We discuss several recent observational results according to which a significant fraction of the Galactic dark matter halo may exist in the form of dense self-gravitating clumps of H2.

We model the large scale mass distribution in the Milky Way and derive in a self-consistent way the associated gravitational potential. The basic constraint in our model is that the shape of the gaseous Galactic halo, observable from synchrotron radiation, γ-rays, soft X-ray background and from H I lines with high-velocity dispersion needs to be explained by the mass distribution.

The resulting mass model has a flat rotation curve. In the solar vicinity surface density, volume density and gravitational acceleration Kz are consistent with all known constraints. We find that the distributions of HI gas and dark matter are closely related to each other. Furthermore, the mass distribution implies a co-rotation of disk and halo for radii R > 5 kpc.

Our analysis supports strongly the hypothesis, that the gaseous halo of the Milky Way traces dark matter in the form of dense self-gravitating H2 condensations as indicated from an analysis of the γ-ray background observed with EGRET (Shchekinov et al., these proceedings). In addition we find evidence for an extended non-baryonic dark matter halo, which co-exists with the baryonic component. We derive for the Milky Way a baryonic mass fraction of 12% in close agreement with cosmological predictions.

H2 and HD molecules provide the cooling needed for the fragmentation and collapse of the first structures in the universe. In this review, we describe the main chemical and physical processes occurring in the primordial gas after the recombination epoch. We also highlight the areas where improvements in the determination of reaction rates and excitation coefficients are necessary to reduce the remaining uncertainties in the predictions of the numerical models. The interaction of primordial molecules with the CBR and the role of H2 and HD cooling in the early universe are discussed. Finally, we comment on the results of recent simulations of the fragmentation and collapse of primordial clouds, with an emphasis on the typical mass scale of the first objects.

Introduction

The formation of the first stellar objects in the universe is a fascinating, yet little understood process. Although we have now observational data on bright quasars and galaxies out to redshifts of about 5 (corresponding to 109 yr after the Big Bang) and on the density fluctuations at redshifts about 1000 (or an age of ∼ 106 yr), there is no direct evidence as to when and how the first structures formed. This unique epoch and the nature of the primordial objects define what has been called the end of the “Dark Ages” (Rees 1999).

According to Big Bang cosmology, there must have been an epoch in the history of the universe during which the original gas mixture was altered by the manufacture of heavy elements inside stars.

Two topics of relevance for H2 formation in the interstellar medium are considered: (i) the interaction of H and H-H with a model-graphite surface (Coronene: C24H12), and (ii) H− formation by charge transfer in the interaction of H with a model-silicate surface (MgO{100} representing forsterite: Mg2SiO4{100}). The first topic is related to the frequently invoked Langmuir-Hinshelwood and Eley-Rideal mechanisms for H2 formation near carbonaceous zones of interstellar dust grains. Ab initio calculations based on Density Functional Theory are used. The second topic proposes a new scenario in which the efficient production of H− ions would subsequently enable the formation of H2 via associative detachment. It stems from recent work of the authors on charge transfer between neutral atoms and ionic insulators.

Introduction

The mechanism of H2 formation in the interstellar medium (ISM) is still an open problem. Owing to the temperature and density conditions existing in the ISM, 3-body recombination and radiative association processes in the gas phase are unable to account for actual H2 abundances. The existence of dust particles in the ISM has attracted the attention of astrophysicists as plausible catalysts or mediators of H-H recombination in space (Hollenbach and Salpeter (1970), Hollenbach and Salpeter (1971)). Current knowledge of interstellar dust particles (IDPs) indicates that they have both a carbonaceous and a silicate composition. This has in particular stimulated the investigation of the role graphitic bonds may have on H2 formation as a result of elementary interactions between H atoms and graphite-like surfaces or platelets.

The bulk of the molecular gas in spiral galaxies is under the form of cold H2, that does not radiate and is only suspected through tracer molecules, such as CO. All tracers are biased, and in particular H2 could be highly underestimated in low metallicity regions. Our knowledge is reviewed of the H2 content of galaxies, according to their types, environment, or star-forming activities. The HI and CO components are generally well-mixed (spiral arms, vertical distribution), although their radial distributions are radically different, certainly due to radial abundance gradients. The hypothesis of H2 as dark matter is discussed, as well as the implications on galaxy dynamics, or the best perspectives for observational tests.

How to observe H2 in galaxies?

