First Results from ISO Spectra of Supernova Remnants Heavily Interacting with the ISM

ISO spectra of the supernova remnant RCW103 are presented. This object is the prototype of a SNR shock heavily interacting with dense ISM (probably a molecular cloud). The spectra are dominated by prominent lines and show very little continuum at λ < 40 μm suggesting that the 12 and 25 μm IRAS emission from these types of remnant could be dominated by lines rather than continuum emission from warm dust heated by the shock as generally believed. The ISO data provide for the first time a simple and reliable estimate of the gas phase abundances of Si and Fe which are found to be close to solar relative to non refractory species such as Ne, S and Ar. This indicates that the shock is very effective in destroying the ISM dust and may therefore explain the absence of warm dust behind the shock. Like the optical Nickel lines, [NiII]6.63 μm yields Ni/Fe abundances a factor ≥ 10 above solar which we conclude results from a large underestimation of the computed Ni+ collision strengths. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaporation and condensation of giant interstellar clouds in a hot-gas environment

Gas phases of the interstellar medium (ISM) coexist locally, penetrate each other and mix by means of dynamical and plasmaphysical processes. E.g. heat conduction from the hot to the cooler gas leads to energy and mass exchange between the gas phases. Analytical solutions exist under which evaporation of cloudy material or condensation of hot gas onto the clouds' surface dominate. Since these results are derived for stationary and static conditions and under ideal assumptions, they do not necessarily hold for a dynamical ISM. On the other hand, the mass and energy exchange between the gas phases is of great importance for the energy budget of the ISM and by this influences the evolution of galaxies. This led us to investigate the evolution of interstellar clouds in a hot gas by means of numerical simulations. At first, we compare static models with the analytical results and found that interstellar clouds with parameters requiring analytically evaporation are, in contrast, accreting surrounding material if self-gravitation and cooling are implied. For the more realistic case, where clouds are embedded in a streaming hot gas, the models show that Kelvin-Helmholtz instability which leads to the disruption of the clouds is suppressed by heat conduction so that the clouds are stabilized to survive. This revised version was published online in September 2006 with corrections to the Cover Date.     

Dense Gas, Chemistry and Star Formation in Luminous Galaxies

The molecular phase of the ISM constitutes the main source of fuel for the activity in starburst and AGNs. The physical conditions and chemical constitution of the molecular gas will change with, and respond to, the evolution of the activity. This paper includes a short discussion of the ¹²CO/¹³CO 1–0 line intensity ratio as a diagnostic tool of the molecular gas properties of luminous galaxies – paired with examples of high-resolution studies of how the line ratio varies within galaxies. A possible connection between the OH megamasers and galaxies with unusually high ¹²CO/¹³CO 1–0 line intensity ratios are also briefly discussed.The relative intensities of the dense gas tracers HNC, HCN, HCO⁺ and CN are a result of both chemistry and starburst evolution. The discussion on the interpretation of HNC 1–0 emission includes the importance of ion-neutral chemistry in a luminous starburst region. Finally, simple cartoon ISM models and how they can be applied to LIRGs and ULIRGs, are presented.     

Interfacial instabilities driven by self-gravity: first numerical results

The evolution of the interstellar medium (ISM) is driven by a variety of phenomena, including turbulence, shearing flows, magnetic fields and the thermal properties of the gas. Among the most important forces at work is self-gravity, which ultimately drives protostellar collapse. As part of an ongoing study of instabilities in the ISM, Hunter, Whitaker & Lovelace have discovered another process driven by self-gravity: the instability of an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the non-linear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh–Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast, such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.     

Molecular absorptions in high-z objects

Molecular absorption lines measured along the line of sight of distant quasars are important probes of the gas evolution in galaxies as a function of redshift. A review is made of the handful of molecular absorbing systems studied so far, with the present sensitivity of mm instruments. They produce information on the chemistry of the ISM at z～1, the physical state of the gas, in terms of clumpiness, density and temperature. The CMB temperature can be derived as a function of z, and also any possible variations of fundamental constants can be constrained. With the sensitivity of ALMA, many more absorbing systems can be studied, for which some predictions and perspectives are described.     

