Abstract. N-body simulations of the formation of clusters of galaxies allow a detailed, quantitative comparison of theory with observations, from which one can begin to address two fundamental and related questions:

INTRODUCTION

A wide range of theories have been proposed to explain the origin of galaxies, clusters of galaxies, and the large-scale structure of the universe. Broadly speaking, these can be divided into two classes. Most currently popular models for the formation of structure in the universe are based on the idea of gravitational instability in an expanding universe, in which it is assumed that structure has grown gravitationally from small-amplitude, Gaussian primordial density fluctuations. A second class of cosmogonic scenarios, which will be referred to here as non-Gaussian models, appeal to other processes besides simple gravitational clustering as the driving force behind the genesis of structure.

Within the basic framework of the gravitational instability picture, there are several rival theoretical scenarios that are viable at present. Depending on the the details of the cosmological initial conditions and dominant mass component of the universe, the sequence of formation of structure may have proceeded in quite different ways. If, for instance, the universe is dominated by weakly interacting, non-baryonic particles (i.e., cold dark matter, hereafter CDM) then the formation of structure is expected to proceed hierarchically from small to large scales, with galaxy and cluster formation preceding the collapse of superclusters.

Abstract. A program is proposed for future optical research on clusters of galaxies. This program includes detailed studies of the internal properties of clusters, the connection between clusters and their environment, and the role of clusters in the study of large-scale structure. It is argued that a digital all-sky survey can be feasibly made with a small telescope and a CCD camera, for studies of nearby and intermediate redshift clusters.

INTRODUCTION

Well, we have now heard and seen a large variety of papers on the properties of clusters of galaxies covering topics which range from determining some of their simple “internal” properties, such as dynamical age and mass, through their use as probes of the large-scale-structure of the Universe. Hearing these, it is quite obvious to me that our knowledge of clusters and their place in the Universe has increased tremendously in the last decade—including what some may call a few backward steps with the realization that many, if not most, clusters are dynamically quite complex and probably “young.”

I have been fortunately given the easy task of describing where to go next—always a lot of fun when you have both found out what you don't know and are preparing many new marvelous tools, like the Hubble Space Telescope and suites of new 8-meter class and survey telescopes, with which to attack the problem.

Abstract. Distant clusters provide ideal samples of galaxies in more-or-less standard environments in which to study the evolution of the galaxies themselves, bound structures, the larger-scale environment, and perhaps eventually to provide data for the classical cosmological tests. We review some of the the observational and theoretical aspects of these topics.

INTRODUCTION

Clusters of galaxies at large redshifts provide, in principle, a set of objects whose evolution can be traced directly from epochs as early as z ≈ 1 with present observational capabilities to the present. It seems almost inconceivable that large clusters are destroyed, and although it is quite clear that clusters are still forming, the inner regions of dense clusters must be quite old. Thus if one looks at galaxies in such regions and takes care to sample clusters whose comoving space densities are roughly the same at all epochs, it would seem as if one could define a quite homogeneous sample of galaxies in which the direct forbears of a set of present-day objects could be studied. We are not quite in a position to do that because the cluster catalogs are in such a sad state, but some progress is being made in this direction; we will discuss this at greater length below.

If one could choose clusters at epochs from the present back to large redshifts in some objective way, it would also be possible to study the evolution of the cluster population itself.

Introduction

Molecules have been found to exist in a rich variety of astrophysical environments, including stellar atmospheres, comets, planetary atmospheres, and the dense and diffuse clouds in the interstellar medium. An understanding of molecular structure, spectroscopy, and photoabsorption processes is thus of critical importance in interpreting many of the current observations, in theoretically modeling these various astrophysical regions, and in judging the reliability of the available molecular data. Even in our own atmosphere, considerations of molecular photoabsorption determine ‘windows’ in the electromagnetic spectrum in which one can carry out ground-based observations. Accurate molecular spectroscopic data derived from laboratory experiments or theoretical calculations are necessary in order both to identify molecular species and to quantify abundances of these species from the observed absorption line wavelengths and intensities, respectively.

