chemistry models and the links between molecular cloud structure and star-formation rates.     

Three components of 3–4 μm absorption bands

We present 2–4 μm spectra of six infrared sources for which the extinction is not purely interstellar, but is dominated by circumstellar or molecular cloud dust. In all cases the absorption bands differ from the interstellar case, though a component of the interstellar absorption is often present. We also present an improved absorption spectrum for the galactic centre source IRS 7, correcting spurious features in our previous spectrum. Three independent components can be identified: (i) The interstellar component, probably of organic origin, and itself not necessarily invariant; (ii) The symmetrical water ice feature at 3.06 μm, found most commonly in molecular clouds; (iii) A component at 3.53 μm possibly identified with solid formaldehyde grains, and seen only in molecular clouds because of its weakness. Other absorption components appear to be unrelated to those in the 3–4 μm region, most notably the 10 μm absorption found in oxygen-rich giants and the interstellar medium, and presumable inorganic in nature. Our observations include the first detection of water ice absorption in a source in the ρ Oph dark cloud. Biological materials provide the best fit to the interstellar case, but do not presently account for the distinct 3.53 μm component. We stress the need for further laboratory experiments using simpler organic materials.     

Surface distribution of dark clouds in M31 absorption gradient along the major axis,I

The distribution of dust clouds in M31 was studied based on a photocopy of this galaxy taken from Hubble'sAtlas. The picture was divided into strips starting from the center of the galaxy on both sides along the major and the minor axis, respectively. The number of dark clouds per square kiloparsec was estimated as a function of the distance from the center of the galaxy along the major axis. If we assume the validity of the standard cloud model in M31, the filling factor introduced by the model was found to vary with distance from the center. It was found furthermore, that the filling factor introduced by the model may be an upper limit. The counts of dark clouds indicate a periodical change in the absorption gradient along the major axis with a very slight systematic variation assumed by the exponential part of the representation.     

Linear kinetic theory of instabilities of a gravitatingstellar disk

Linear kinetic theory is developed to describe collective oscillations (and their instabilities) propagating in a rapidly rotating disk of stars, representing a highly flattened galaxy. The analysis is carried out for the special case of a self-gravitating, infinitesimally thin, and spatially inhomogeneous system, taking into account the effects both of thermal movements of stars and of gravitational encounters between stars and giant molecular clouds of an interstellar medium. The star–cloud encounters are described with the use of the Landau collision integral. The dynamics of gravity perturbations with rare interparticle encounters is considered. Such a disk is treated by employing the well elaborated mathematical formalisms from plasma perturbation theory using normal-mode analysis. In particular, the method of solving the Boltzmann equation is applied by integration along paths, neglecting the influence of star–cloud encounters on the distribution of stars in the zeroth-order approximation. We are especially interested in important kinetic effects due to wave–star resonances, which we have little knowledge about. The kinetic effects are introduced via a minor drift motion of stars which is computed from the equations of stellar motion in an unperturbed central force field of a galaxy. The dispersion laws for two main branches of disk's oscillations, that is the classical Jeans branch and an additional gradient branch, are deduced. The resonant Landau-type instabilities of hydrodynamically stable Jeans and gradient gravity perturbations is considered to be a long-term generating mechanism for propagating density waves, thereby leading to spiral-like and/or ring-like patterns in the flat galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Star formation in a multi-phase ISM

We present a 3d code for the dynamical evolution of a multi-phase interstellar medium (ISM) coupled to stars via star formation (SF) and feedback processes. The multi-phase ISM consists of clouds (sticky particles) and diffuse gas (SPH): exchange of matter, energy and momentum is achieved by drag (due to ram pressure) and condensation or evaporation processes. The cycle of matter is completed by SF and feedback by SNe and PNe. A SF scheme based on a variable SF efficiency as proposed by Elmegreen and Efremov (1997) is presented. For a Milky Way type galaxy we get a SF rate of ∼1 M_⊙ yr^-1 with an average SF efficiency of ∼5%. This revised version was published online in August 2006 with corrections to the Cover Date.     