The bulk of molecular hydrogen in a galaxy is cold, around 10–20K, and therefore invisible. The first rotational level, accessible only through a quadrupolar transition, is more than 500 K above the fundamental. The presence of H2 is inferred essentially from the CO tracer. The carbon monoxide is the most abundant molecule after H2; its dipolemoment is small (0.1 Debye) and therefore CO is easily excited, the emission of CO(1–0) at 2.6mm (first level at 5.52K) is ubiquitous in the Galaxy.

The H2/CO conversion ratio

To calibrate the H2/CO ratio, the most direct and natural is to compare the UV absorption lines of CO and H2 along the same line of sight (Copernicus, e.g. Spitzer & Jenkins 1975; ORFEUS, cf Richter et al., this conference).

This book gathers all contributions to the International Conference on H2in Space held in Paris, France, on September 28- October 1st, 1999. The attendance was 106 participants from 16 countries. The goal was to gather together representatives of three communities: experts in the physics of the molecule, including experimentalists, observers of the interstellar medium, in particular of the warm H2 detected in infrared lines, and theoreticians studying H2 formation and cooling in astrophysical objects, from the early universe to the present galaxies.

The electronic structure of the H2 molecule has been well studied in the past, but it was shown that recent progress has been made on the theory of highly excited and long-range Rydberg states, both from calculations of line-strengths and comparison with experiments. New and more accurate data are now available for the rates of collisional excitation of rovibrational transitions by neutrals, as well as protons or electrons. It has been known for several decades that interstellar H2 is formed on dust grains, but, until now, the efficiency of this process was poorly known. Recent experiments have reproduced this formation process on silicates and amorphous carbon and shown that the efficiency is strongly dependent on temperature. In particular, the mobility of H atoms on grains is much lower than previously thought. Laboratory experiments with trapped ions and nanoparticles have opened new avenues of investigation. The relationship of H2 with PAHs is being addressed both in the laboratory and by means of astronomical observations.

We review laboratory studies of the formation of molecular hydrogen on surfaces and in conditions of astrophysical interest. Theoretical analysis and predictions are given on how experimental results can shed light on actual physico-chemical processes occuring in the interstellar medium. Preliminary measurements of H atom sticking are also shown.

Introduction

Molecular hydrogen is the most abundant molecule in the Universe. In space it plays two main roles that render it of incomparable importance:

1. – once formed it becomes a very efficient coolant that increases the rate of collapse of interstellar clouds contributing to shape galaxies and to regulate their dynamics;

2. – once ionized by UV photons or by cosmic rays H2 enters and triggers all reactions schemes that form most of molecular species in the gas phase.

Molecular hydrogen has always been a source of puzzles for astrophysicists in spite of its simplicity. The mechanism of its formation in the extreme conditions encountered in the interstellar medium is certainly one of them. When a sufficient abundance of electrons and ions exists, ion-atom reactions may be quite effective in producing H2 in the gas phase (Stancil & Dalgarno 1998; when, on the contrary, physical conditions allow almost only the presence of neutral hydrogen atoms, the radiative association of two of them is highly inefficient and cannot explain the observed abundances. The main reason is that the time-scale for the release of a consistent fraction of the formation energy (4.5 eV) through the emission of a photon via forbidden transitions is too long and the proto-molecule that is formed in a repulsive state almost invariably breaks up.

We describe spectro-imaging observations of the bright western ridge of the supernova remnant IC443 obtained with the ISOCAM circular variable filter (CVF) on board the Infrared Space Observatory (ISO). The CVF data show that the 5 to 14 µm spectrum is dominated by the pure rotational lines of molecular hydrogen (v = 0–0, S(2) to S(8) transitions). We compare the data to a new time-dependent shock model.

Introduction

The supernova remnant IC443 is a prime example of the interaction of a supernova blast wave with an ambient molecular cloud. On optical plates, IC443 appears as an incomplete shell of filaments (Fig. 1) with a total extent of about 20 arcmin, i.e. ∼ 9pc for an adopted distance of 1,500 pc. The shock generated by the supernova explosion, that occurred (4–13) × 103 years ago, encountered nearby molecular gas which is mainly found along a NW-SE direction across the face of the optical shell. IC443 has been the subject of numerous studies from X-rays, visible, infrared to radio wavelengths (e.g., Mufson et al. 1986 and references therein). Studies of the interaction between the shock and the ambient molecular gas were done by observing molecular hydrogen in the rotational–vibrational transitions (Burton et al. 1988, 1990 – see Fig. 1 – and Richter et al. 1995a), in the pure rotational S(2) transition (Richter et al. 1995b).

We derive and discuss the strong dependence on metallicity of the CO to H2 conversion factor X = N(H2)/Ico = 12.2 – 2.5log[O]/[H] appropriate to extragalactic objects, as well as the weaker dependence found for such objects from interferometer measurements.