The vibrational and rotational nuclear degrees of freedom add significantly to the spectroscopic complexity of a molecule compared with an atom. As in an atom, transitions between different electronic energy levels in a molecule are generally observed at optical (4000–6000 Å) and ultraviolet (800–4000 Å) wavelengths, whereas transitions between vibrational energy levels and rotational energy levels generally take place in the infrared region (2–20 µm) and microwave region (λ > 0.2 mm) of the electromagnetic spectrum, respectively. This range of energies over which molecules are responsive to radiation makes them valuable as sensitive probes of the physical conditions of the astrophysical environments in which they are found.

Introduction

The interior of a dense molecular cloud is efficiently shielded from the ultraviolet photons of the interstellar radiation field by the grains. Accordingly, the effects of the ultraviolet photons on the physical and chemical states of dense clouds are in general, neglected. However, several internal sources of ultraviolet photons may be present, including the emission from embedded stars and internal shocks driven by mass loss from young stars. Here the discussion is focused on a diffuse source of internal ultraviolet photons arising from energetic cosmic rays capable of penetrating dense clouds. Cosmic ray particles with energies between 10 and 100 MeV ionize molecular hydrogen in the interior of the clouds and generate secondary electrons with a mean energy of around 30 eV (Cravens and Dalgarno 1978). Because the fractional ionization is generally low, the secondary electrons degrade mainly through excitations of various electronic states of H2. Typical excitation energies are 10–15 eV. Ionization of H2 by the secondary electrons may also occur. The subsequent decays of the electronically excited states of H2 produce ultraviolet photons within the clouds.

The idea of molecular hydrogen emission inside dense clouds was invoked by Prasad and Tarafdar (1983) to explain the large abundance of atomic carbon which exists in several molecular clouds. Observations of emission in the 3P1–3P0 and 3P2–3P1 fine-structure lines of neutral atomic carbon in dense interstellar clouds (Phillips and Huggins 1981, Keene et al. 1985, Zmuidzinas, Betz and Goldhaber 1986) have demonstrated the existence of C with abundances relative to CO exceeding 0.1.

Introduction

It's a great pleasue for me to contribute to this volume in honor of Alex Dalgarno's 60th birthday. I first met Alex when I came to Harvard as an assistant professor in 1968. From these turbulent times up to now, Alex has been a very good friend and mentor to me. From Alex I gained an interest in atomic and molecular processes that has influenced my research ever since. My writing skills improved considerably by virtue of working with him. Most important, I am grateful to Alex for setting an example of what a good professor should be, not only as a scholar and teacher, but also as a generous and loyal friend to his students and colleagues.

With Supernova 1987A (February 23, 1987), nature has provided some birthday fireworks that will be a festive reminder of Alex's many important contributions to astrophysics. The brightest supernova since SN1604 (Kepler's supernova), SN1987A is the first one that has been observed in every electromagnetic wavelength band and it is the first one that will remain observable for several years as the debris clears away to allow a detailed view of its interior. Thus, SN1987A offers an unprecedented opportunity to infer details of supernova explosion dynamics and nucleosynthesis. This task presents fascinating and challenging problems in atomic and molecular astrophysics because, as I will describe, SN1987A is remarkably cool (≥7000 K) throughout its interior and there is good evidence that CO and SiO molecules have already formed there.

Introduction

The subject of this article is dust in dense interstellar clouds – its composition and its chemical evolution. The relevant astronomical data include extinction and polarization in the infrared, visible and ultraviolet spectral ranges. Identification of the carriers of the observed spectral features is a non-trivial task. The features appear to be notoriously non-unique; significant fractions of them have been assigned to two or more dissimilar materials. Also, it now appears that grains are made largely of highly disordered and/or composite materials. In disordered mixtures spectral features of molecules can be altered considerably with respect to known spectra of pure crystalline materials, which also complicates the identification.