A model of supernova feedback in galaxy formation

A model of supernova feedback during disc galaxy formation is developed. The model incorporates infall of cooling gas from a halo, and outflow of hot gas from a multiphase interstellar medium (ISM). The star formation rate is determined by balancing the energy dissipated in collisions between cold gas clouds with that supplied by supernovae in a disc marginally unstable to axisymmetric instabilities. Hot gas is created by thermal evaporation of cold gas clouds in supernova remnants, and criteria are derived to estimate the characteristic temperature and density of the hot component and hence the net mass outflow rate. A number of refinements of the model are investigated, including a simple model of a galactic fountain, the response of the cold component to the pressure of the hot gas, pressure-induced star formation and chemical evolution. The main conclusion of this paper is that low rates of star formation can expel a large fraction of the gas from a dwarf galaxy. For example, a galaxy with circular speed 50 km s¹ can expel 6080 per cent of its gas over a time-scale of 1 Gyr, with a star formation rate that never exceeds 0.1 M yr¹. Effective feedback can therefore take place in a quiescent mode and does not require strong bursts of star formation. Even a large galaxy, such as the Milky Way, might have lost as much as 20 per cent of its mass in a supernova-driven wind. The models developed here suggest that dwarf galaxies at high redshifts will have low average star formation rates and may contain extended gaseous discs of largely unprocessed gas. Such extended gaseous discs might explain the numbers, metallicities and metallicity dispersions of damped Lyman systems.     

The Evolution of the Interstellar Medium over Cosmic Time

C23 UV spectroscopy of the PG1159-type star NGC7094 C26 Variations of the radio synchrotron spectral index in the interstellar medium of M33 C38 Angular Momentum Evolution of Young Brown Dwarfs and Low Mass Stars C48 The radio halo of the nearby starburst galaxy NGC 253 C95 Signatures of early metal enrichment in Damped-Lyman Alpha systems C113 CO 4 → 3 and [CI] 1 → 0 in the centers of NGC4945 and Circinus C115 Ratio of atomic and molecular gas and gravitational stabilty in the disk of M51 C130 The Interstellar Mediumat Early Cosmic Times: Molecular Gas in Distant Quasar Host Galaxies C188 Probing the interstellar medium in distant galaxies with SPICA/ESI C191 The evolution of spectral energy distributions of galaxies over cosmic times C197 Observations of 60Fe in the Galaxy with INTEGRAL/SPI C204 Evolution of Interstellar Clouds in a hot Gas Environment C205 The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies C206 Energy and element deposit into the interstellar medium during the lives of massive stars C209 The distribution and kinematics of massive stars in the inner Galaxy mapped with SPI/INTEGRAL 26Al 1.8 MeV line observations C213 PDR modelling of the Galactic FIR line emission C239 Towards a complete picture of the molecular ISM in local Luminous Infrared Galaxies: first results from the JCMT/IRAM line survey C242 The Search for the Very High-redshift Tail of Submillimeter Galaxies     

The mean lifetime of interstellar molecular clouds

We describe the results of a study of the mean lifetime of molecular clouds based on actual observations. Using the model of growth from cloud-cloud collision and observations of ¹³CO along the galactic equator between longitudes 27°.85 and 40°, we derive a lower limit of 1 × 10⁹ yr for the lifetime of the interstellar molecular clouds     

Effects of shock processing on Serkowski polarization curves

This paper presents a semi-empirical model for variations of interstellar polarization curves based upon the Serkowski-Wilking law for optical and near-infrared wavebands. The model assumes that nonspherical dust grains producing interstellar polarization are core-mantle particles shaped like oblate spheroids. The physical picture is one in which large (a ₀ 0.1µm) particles in the dense cloud phase are deposited into the diffuse cloud medium and thereafter undergo mantle processing by galactic shocks and UV starlight. It is shown that polarization curves vary their widths mainly as a consequence of the nonthermal sputtering of mantles by low-velocity shocks. Mantle sputtering by shocks in low density clouds tends to broaden the curves, whereas mantle sputtering by shocks in denser clouds produce narrow curves. Hence, shock processing of grain mantles can explain the observed correlation between the width of polarization curves and the dust grain environment.