Photo's from Proceedings of the Third MicroQuasar Workshop; Granada Worshop on Galactic Relativistic Jet Sources – Granada,Spain, 11–13 September 2000

Knowledge of the molecular component of the ISM is fundamental to understand star formation. The H₂ component appears to dominate the gas mass in the inner parts of galaxies, while the HI component dominates in the outer parts. Observation of the CO and other lines in normal and starburst galaxies have questioned the CO-to-H₂ conversion factor, and detection of CO in dwarfs have shown how sensitive the conversion factor is to metallicity. Our knowledge has made great progress in recent years, because of sensitivity and spatial resolution improvements. Large-scale CO maps of nearby galaxies are now available, which extend our knowledge on global properties, radial gradients, and spiral structure of the molecular ISM. Millimetric interferometers reveal high velocity gradients in galaxy nuclei, and formation of embedded structures, like bars within bars. Galaxy interactions are very effective to enhance gas concentrations and trigger starbursts. Nuclear disks or rings are frequently observed, that concentrate the star formation activity. Since the density of starbursting galaxies is strongly increasing with redshift, the CO lines and the mm dust emission are a privileged tool to follow evolution of galaxies and observe the ISM dynamics at high redshift: they could give an answer about the debated question of the star-formation history, since many massive remote starbursts could be dust-enshrouded.     

A statistical description of astrophysical turbulence

The properties of the ISM indicate that it is turbulent. However, the ISM turbulence is radically different from that in incompressible fluids. That is why it is so important to study it through observations. The relevant study still poses a challenging problem. In the present paper recent results based on a statistical approach to the problem are surveyed. Although it was pointed out long ago (see Kaplanet al., 1970) that random 3D motions of the ISM gas result in fluctuations of the observed electromagnetic emission, it is only recently that the problem of recovering statistical properties of the ISM turbulence from the line integrated data was given an adequate mathematical treatment. Here by the example of studying turbulence in HI, it is shown that the inverse problem can be solved uniquely using a realistic model of the ISM. The application of theoretical conclusions to existing data explains some facts which used to be considered inconsistent with turbulent behaviour and reveals unexpected features of the ISM turbulence.     

Precondensed matter: Key to the early solar system

Chemical and isotopic anomalies in meteorites may be understandable in terms of the chemical fractionation routinely expected in the interstellar medium (ISM). Dust of distinct composition is idealized as being of three types: (1) thermal supernova condensates (SUNOCONS), (2) thermal condensation during other stellar mass-loss processes (STARDUST), and (3) nonthermal sticking processes in cold nebulae (NEBCONS). Great depletions in ISM of Ca Al Ti are due to SUNOCONS, although STARDUST is about twice as abundant. An abundance table of interstellar SUNOCONS is presented. Parent bodies in the solar system are accumulated directly from the ISM. No hot solar condensation sequence is assumed. Only relatively volatile elements within NEBOCONS are vaporized in the warm solar accretion disk. Variations in the relative amounts of these components during accumulation processes plus subsequent solid chemistry may have produced such chemical anomalies as the meteoritic fractionation patterns and the Ca Al-rich inclusions. Isotopic anomalies result from four processes that selectively site specific isotopes: (1) extinct radioactivities, (2) distinct supernova shells, (3) gas-dust separation, and (4) gas-dust age difference. Planetary accumulation will have been fingerprinted by the chemical state of the ISM if this picture is correct.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Phase transitions in the ISM a source of dissipative behaviour

By considering a simple fluid model, we investigate the role of phase transitions in the ISM on the galaxy- scale gas dynamics. Cooling and heating timescales in the ISM are typically shorter than typical galactic rotation timescales, so the individual phases in the ISM can be assumed to be in temperature equilibrium with the radiation field. Using this approximation we can construct an equation of state which depends upon the average density and mass fractions in the individual phases. Previous studies suggest that there is an equilibrium phase fraction as a function of pressure. We incorporate evolution towards this equilibrium state as a relaxation term with a time to obtain equilibrium . We derive a condition in terms of a critical Mach number when one dimensional shocks should be continuous. For small values of the relaxation time we show that the relaxation term acts like a viscosity. We show with one dimensional simulations that increasing causes shocks to become smoother. This work suggests that phase changes can strongly effect the gas dynamics of the ISM across spiral arms and bars.     