Introduction

The difficulty of directly observing molecular hydrogen (H2), the major constituent of the interstellar medium in galaxies, and ways of doing so indirectly are reviewed elsewhere in this volume (Combes 2000). Usually, H2 cloud properties are derived by extrapolation from more easily conducted CO observations. For instance, observed CO cloud sizes and velocity widths yield total molecular gas masses under the assumption of virial equilibrium. However, in extragalactic systems especially, this method is beset by pitfalls (see Israel, 1997, hereafter Is97) and requires high linear resolutions (i.e. use of interferometer arrays). More seriously, the fundamental assumption of virialization appears to be false. As individual components (‘clumps’) have velocities of only a few km s−1 and CO complex sizes are 50–100 pc, crossing times are comparable to CO complex lifetimes of only a few times 107 years or less (Leisawitz et al. 1989; Fukui et al. 2000; see also Elmegreen 2000). As equilibrium cannot be reached in a single crossing time or less, the virial theorem is not applicable to such complexes. Indeed, the elongated and interconnected filamentary appearance of many large CO cloud complexes do not suggest virialized systems (see also Maloney 1990).

We have studied molecular hydrogen emission in a sample of 21 YSOs using spectra obtained with the Infrared Space Observatory (ISO). H2 emission was detected in 12 sources and can be explained as arising in either a shock, caused by the interaction of an outflow from an embedded YSO with the surrounding molecular cloud, or in a PDR surrounding an exposed young earlytype star. The distinction between these two mechanisms can not always be made from the pure rotational H2 lines alone. Other tracers, such as PAH emission or [SI] 25.25 µm emission, are needed to identify the H2 heating mechanism. No deviations from a 3:1 ortho/para ratio of H2 were found. Both shocks and PDRs show a warm and a hot component in H2, which we explain as thermal emission from warm molecular gas (warm component), or UV-pumped infrared fluorescence in the case of PDRs and the re-formation of H2 for shocks (hot component).

Introduction

Molecular hydrogen is expected to be ubiquitous in the circumstellar environment of Young Stellar Objects (YSOs). It is the main constituent of the molecular cloud from which the young star has formed and is also expected to be the main component of the circumstellar disk. Most of this material will be at temperatures of 20–30 K and difficult to observe. However, some regions may be heated to temperatures of a few hundred K and produce observable H2 emission.

Observations of the near-infrared spectrum of molecular hydrogen in photo-dissociation regions has become a standard tool for revealing the detailed physical conditions and complex density structures of molecular clouds. Most recently, consideration has been give to the detailed behaviour of the ratio of ortho-to-para excited states, and the information that this ratio may contain regarding the history of the molecular cloud (Draine & Bertoldi 1996, Sternberg & Neufeld 1999). This paper will review NIR observations of the H2 spectrum with particular reference to the ortho-para ratios observed. Recent spectroscopy of both galactic and extragalactic sources provide some interesting constraints on the models.

Introduction

Modelling of the H2 emission from photodissociation regions (PDRs) has reached a very high level of sophistication a decade after the first observations of H2 fluorescent emission, from the planetary nebula NGC2023. The earliest models, which predicted the response of low density H2 gas to a moderate intensity UV field (Black & van Dishoeck 1987, Sternberg & Dalgarno 1998) have been expanded to include the effects of collisional excitation of the lowest H2 energy levels (Burton, Hollenbach & Tielens 1990, Sternberg 1991) and of self-shielding of dense H2 (Draine & Bertoldi 1996). Observations of the H2 far-red and near-infrared spectrum confirm the model results for emission arising in energy levels as high as Ek > 40,000K (Draine 2000). Recently, theoretical attention has turned to the observed ortho-para ratio of H2 and the potential that this measure may hold for furthering our understanding of the past and present physical conditions in the PDR.

Present day laser technology has advanced such that multiple resonance excitation can be performed using several lasers of various wavelengths. Also narrowband tunable extreme ultraviolet laser radiation is readily available, to bridge the gap between the low-lying electronic ground state and the excited singlet states in molecular hydrogen. These methods have been employed to investigate a new class of excited states of H2 that are confined to large internuclear separation.