Clearly, additional sources of information are needed. The sources which I chose to employ are the recently available data on the composition of Halley's comet, and on the structure, the composition and the spectral properties of the interplanetary dust particles (IDPs). I thus assume that cometary and IDP materials preserve many of the characteristics of the dust in the original interstellar cloud. As a rationale I quote a recent review of Geiss (1987) on the results of exploration of Halley's comet: (a)‘… The abundance data show that a large fraction of material in Halley's nucleus condensed at very low temperature’ (b)‘… comets are regular members of the solar system which have preserved the original charactistics of the condensed and accreted matter better than other bodies in this system.’

Introduction

A detailed knowledge of collisional excitation processes is important in various aspects of the study of interstellar clouds. Because local thermodynamic equilibrium rarely obtains in such environments, the diagnosis of physical conditions, such as temperature, particle density, and radiation density, requires a quantitative understanding of all microscopic processes (i.e. collisional excitation and deexcitation and radiative decay and absorption) which influence the excitation conditions. Quite often, only rotational excitation of simple molecules need be considered. For instance, dense, cold cloud gas is studied primarily by observing millimeter and submillimeter emission features arising from transitions between different rotational levels of molecules in their ground electronic and vibrational states. In diffuse clouds, simple diatomics such as H2, CN and C2 are observed through electronic absorption transitions involving different rotational levels of the ground vibrational and electronic state. Even some atomic species such as C and C+ are observed in various fine structure states. This information is important for the diagnosis of diffuse clouds. Observable emission from vibrationally excited molecules arises in hotter gas or in regions exposed to a strong ultraviolet radiation field, but the collisional excitation of vibrational states is not well understood for the appropriate temperature range. However, the observed emissions due to the decay of collisionally excited fine structure levels of atomic species, such as C and C+ can be used to investigate clouds.

Second, collisional excitation of atomic and molecular species is always followed by spontaneous radiative emission leading to a loss of energy from the medium which is an important cooling process of the interstellar gas.

The present volume has been designed to be a self-contained introduction to the field of molecular astrophysics. It can serve as the text for a one semester postgraduate course concerning that subject exclusively or as a supplementary text in a postgraduate course on the interstellar medium. It can also be used by research astronomers, atomic and molecular physicists, chemists, and atmospheric scientists who have interests in weakly ionized plasmas and the physical and chemical processes which occur in them and who wish to become familiar with recent work in molecular astrophysics.

Many of the articles concern theoretical studies and modelling. Part I, consisting of two chapters written by observers, provides a general description of the astronomical context within which much of the remainder of the volume should be considered. The contribution by Per Friberg and Åke Hjalmarson reviews briefly our understanding of the global physical properties of the Galactic interstellar medium, describes the wide range of conditions within the dark molecular clouds (those having visual extinctions greater than about 1) and the distribution in the Galaxy of the dark clouds, and compares the chemical contents of different regions in various dark clouds. Leo Blitz has written about observations of molecules in other galaxies including some with active nuclei.

The study of chemistry in diffuse molecular clouds (those having a visual extinction of about 1 or less) constitutes the subject of Part II and is perhaps the most fundamental area in molecular astrophysics.

Introduction

Our understanding of the structure of the interstellar medium has progressed enormously in the last 15 years largely as a result of the development of radio emission and ultraviolet absorption observations. From CO radio surveys we now know that about one-half of the interstellar gas is in the form of giant molecular clouds that are the formation sites of O and B stars, while measurements of highly excited gas, such as hot bubbles and expanding shells, indicate that a significant volume of the interstellar medium contains dilute, hot gas. The physical properties and dynamics of the interstellar gas are strongly influenced by the interaction of the massive O and B stars and supernovae. Supernova shells sweep up interstellar matter creating clouds and also impact them causing compression and fragmentation. The O and B stars produce large HII regions which expand and compress the interstellar gas. Thus on a theoretical basis we expect to find complex velocity fields, shock structures, and density inhomogeneities in the diffuse and translucent component of the interstellar clouds.

Today it is hard to imagine studying the diffuse interstellar clouds without relying on observations of molecular lines. In an excellent review article by Dalgarno and McCray (1972) on the heating, ionization and cooling processes of HI regions, these authors stated that ‘The processes leading to interstellar molecule formation are only partially understood …’. The central role of molecules as both important components and physical probes was just becoming evident.