Starburst galaxies – An observer’s view

Starbursts are the most efficient producers of metals in the Universe at low redshifts. They produce enough energy to driveoutflows of material from their disks.This makes them important objects to study in order to understand the chemical evolution not only of the interstellar medium (ISM) in the starburst galaxies themselves, but also of the intergalactic medium (IGM) in their vicinity. However, several key quantities of starbursts that are neededas input to models of their ISM are still ill-constrained. Some of these critical parameters are e.g. the metalabundances of hot ionized gas, the ionization state ofwarm ionized gas, the amount of energy deposited intothe ambient by a starburst, the efficiency of itsconversion into mechanical energy and thus the totalkinetic energy of the star formation-driven outflowsand their kinematics. The latter are important when considering under whichcircumstances matter energized by a starburst will reach the so-called ‘blowout’ condition, i.e. supersede the threshold energy starting at which local energy injection into the ISM can drive an outflow first into the halo (where metal re-distribution might be very efficient) and eventually out into intergalactic space. I will discuss here a few of these quantities, how we canmeasure them better than in the past, and in which way some of our observing techniques need to be improved in order toobtain better constraints from the data.     

Dynamical and chemical evolution of gas-rich dwarf galaxies

We study the effect of a single, instantaneous starburst on the dynamical and chemical evolution of a gas-rich dwarf galaxy, the potential well of which is dominated by a dark matter halo. We follow the dynamical and chemical evolution of the interstellar medium (ISM) by means of an improved two-dimensional hydrodynamical code coupled with detailed chemical yields originating from type II SNe, type Ia SNe and single low- and intermediate-mass stars (IMS). In particular we follow the evolution of the abundances of H, He, C, N, O, Mg, Si and Fe. We find that for a galaxy resembling IZw18, a galactic wind develops as a consequence of the starburst and it carries out of the galaxy mostly the metal-enriched gas. In addition, we find that different metals are lost differentially in the sense that the elements produced by type Ia SNe are lost more efficiently than others. As a consequence of that, we predict larger [ α /Fe] ratios for the gas inside the galaxy than for the gas leaving the galaxy. A comparison of our predicted abundances of C, N, O and Si in the case of a burst occurring in a primordial gas shows a very good agreement with the observed abundances in IZw18 as long as the burst has an age of ∼31 Myr and IMS produce some primary nitrogen. However, we cannot exclude that a previous burst of star formation had occurred in IZw18, especially if the pre-enrichment produced by the older burst was lower than Z =0.01 Z_⊙. Finally, at variance with previous studies, we find that most of the metals reside in the cold gas phase already after a few Myr. This result is mainly caused by the assumed low SN II heating efficiency, and justifies the generally adopted homogeneous and instantaneous mixing of gas in chemical evolution models.     

Interstellar deuteroammonia

Close to 30 deuterated molecules have now been detected in the ISM, including doubly-deuterated species D₂H⁺, ND₂H, D₂CO, CHD₂OH, D₂S, and D₂CS, as well as triply-deuterated ammonia and methanol. We review the current understanding of depletion and deuteration processes in cold, dense interstellar medium (ISM) and discuss the utility of deuteroammonia as a tracer of the physical conditions and kinematics of cold, dense gas.     

Mapping the roots of the galactic outflow in NGC1569

The exact nature of the interaction between hot, fast-flowing star-cluster winds and the surrounding clumpy interstellar medium (ISM) in starburst galaxies has very few observational constraints. Besides furthering our knowledge of ISM dynamics, detailed observations of ionised gas at the very roots of large-scale outflows are required to place limits on the current generation of high-resolution galactic wind models. To this end, we conduct a detailed investigation of the ionised gas environment surrounding the young star clusters in the starburst galaxy NGC1569. Using high spatial and spectral-resolution Gemini/GMOS integral-field unit observations, we accurately characterise the line-profile shapes of the optical nebular emission lines and find a ubiquitous broad (～300 km?s^?1) component underlying a bright narrower component. By mapping the properties of the individual line components, we find correlations that suggest that the broad component results from powerful cluster wind–gas clump interactions. We propose a model to explain the properties of the line components and the general turbulent state of the ISM.     