Introduction

Molecular hydrogen, the smallest neutral chemical entity, is often considered to be the simplest molecule. For a spectroscopist, however, H2 brings about a number of complications which make it a difficult object to study. First of all, from an experimental perspective, the electronic ground state is separated from the excited states by a large energy gap, which can be bridged only by photons in the domain of the extreme ultraviolet (XUV). Furthermore hydrogen is a light molecule with a very open rotational structure; the rotational lines are often so widely spaced that it is not obvious that they form a progression. Also, as a consequence of the small mass, deviations from the Born-Oppenheimer are most prominent and strongest in H2. Non-adiabatic interactions shift the energy levels over several tens of cm−1, so that the rovibronic structure becomes confused. As a result assignment of observed spectra, even with rotational quantum numbers only, is not straightforward. This point is illustrated by the Dieke atlas (Crosswhite 1972), a compilation of spectra pertaining to transitions between excited states, recorded in the visible domain with a classical spectrometer.

This paper reports the theoretical and experimental work on H2 formation on interstellar dust mimics. These studies are being carried out under the auspices of the UCL Centre for Cosmic Chemistry and Physics.

Introduction

The purpose of this article is to report on the current state of work at the UCL Centre for Cosmic Chemistry and Physics, a consortium of scientists at University College London addressing problems of chemistry arising in astronomy. All the work currently in progress in this consortium is concerned with H2 formation on surfaces, and it consists of both theoretical and experimental programmes.

The Centre was formed a few years ago when it was realised that advances in both experimental and theoretical techniques now make it possible to address in a realistic manner some problems of longstanding and fundamental interest in astronomy. The expertise at UCL, both in theory and experiment, is very strong on surface reactions; the current motivation from astronomy also emphasises the gas/dust interaction (Williams 1998). It was decided, therefore, to undertake a long-term and coordinated programme on surface processes of relevance to astronomy. Of course, the most fundamental interaction is that leading to H2 formation on dust. There is currently some important experimental and theoretical work being carried out in this particular area, and much of this work has been reported at this meeting. Nevertheless, it was felt that the UCL consortium could make a useful contribution without simply replicating the experiments and calculations of others.

Cool fronts originated by H2 formation and supported by non saturated thermal conduction in the pregalactic gas, are analyzed. The pressure (p2), number density(n2), temperature (T2) and flow velocity (v2) behind the front are found as functions of the temperature ahead the cool front T1 and the intake Mach number M1. Compression behind the cool front occur for both, supersonic and subsonic intake flows providing that M1 is larger than a threshold value, the exact value of which depends on T1. But strongly compressed subsonic flows are left for larger values of M1. Quasi-isobaric cool fronts (p2/p1 ≈ 1) occur when the ratio n1/n2 is closed to the maximum value, where the compressional branch just emerges, beyond which the pressure of the flow behind the front increases when n1/n2 decreases, i.e. for denser subsonic flows behind the cool front. Implications of the above results on the formation of cool condensations in the primordial gas are outlined.

Introduction

Previous studies (Field 1965, Yoneyama 1973, Ibáñez & Parravano 1983, Fall & Rees 1985, Corbelli & Ferrara 1995, Puy et al. 1998) have showed that thermal instability can originate cool condensations in hot plasmas. Also it is believed that at large scales such cold structures are the precursors of the gravitational instability, because if a thermal instability is triggered, in cool regions the temperature decreases and the density increases, i.e. the Jeans mass (∼ T3/2ρ−1/2) could decrease below the value of the actual mass and therefore such regions should gravitationally collapse likely forming stars, globular clusters and galaxies.

Observations and interpretation of extragalactic rotational and rovibrational H2 emission are reviewed. Direct observations of H2 lines do not trace bulk H2 mass, but excitation rate. As such, the H2 lines are unique diagnostics, if the excitation mechanism can be determined, which generally requires high-quality spectroscopy and suitable additional data. The diagnostic power of the H2 lines is illustrated by two cases studies: H2 purely rotational line emission from the disk of the nearby spiral galaxy NGC891 and high resolution imaging and spectroscopy of H2 vibrational line emission from the luminous merger NGC6240.

Introduction

Direct observations of H2 emission from external galaxies have become standard practice in the past decade through the revolution in ground-based near-infrared instrumentation. As a result, the near-infrared H2 rovibrational lines are now readily detectable throughout the local universe (e.g., Moorwood & Oliva 1988, 1990; Puxley et al. 1988, 1990; Goldader et al. 1995, 1997; Vanzi et al. 1998). More recently, the Short Wavelength Spectrograph (SWS) on the Infrared Space Observatory (ISO) has for the first time allowed detection of the purely rotational H2 lines in the mid-infrared spectral regime. For instance, the first detection (outside the solar system) of the H2S(0) line at 28.21 µm was reported by Valentijn et al. (1996) from the star forming nucleus of the nearby spiral galaxy NGC 6946.