Radiation drag driven mass accretion in a clumpy interstellar medium: implications for the supermassive black hole-to-bulge relation

We quantitatively scrutinize the effects of the radiation drag arising from the radiation fields in a galactic bulge in order to examine the possibility that the radiation drag could be an effective mechanism to extract angular momentum in a spheroidal system like a bulge and allow plenty of gas to accrete on to the galactic centre. For this purpose, we numerically solve the relativistic radiation hydrodynamical equation coupled with accurate radiative transfer, and quantitatively assess the radiation drag efficiency. As a result, we find that in an optically thick regime the radiation drag efficiency is sensitively dependent on the density distributions of the interstellar medium (ISM). The efficiency drops according to     in an optically thick uniform ISM, where τ _T is the total optical depth of the dusty ISM , whereas the efficiency remains almost constant at a high level if the ISM is clumpy . Hence, if bulge formation begins with a star formation event in a clumpy ISM, the radiation drag will effectively work to remove the angular momentum and the accreted gas may form a supermassive black hole. As a natural consequence, this mechanism reproduces a putative linear relation between the mass of a supermassive black hole and the mass of a galactic bulge, although further detailed modelling for stellar evolution is required for a more precise prediction.     

Self-gravity driven instabilities of interfaces in the interstellar medium

In order to understand star formation it is important to understand the dynamics of atomic and molecular clouds in the interstellar medium (ISM). Non-linear hydrodynamic flows are a key component to the ISM. One route by which non-linear flows arise is the onset and evolution of interfacial instabilities. Interfacial instabilities act to modify the interface between gas components at different densities and temperatures. Such an interface may be subject to a host of instabilities, including the Rayleigh–Taylor, Kelvin–Helmholtz, and Richtmyer–Meshkov instabilities. Recently, a new density interface instability was identified. This self-gravity interfacial instability (SGI) causes any displacement of the interface to grow on roughly a free-fall time-scale, even when the perturbation wavelength is much less than the Jeans length. In previous work, we used numerical simulations to confirm the expectations of linear theory and examine the non-linear evolution of the SGI. We now continue our study by generalizing our initial conditions to allow the acceleration due to self-gravity to be non-zero across the interface. We also consider the behaviour of the SGI for perturbation wavelengths near the Jeans wavelength. We conclude that the action of self-gravity across a density interface may play a significant role in the ISM either by fuelling the growth of new instabilities or modifying the evolution of existing instabilities.     

Gas phase processes affecting galactic evolution

Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities, suggesting that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form. If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can be calculated and the star formation rate determined from first principles. The results suggest that only 10^-4 of the ISM mass actively participates in the star formation process and that this fraction does so because its density is larger than 10⁵ cm^-3, at which point several key processes affecting dynamical equilibrium begin to break down. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

银河系化学元素的丰度特征和合成图像(I):星际介质中轻元素丰度的观测及其特征

给出并解释了星际介质中轻元素D，3He，4He和Li的最新观测数据．星际介质中轻元素的丰度观测结果可以用来检验标准大爆炸核合成理论，因此对这些元素的丰度研究具有重要的天体物理意义．到目前为止，轻元素丰度的观测结果基本上支持开放宇宙的观点．根据最新的观测结果，在本地星际介质中D丰度可能存在小尺度不均匀性，而对类星体吸收云的观测表明不同观测者所获得的原初D丰度结果最大差别可达一个量级．如果观测是可靠的，那么在目前的标准大爆炸核合成理论和星系化学演化模型框架下还不能解释这种结果．另外种种迹象表明太阳系丰度可能不代表45亿年前本地星际介质的丰度．     

Matrix isolation spectroscopy study of iron reactivity towards PAHs

The hypothesis of organometallic chemistry between polycyclic aromatic hydrocarbons (PAHs) and metals in the interstellar medium (ISM) has been recently proposed to contribute both to the unidentified interstellar infrared (UIR) emission and to the depletion of transition metals in the ISM. Since temperature and pressure conditions of the ISM can be easily simulated by matrix isolation techniques, we have undertaken the preparation of such species through cocondensation reactions of metal atoms (iron) with some PAHs (pyrene, coronene) in low temperature argon and dinitrogen matrices. The identification of species is achieved by means of FTIR and UV-visible spectroscopies. First results show that a weak reactivity occurs between neutral species at lOK: π-complexes, hydrogen-bonded compounds, and one hydride species have been observed.